**The Legacy of Uranium Development on or Near Indian Reservations and Health Implications Rekindling Public Awareness**

#### **Anita Moore-Nall**

**Abstract:** Uranium occurrence and development has left a legacy of long-lived health effects for many Native Americans and Alaska Natives in the United States. Some Native American communities have been impacted by processing and development while others are living with naturally occurring sources of uranium. The uranium production peak spanned from approximately 1948 to the 1980s. Thousands of mines, mainly on the Colorado Plateau, were developed in the western U.S. during the uranium boom. Many of these mines were abandoned and have not been reclaimed. Native Americans in the Colorado Plateau area including the Navajo, Southern Ute, Ute Mountain, Hopi, Zuni, Laguna, Acoma, and several other Pueblo nations, with their intimate knowledge of the land, often led miners to uranium resources during this exploration boom. As a result of the mining activity many Indian Nations residing near areas of mining or milling have had and continue to have their health compromised. This short review aims to rekindle the public awareness of the plight of Native American communities living with the legacy of uranium procurement, including mining, milling, down winders, nuclear weapon development and long term nuclear waste storage.

Reprinted from *Geosciences.* Cite as: Moore-Nall, A. The Legacy of Uranium Development on or Near Indian Reservations and Health Implications Rekindling Public Awareness. *Geosciences* **2015**, *5*, 15-29.

#### **1. Introduction**

Native American communities on American Indian reservations located with natural resources on or near their lands may be at a greater risk for environmentally induced ailments [1]. The impact of natural resource development has not always been fully recognized with respect to the cultural and health effects of the people and animals of these lands. Sometimes the effects are not realized until after the fact when problems associated with resource extraction or cleanup may already be impacting the health of the population [2–8]. On some reservations a lack of education and knowledge about the effects of geologic materials such as uranium and coal led to long term health problems when resources were developed [7]. In this short review the effects of uranium procurement will be addressed, though many other factors may also be contributing to poor health of the Native American populations with natural resources on or near their lands.

Technologically-enhanced, naturally-occurring radioactive material (TENORM) is produced when activities such as uranium mining or milling concentrate or expose radioactive materials that occur naturally in ores, soils, water, or other natural materials [9]. Radioactive materials can be classified under two broad headings: man-made and naturally occurring radioactive materials (NORM). Both of these materials affect many Americans but especially the Native American populations in the United States and Canada, whose designated lands host uranium deposits. Mining of uranium by underground and surface methods produces bulk waste material, including tailings and overburden. During mining the waste rock and soil have little or no practical use, they are generally stored on land near the mine site [10,11]. These materials contain NORM which may become dispersed in the environment through airborne dust and contaminated water. Continued exposure to these materials can cause severe health problems [10,12]. Abandoned conventional uranium mines often contain other hazardous contaminants, such as metals. For example, the carcinogen arsenic may be a problem at some uranium mines, contributing to increased health risks [11].

#### *1.1. The Quest for Uranium*

The origin of the Department of Energy is traced to World War II and the Manhattan Project effort to build the first atomic bomb [13]. The "Manhattan Project" was conducted mainly at the Los Alamos National Scientific Laboratory, a huge fortified compound created in 1943 [14] on the Pajarito Plateau, northwest of Santa Fe, New Mexico, on land supposedly reserved for the exclusive use and occupancy of the San Ildefonso Pueblo [15]. Uranium, the key material used in the lab's experiments and eventual fabrication of prototype nuclear weapons, was mined and milled in four centers of the nearby Navajo Reservation [6,9,16] including reservation land near Shiprock, New Mexico; Monument Valley, Utah; Church Rock, New Mexico; and Kayenta, Arizona. Hanford, a uranium enrichment/plutonium manufacturing facility, was added in 1943, near the town of Richland, on Yakima land in eastern Washington [16,17]. The Hanford area bordering the Columbia River was home to several tribes of Native Americans for centuries. Remnants, artifacts, and burial sites associated with historical Native American activity are found throughout the Site and are protected by law [16]. On 16 July 1945, the world's first atomic bomb was detonated 200 miles south of Los Alamos at Trinity Site on the Alamogordo bombing range [13,14], now the White Sands Test Range, adjoining the Mescalero Apache Reservation. It is this quest for uranium and these different aspects of the procurement plus the disposal and storage of waste that continues to contribute to poor health among many Native American populations. Many cancer clusters and other ailments are attributed to this quest.

#### *1.2. Uranium Production on Native American Lands*

The uranium production peak spanned from approximately 1948 to the early 1980s primarily to produce uranium for weapons and later for nuclear fuel [9,10]. Thousands of mines, mainly on the Colorado Plateau, were developed in the western U.S. during the uranium boom. Native Americans in the area including the Navajo, Southern Ute, Ute Mountain, Hopi, Zuni, Laguna, Acoma, and several other Pueblo nations, with their intimate knowledge of the land often led miners to uranium resources during this exploration boom [5,8]. There are about 4000 uranium mines with documented production [10]. With information provided by other federal, state, and tribal agencies, the Environmental Protection Agency (EPA) has identified 15,000 abandoned uranium mine locations with uranium occurrence in 14 western states with about 75% of those on federal and tribal lands [10]. The majority of these sites were conventional (open pit and underground) mines [10]. Between 1950 and 1989 surface and underground mines in the U.S. produced more than 225 million tons of uranium ore [8]. Figure 1 shows the abandoned uranium mines in the western United States.

**Figure 1.** Uranium locations from Environmental Protection Agency (EPA) database and Federal Lands. The green federal lands are Native American reservations. About three-fourths of the uranium locations in the EPA Uranium Location Database are on Federal Lands. Figure is modified from Geographic Analysis on the Location of Uranium Mines [9].

#### 1.2.1. The Navajo Nation

The Navajo Nation was one of the Indian nations heavily affected by this activity with more than a thousand mines and four uranium mills on the reservation lands [5,6,8]. When mining came to the reservation the Navajo men were ready to gain employment and the close work seemed ideal. What they didn't realize was that they were being exposed to radiation when they worked and brought it home with them in their clothing to their families [6]. Energy material may contain harmful chemical substances that, if mobilized into air, water, or soil, can adversely impact human health and environmental quality [18]. As a result of the mining activity much of the population of the Navajo Nation residing near the areas of mining or milling has had their health compromised. Many of the miners developed cancers; some were lung cancer from inhalation of radioactive particles, *i.e.*, exposure to radon [6]. Of the 150 Navajo uranium miners who worked at the uranium mine in Shiprock, New Mexico until 1970, 133 died of lung cancer or various forms of fibrosis by 1980 [19]. Other potential health effects include bone cancer and impaired kidney function from exposure to radionuclides in drinking water [12]. The government and the mining companies failed to inform the people of the Navajo Nation that working with uranium might be hazardous to one's health [2–8]. The Public health Service even conducted a study to document the development of illnesses as the mining progressed without consent or presenting the data to the miners involved [5,8]. Most of the 1000 unsealed tunnels, unsealed pits and radioactive waste piles still remain on the Navajo reservation today, with Navajo families living within a hundred feet of the mine sites [9,20]. Some of the homes were built with tailings material and much of the water is contaminated on the reservation [20]. Figure 2 shows a sign erected by the Navajo and U.S. EPA which is typical for many of the water sources on the reservation.

**Figure 2.** Sign erected by the Navajo and U.S. EPA which is typical for many of the water sources on the reservation. Figure from EPA Pacific Southwest Region 9 Addressing Uranium Contamination on the Navajo Nation [21].

#### 1.2.2. Laguna Pueblo Tribe

The Village of Paguate (Laguna Pueblo), 40 miles west of Albuquerque, New Mexico was host to the largest open-pit uranium mine in the United States, the Jackpile Mine [22,23]. The mine was the largest producer of uranium ore in the Grants District [24]. Though the site was officially reclaimed in 1995 it is being considered for a National Priorities Listing (NPL) with the EPA after a Record of Decision (ROD) Compliance Assessment for Jackpile-Paguate Uranium Mine was performed to determine if the post-reclamation had met the requirements of the Environmental Impact Statement and ROD. This report concluded that reclamation of the mine was still not complete. The Laguna Pueblo, representing a population of about 8000, rejected mining company offers to operate a uranium mill on tribal land. The mill was built just down the road at Bluewater, now another Superfund site [25].

#### 1.2.3. The Eastern Shoshone and Northern Arapaho Nations

Uranium mining and processing has also left a legacy of contaminated groundwater and tailings on the Wind River Reservation, Wyoming, home to Eastern Shoshone and Northern Arapaho Indians. Increased incidences of cancers among its peoples are attributed to the old Susquehanna-Western uranium mill tailings site [26]. The site is a few miles southwest of Riverton, the ninth most-populated city in Wyoming. In some areas of the Wind River Indian Reservation groundwater contamination is so bad that the Department of Energy (DOE) estimates drinking water from contaminated aquifers could make residents up to 10 times more likely to develop cancer than the general population [26]. Uranium was not mined on the Wind River Reservation but uranium mined in the Pryor Mountains, Montana and Northern Bighorn Mountains, Wyoming was some of the ore processed there.

#### 1.2.4. The Sioux Nations

Uranium mining in South Dakota, Wyoming, Montana, and North Dakota began in the middle of the 1950s [1]. More than 1000 open-pit uranium mines and prospects can be found in the four state region according to U.S. Forest Service maps. There were numerous uranium mines throughout the southern Black Hills National Forest as well as in Custer National Forest near the Lakota-Sioux lands in the Black Hills of South Dakota, which also had mines [1]. Most of these have not been reclaimed.

#### 1.2.5. The Spokane Nation

The only uranium mining in Washington State was on the Spokane Indian Reservation. The mines were the Sherwood Uranium Mine and the Midnite Uranium Mine, which opened in the 1950s to produce uranium for the U.S.-Soviet nuclear arms race [27]. Just as on the Navajo reservation the mines brought needed employment to the reservation at that time and the miners were not informed of the dangers uranium mining [28]. About 33 million tons of radioactive waste rock and ore remain at the 350-acre site above the Spokane River [27]. The mines have been closed since the 1980s. The Midnite Mine site, the larger of the two uranium mines on the reservation is a superfund site [27]. Newmont Mining Co. (Greenwood Village, Colorado, USA) and its subsidiary, Dawn Mining Co. (Ford, Washington, USA) expects to begin cleanup of the Midnite Mine in 2015 [27]. "The plan is to fill in open pits left from the mine excavations with the waste rock and ore. The pits would be capped to keep radon gas from escaping. Groundwater in the pits will be collected and treated and then piped 7 miles to the Spokane River" stated a consultant from Newmont [27]. "Newmont already collects and treats water at the old mine site, but that water is currently discharged into Blue Creek, a tributary of the Spokane River. Discharging the treated wastewater directly into the Spokane River will reduce the impacts to Blue Creek, where the tribe is working to re-establish a native redband trout run" stated the Superfund director for the Spokane Tribe [27]. "The water discharged into the Spokane River must meet the tribe's water quality standards for radionuclides, heavy metals and other pollutants, which are stricter than state and federal standards. The cleanup work is also subject to permits from the U.S. Environmental Protection Agency" [27] Members of the Spokane Tribe who worked at the mine or who live on the reservation are questioning the high rates of cancer on the reservation [28].

The Spokane Tribe teamed up with the Washington Department of Health and the Northwest Indian Health Board to track cancer rates among the tribe's 2700 members living with the legacy of the mining from the Sherwood Uranium Midnite Uranium Mines [28]. Study results are pending [28].

#### *1.3. Nuclear Weapons Development*

Pacific Northwest tribal groups on nine reservations in Washington, Idaho and Oregon were impacted by Hanford Nuclear reservation activities [29]. The Hanford Nuclear site is located on 1518 square km of shrub-steppe desert in southeastern Washington State [17] surrounded by these nine reservations (Figure 3).

**Figure 3.** Hanford Nuclear Reservation, shown in red, was located in the state of Washington. Nine Native American reservations surround it. Figure modified from Edward Liebow in Hanford, Tribal Risks, and Public Health [29].

The people of these reservations traditionally used [17] and continue to use the lands and resources from the Columbia River Plateau region including land that was ceded to the government for which they retained hunting and gathering privileges [29]. Thus, they may have been exposed to more radiation and contaminants than the general public in practicing traditional lifestyles while fishing, hunting game, food gathering (berries, root plants, *etc.*) harvesting medicinal plants and traditional practices (*i.e.*, sweats), as well as social and spiritual interaction networks [29]. This region was contaminated by Hanford activities through primarily two distinct forms: airborne and river-borne releases, both normal operations and some accidental releases [29]. During the period from 1944 to 1972 Hanford released 25 million curies of radioactive contamination into the environment as a comparison the Chernobyl plant released between 35 and 49 million curies of iodine-131 (I-131) [29]. Five of the reservations, the Colville Confederated Tribes, Spokane, Kalispel, Kootenai, and Coeur

D'Alene are primarily downwind of Hanford Nuclear site's 1450 square kilometer area (Figure 3) and would have been exposed to the airborne release of radioactive contamination for the most part normal by-product of chemical reactions used to separate weapons-grade plutonium from enriched uranium reactor rods, *i.e.*, I-131 with less of a contribution of the river borne releases [29]. The other four reservations, the Nez Perce, Confederated Tribes of the Umatilla, Confederated Tribes and Bands of the Warm Springs and the Yakama Nation are known to consume large quantities of fish and likely received higher doses of river borne releases which resulted from both accidental releases and normal operations that used Columbia River water to cool weapon-production reactor cores [29]. Additionally, liquid waste that had been poured onto the ground or held in ponds or trenches at the Hanford reservation evaporated or soaked into the soil on the site [30]. The waste contaminated some of the soil and is thought to have also created underground "plumes" of contaminants [30] which could also affect the tribes who consumed native food sources in the area. These nine reservations were all part of the Intertribal Council on Hanford Health Projects established in 1994 when all tribal governing bodies involved agreed on bylaws and operations plans for the council [29]. The group sought to give the tribal perspective of the information needed for estimating radiation doses from distinctive traditional lifestyles of the represented tribes and protect their sovereignty in public health research while also ensuring the scientific integrity of the research involving their people and land [29]. The final report of the federal government's Hanford Thyroid Disease Study (HTDS), a dose-based analysis epidemiological study conducted under contract by researchers at the Fred Hutchinson Cancer Research Center in Seattle from 1989 to 2002 [30], showed northwest U.S. residents with childhood radiation exposures from Washington State's Hanford nuclear site had similar risk levels for thyroid cancer and other thyroid disease regardless of their radiation dose [31]. Many people were dissatisfied with the results of the report and have lawsuits pending [30]. The study was not specific to Native American communities though "the authorizing language which provided funding for the study specifically required that thyroid disease among Native Americans be studied. However, no study focusing on thyroid disease among Native Americans was ever completed." ([29], p. 152). According to the HTDS summary report: "based on information from Native American Tribes and Nations, a study such as the HTDS in Native American populations alone was not feasible because it would have too little chance of detecting any health effects from Hanford's iodine-131" [32]. Native Americans were included in the HTDS if they were identified in the group that made up the study cohort [32]. The study used computer programs from Hanford Environmental Dose Reconstruction Project (1987–1994) and interviews with participants to estimate I-131 doses for 3440 people born between 1940 and 1946 to mothers living in seven Washington counties, took nearly 13 years to complete and cost \$18 million dollars [31]. The site is an environmental cleanup project that approximately 11,000 Hanford employees are involved with today [30]. 

#### *1.4. Milling and Abandoned Mills*

Over ninety percent of all milling done in the U.S. occurred on or just outside the boundaries of American Indian reservations [33]. Mills logically would be located near the production or mine sites for infrastructure, thus many mills were on or near the reservations where uranium mining was operating. A disaster of huge consequence for the Navajo Nation occurred at the Church Rock uranium mill spill on 16 July 1979, in New Mexico when United Nuclear Corporation's Church Rock uranium mill tailings disposal pond breached its dam [34]. Over 1000 tons of solid radioactive mill waste and 93 million gallons of acidic, radioactive tailings solution flowed into the Puerco River, and contaminants traveled 130 km downstream onto the Navajo Nation [35]. The mill was located on privately owned land approximately 27 km north of Gallup, New Mexico, and bordered to the north and southwest by Navajo Nation Tribal Trust lands [35]. Local residents, who were mostly Navajos, used the Puerco River for irrigation and livestock and were not immediately aware of the toxic danger [34]. The Navajo Nation asked the governor of New Mexico to request disaster assistance from the U.S. government and have the site declared a disaster area, but he refused, limiting disaster relief assistance to the Navajo Nation [34]. In terms of the amount of radiation released, the accident was larger in magnitude than the Three Mile Island accident of the same year [34] but received little public attention. This was likely due to the remoteness and sparsely populated area of the Navajo Nation which was impacted by the spill. The area was inhabited by mainly Navajo people, many who only spoke their native tongue [34]. This is in contrast to the highly populated area of Middletown, Pennsylvania located three miles from the Three Mile Island Nuclear Generating Station where the TMI accident occurred. Possibly the greater significance of a nuclear power plant *versus* a tailings dam may also have influenced media coverage.

#### **2. Indian Health Service, Census Data and Health Disparities**

#### *2.1. Indian Health Service*

To evaluate health issues of Native American populations one must be aware of the unique relationship that American Indians and Alaska Natives (AI/AN) have with the federal government. The provision of health services to members of federally-recognized tribes grew out of the government-to-government relationship, established in 1787, between the federal government and Indian tribes in exchange for tribal lands. This relationship is based on Article I, Section 8 of the Constitution, and has been given form and substance by numerous treaties, laws, Supreme Court decisions, and Executive Orders [36,37]. The Indian Health Service (IHS), an agency within the Department of Health and Human Services, is responsible for providing federal health services to AI/AN. Approximately 2 million of the 3.4 million AI/AN, members of 566 federally recognized tribes across the U.S., are served by the IHS [36,37]. The organization has fulfilled the federal government's responsibility since 1955. The AI/AN health system has evolved greatly since then and now consists of IHS hospitals and health centers managed by the federal government, tribally managed services, and urban Indian health programs [38]. There are 12 Area offices, which are further divided down into 168 Service Units that provide care at the local level; most of these are rural primary care systems and are staffed by 70% native employees [36].

Tribal involvement and collaboration is an important aspect of the IHS in meeting the health needs of its service population [37]. Tribal delegation meetings are a form of tribal consultation where elected tribal officials meet with the IHS Director or senior staff to discuss health policy and program management issues related to the provision of health services to the Indian population. The IHS has an official Tribal Consultation Policy [37]. There are also programs with committees, task forces, boards and workgroups set up within the IHS system to address different aspects of policy and communication between the tribes and the federal government.

One of these programs is the Environmental Health Services (EHS) program. It includes the specialty areas of injury prevention and institutional environmental health. The IHS EHS program identifies environmental hazards and risk factors in tribal communities and proposes control measures to prevent adverse health effects. These measures include monitoring and investigating disease and injury in tribal communities; identifying environmental hazards in community facilities such as food service establishments, Head Start Centers, community water supply systems, and health care facilities; and providing training, technical assistance, and project funding to develop the capacity of tribal communities to address their environmental health issues [38]. The current IHS director is Yvette Roubideaux, M.D., M.P.H., a member of the Rosebud Sioux Tribe, South Dakota; she has served since May of 2009.

#### *2.2. Census Data and Disparities*

In the 2010 U.S. Census, 5.2 million people (about 1.7% of the U.S. population) identified themselves as AI/AN, solely or in combination with one or more racial/ethnic groups [39]. This population is concentrated in the west and south and proximate to AI/AN areas (reservations/trust lands) for most of the population [39]. AI/AN people consistently experience lower health status when compared with other Americans. The health status of AI/AN is affected by a number of environmental hazards, such as living in remote and isolated locations that expose residents to severe climatic conditions, hazardous geography, and disease-carrying insects and rodents, limited availability of housing and extensive use of sub-standard housing, unsanitary methods of sewage and waste disposal, and unsafe water supplies [40]. Lower life expectancy and the disproportionate disease burden exist possibly due to inadequate education, disproportionate poverty, discrimination in the delivery of health services, cultural differences and geographic location [40]. This population is concentrated in the west and south and proximate to AI/AN areas (reservations/trust lands) for most of the population [39]. AI/AN have the highest national poverty at 27.0%; nine states had poverty rates of about 30% or more for AI/AN: Arizona, Maine, Minnesota, Montana, Nebraska, New Mexico, North Dakota, South Dakota, and Utah [39]. There are many interwoven quality of life issues associated with life in native settings. These are broad issues deeply rooted in economic adversity, poor social conditions and a struggle to maintain a cultural identity while assimilating with U.S. society. A major cause of poverty in Native American communities is the persistent lack of opportunity; even most of the communities with natural resources on their lands are faced with high poverty. The Economic Research Service reports that Native American communities have fewer full-time employed individuals than any other high-poverty community. Mortality rates in AN populations are 60% higher than those of the U.S. white population [41,42], and mortality in AI populations are about twice that of the general U.S. population [43]. In addition, AI/AN have the lowest cancer survival rates among any racial group in the United States [44]. Native Americans in the Northern Plains region have a cancer mortality rate approximately 40% higher than that of the overall population [45].

There is sufficient evidence of disparities in health care financing, access to care, and quality of care to conclude that American Indians and Alaska Natives are disadvantaged in the health care system [33,34]. Comparing per capita personal health care expenditures on user population the IHS expenditure is \$2741 while the total U.S. population expenditure is \$7239 [37]. Due to the remoteness of many of the IHS facilities and funding available not all IHS facilities have the capabilities to address all the needs of the population. Contract health services (CHS) are purchased based on a priority system. The IHS is the Payor of Last Resort which requires patients to exhaust all health care resources available to them from private insurance, state health programs, and other federal programs before IHS can pay through the CHS program [40].

#### 2.2.1. Toxic Waste Storage

When conditions exist of such extreme poverty for many Native American communities they have been approached by companies wanting to store toxic wastes on their lands. This creates hard situations for some tribes who would like the added "income" but desire to have a safe environment to live in. The Mescalero Apache, Prairie Island Mdewakanton, Minnesota Sioux, Skull valley Goshutes, Lower Brule, two Alaskan native communities, Chickasaw, Sac and Fox, Eastern Shawnee, Quassarie, Ponca. Tribes have all applied to be sites for Monitored Retrievable Storage (MRS), a temporary solution to the problem of storing vast amounts of high-level nuclear waste [46]. The safety of these sites is still under question.

#### 2.2.2. Radiation Exposure Compensation Act

The Radiation Exposure Compensation Act passed in 1990 and amended in 2000 [46], was set up to make partial restitution to the people who contracted cancer and a number of other specified diseases as a direct result of mining, mill working or their exposure to atmospheric nuclear testing undertaken by the United States during the Cold War [6]. For miners the requirements such as whether the miner was a smoker, how long they had worked, whether the mine(s) they were employed in had radon exposure monitoring, medical proof of lung cancer or a nonmalignant respiratory disease, *etc.* made it very difficult for the people to be accepted in the program. Many miners were designated as smokers even though they may have only smoked the equivalent of a pack of cigarettes over a year, in ceremonial practices which increased the WLM (working level months—a measure of radon exposure based on Social Security work records and exposure to radon) required to 500 unless they contracted lung cancer under age 45 then it was 300 WLM [6]. The stringent, often impossible requirements made attaining this compensation hard for most of the victims involved. Many of the people are still trying to be compensated.

#### **3. Discussion and Conclusions**

The legacy of uranium procurement has left a legacy of long-lived health effects for many Native Americans and Alaska Natives in the United States. There have been a number of studies that are starting to address the health impacts of this legacy. The largest population and some of the most impacted people are the tribes living in the southwestern USA, especially the Navajo. A consortium of federal and tribal agencies reported that a five-year, \$110 million project to clean up uranium contamination in the Navajo Nation had addressed the most urgent risks there [47]. But the report also said that in the last five years the agencies have learned much more "about the scope of the problem and it is clear that additional work will be needed" [47]. The consortium included the U.S. Environmental Protection Agency, the Bureau of Indian Affairs, Nuclear Regulatory Commission, Department of Energy and Indian Health Service. The project started in 2007 to tackle the widespread uranium contamination on Navajo lands left over from the nation's atomic weapon production programs. Among their accomplishments, the agencies reported that they have cleaned up nine abandoned uranium mines, rebuilt 34 homes and replaced contaminated soil at 18 sites, many near homes. The agencies also assessed the status of 520 mines, 240 water sources and 800 homes and public structures, exceeding goals set in the five-year plan, the report said [48,49]. It added that officials shut down three contaminated wells and hauled clean water to affected areas of the Navajo Nation or started projects to pipe in water.

Another study in the Eastern Agency of the Navajo Nation in New Mexico [50], studied environmental uranium contamination in a former mining and milling area. Despite decades of inactivity in the mines and mills, environmental contamination was widespread, often in proximity to homes, areas grazed by livestock, and locations frequented by children and families. The uranium contamination in this area was predominantly in the highly soluble chemical forms that could be spread when disturbed or by the bursts of precipitation that occur in this semiarid region at certain times of the year [50].

The Navajo Birth Cohort Study will use Community Based Participatory Research (CBPR) methods [51] and is a collaborative effort to better understand the relationship between uranium exposures and early developmental delays on the Navajo Nation [52]. The five-year Study is funded by Congress at the request of the Navajo Nation and in response to concerns expressed by women about health impacts of living near abandoned uranium mines [53]. Partners in the Study include the Centers for Disease Control and Prevention/Agency for Toxic Substances and Disease Registry, Navajo Area Indian Health Service, Navajo Nation Division of Health, University of New Mexico Community Environmental Health Program, UNM Pediatrics Department Center for Development and Disability, and Southwest Research and Information Center [53]. Eligible women are between the ages of 14 and 45 who have lived on the Navajo Nation for five years, are pregnant, and will deliver at the designated hospitals in Chinle, Gallup, Shiprock, Ft. Defiance, and Tuba City [53].

This short review only provides a quick glance at the continuing legacy of long-lived health effects for many Native American populations due to uranium procurement in the United States. The reader is encouraged to explore some of these topics and become aware of the issues. Native American communities, those on and near reservations, consistently experience lower health status when compared with other Americans. To be able to set up medical geology studies as collaborations within these separate nations, it is important that traditional knowledge is incorporated into the study plans. The non-Native American exposure doses and other variables within each unique study may underestimate necessary values within a study related to Native American communities. Without weighting standard values differently in applied models, or considering different types of variables in GIS and other geospatial tools, the results of the studies may not truly represent the Native American or Alaska Native populations and the effects of the environment or toxic indicator which is studied.

Medical Geology has been shown to be an effective tool in many applications around the world. The sample size of many of the Native American and Alaska Native communities is small for large statistical studies, but increased homogeneity in the small sample sizes due to cultural and traditional values may provide good results, which can be implemented to improve health conditions of the people involved. Funding for projects will also need to be collaborative. Working with programs like the Tribal ecoAmbasadors Program, National Institute of Health, National Science Foundation, the U.S. Geological Survey, IHS and Tribal colleges may yield productive studies that can be used to help improve the overall health of these communities. The use of community-based participatory research approaches that incorporate Native American social networks can be effective in helping to achieve policy changes to address health issues.

#### **Acknowledgments**

Funding for research: Alfred P. Sloan Graduate Scholarship Programs—Minority Ph.D. Component/Sloan Indigenous Graduate Partnership; Montana State University—Dennis and Phyllis Washington Foundation Native American Graduate Fellow; HOPA Mountain Program. Note: The content is solely the responsibility of the author; it has not been formally reviewed by any of the funders.

#### **Conflicts of Interest**

The author declares no conflict of interest.

#### **References**



## **Exposure to Selected Geogenic Trace Elements (I, Li, and Sr) from Drinking Water in Denmark**

## **Denitza Dimitrova Voutchkova, Jörg Schullehner, Nikoline Nygård Knudsen, Lisbeth Flindt Jørgensen, Annette Kjær Ersbøll, Søren Munch Kristiansen and Birgitte Hansen**

**Abstract:** The naturally occurring geogenic elements iodine (I), lithium (Li), and strontium (Sr) have a beneficial effect on human health. Iodine has an essential role in human metabolism while Li and Sr are used, respectively, as a treatment for various mental disorders and for post-menopausal osteoporosis. The aim here is to evaluate the potential for future epidemiological investigations in Denmark of lifelong and chronic exposure to low doses of these compounds. The drinking water data represents approximately 45% of the annual Danish groundwater abstraction for drinking water purposes, which supplies approximately 2.5 million persons. The spatial patterns were studied using inverse distance weighted interpolation and cluster analysis. The exposed population was estimated based on two datasets: (1) population density in the smallest census unit, the parishes, and (2) geocoded addresses where at least one person is residing. We found significant spatial variation in the exposure for all three elements, related mainly to geochemical processes. This suggests a prospective opportunity for future epidemiological investigation of long-term effects of I, Li, and Sr, either alone or in combinations with other geogenic elements such as Ca, Mg or F.

Reprinted from *Geosciences.* Cite as: Voutchkova, D.D.; Schullehner, J.; Knudsen, N.N.; Jørgensen, L.F.; Ersbøll, A.K.; Kristiansen, S.M.; Hansen, B. Exposure to Selected Geogenic Trace Elements (I, Li, and Sr) from Drinking Water in Denmark. *Geosciences* **2015**, *5*, 45-66.

#### **1. Introduction**

Although most chemical elements do not occur exclusively in drinking water (DW), exposures via DW, even at low concentrations, may have important consequences across the entire population [1].

The three trace elements which are in focus here (I, Li, and Sr) have in common that they are essential (I) or possibly essential (Li, Sr) for humans and are currently used as part of pharmaceuticals or dietary supplements. Also, their concentrations in DW can vary spatially, and lifelong exposure to different naturally occurring levels may have an impact on public health in various ways.

#### *1.1. Sources of I, Li, and Sr in Ground- and Drinking Water*

Iodine (I) is a trace element from the halogen group and occurs in oxidation states í1, 0, +1, +3, +4, +5, and +7 [2]. However, in the hydrogeochemical cycle, I is found in the stable inorganic forms iodide (I<sup>í</sup> ) and iodate (IO3 í ), as well as in various dissolved organic iodine compounds. The speciation data of I in Danish DW reported in [3] showed that there were six speciation combinations. Also, the complex spatial distribution of DW-I was attributed to differences in geological layers, hydrogeochemical reactions, and/or treatment procedures at the waterworks [3]. Iodine concentrations in Danish groundwater are characterised by both small-scale heterogeneity and large-scale spatial trends [4]. Voutchkova *et al.* [4] found that elevated groundwater-I concentrations originate from Palaeocene and Cretaceous limestone/chalk aquifers, and saw an association between I, Li, Ba, and Br. These elevated concentrations of I in Danish groundwater seemed to be caused by leaching from soil, the marine origin of the aquifers, and/or saline water influence; however, the processes governing the I concentrations were site and depth specific [5].

The lithium (Li) ion at +1 oxidation state is generally soluble and mobile in groundwater; however, sorption onto clay minerals and zeolite occurs [2]. Lithium in groundwater may have multiple geogenic sources. It occurs in the minerals spodumene (LiAlSi2O6) and lepidolite (K2Li3Al4Si7O21(OH,F)3), but also in many other minerals. Pegmatite and brines especially are strongly enriched in Li [6]. In a European study, a median value of 2.6 ȝg/L Li (min. <0.2 ȝg/L and max. 75 ȝg/L) in tap water was found based on 579 samples from all over Europe [2]. To our knowledge, there are no studies focusing on Li sources in Danish groundwater, except for a baseline study, where Hinsby *et al.* [7] concluded that Li in Miocene aquifers was of natural origin.

Strontium (Sr) occurs in nature in the +2 oxidation state and is the 15th most abundant element on Earth [2]. The size of the Sr2+ ion is intermediate between those of Ca2+ and K+ ; thus, it substitutes them in many rock-forming minerals [2]. High concentrations of Sr in Danish groundwater were studied by Bonnesen *et al.* [8], who found that Sr concentrations increased with depth more than would be expected from diffusion of deep connate seawater alone. They concluded that dissolution of small amounts of Sr-rich aragonite (Ca1í*x*Sr*x*CO3) or equilibrium with the Sr-rich minerals celestite (SrSO4) and strontianite (SrCO3) was the cause of elevated Sr in these chalk formations. Ramsay [9] found a correlation between Mg and Sr, and concluded that recrystallization of Sr-containing aragonite to pure calcite was the main cause of observed elevated Sr concentrations in the chalk aquifers in eastern Denmark. Strontium-enriched groundwater is hence an indicator of limited hydrogeological flushing with fresh water, as carbonate recrystallization takes place on a millennial time scale. Strontium (together with B, Br, Cs, Ge, Li and Rb) is especially enriched in hydrogeochemically mature groundwater [2].

#### *1.2. Public Health and I, Li, and Sr in Drinking Water*

Although there are no EU or U.S. standards for I, Li, or Sr, there are national standards for some European countries [2]. A DW standard (maximum values) for I exists in Russia (125 ȝg/L), for Li in both Russia and Ukraine (30 ȝg/L), and for Sr in Bosnia and Herzegovina (2 mg/L), Russia (7 mg/L), and Ukraine (7 mg/L) (see references in [2]). The Danish DW standard is 1 mg/L for Li, and 10 mg/L for Sr (provisional), while there is no standard for I [10].

#### 1.2.1. Iodine

Iodine has an essential role in human metabolism [11]. Both insufficient and excessive dietary I intake can cause health problems. Worldwide, the focus is on iodine deficiency (ID), as it is "*the*  *single most important preventable cause of brain damage*" [12]. Lower IQ, learning capacities, quality of life, and economic productivity are just a few of the adverse effects of severe ID [12]. Even mild ID can result in learning disabilities, poor growth and diffuse goitre in school children [13]. ID is not confined to developing countries [14]: Zimmermann and Andersson [15] estimated that 43.9% (*n* = 30.5 million) of 6–12-year-old children and 44.2% (*n* = 393.1 million) of the general population in the World Health Organization (WHO) European Region have insufficient I-intake. Denmark is amongst the 30 countries with ID status worldwide; however, subnational surveys are used for estimating the status in Denmark, as recent nationwide ones are lacking [16].

The recommended daily nutrient intake (RNI) for I is 150 ȝg for adults (250 ȝg at pregnancy and lactation), 120 ȝg for 6–12-year-old children, and 90 ȝg for babies 0–59 months old [11]. Iodine in the human body originates mainly from food and DW; however, most foods (except sea products) are naturally I low. Therefore, universal salt iodization (USI) programs have been established in many countries, as this is the strategy officially recommended by WHO and United Nations Children's Fund (UNICEF) for elimination of ID worldwide [11,12]. The sustainability of USI as an ID prevention measure depends on continuous monitoring [12], as well as its integration/coordination with the nationwide strategies for reduction of salt consumption [17]. In the context of the ongoing debate on how to address this issue [14,16,18,19], it is important to focus on regional (local) differences in other I-rich products [18] such as water, milk, *etc.*

Generally, DW is not considered a major dietary I source, providing only 10% [20]. After the mandatory USI was introduced in Denmark in 2001 [21], about 14% of the dietary I intake was derived from DW and other beverages (*w*/*o* juices and milk) [22]. Before the mandatory USI this percentage was 24%–25% [23,24]. It has been shown that local or regional geographical variation of I in DW exists and can be important for the I intake of the population, especially in areas where DW is of groundwater origin, as in Denmark [3,21,25] or China [26,27]. As part of our previous study, the DW contribution to dietary I intake in Denmark was estimated to vary in different parts of the country from 0% to above 100% (adults) or 50% (adolescents) of the RNI [3].

#### 1.2.2. Lithium

Lithium has been used as a treatment for various mental disorders for more than 60 years. The therapeutic doses are much higher than naturally occurring Li levels in DW and typically within a clinical range of 300–1200 mg Li2CO3 per day [28,29]. A substantial amount of studies and meta-analyses investigating patients with mood disorders show that Li significantly reduces suicide mortality in both long-term and short-term treatment [30,31]. It has been hypothesised that if Li in therapeutic doses was effective in preventing suicide in people who already suffer from a mental illness, perhaps Li in small doses over the course of a lifetime could prevent suicide in the general population. This idea has been investigated in several ecological studies on aggregated data. A study from Texas, USA [32] found that counties with high Li levels in DW were associated with significantly lower suicide rates. More recent studies from Japan [33], the U.S. [34], and Austria [35,36]—the latter three also accounting for socioeconomic factors that are closely related to suicide—suggest that long-term intake of small doses of Li via DW may reduce the risk of suicide. Another study in the east of England found no correlation between Li levels in DW and suicide rates [37]. The biochemical mechanisms of action of Li are complex and not fully understood. Studies suggest that Li has a direct antisuicidal effect through a reduction of aggressivity and impulsivity, which are both associated with an increased risk of suicide [38].

#### 1.2.3. Strontium

Osteoporosis is characterised by reduced bone mass and disruption of bone architecture, resulting in increased bone fracture and fragility, and thereby imposing a significant burden on both the individual and society [39]. Hernlund *et al.* [39] estimated, using a diagnostic criterion from WHO, that approximately 22 million women and 5.5 million men residing in the EU in 2010 had osteoporosis. Of these, approximately 0.28 million were from Denmark (female: 0.22 million, male: 0.06 million) [39,40]. The beneficial effects of stable Sr in the treatment of post-menopausal osteoporosis was reported as early as in the 1950–1960s; however, perhaps because of undue association of the stable naturally occurring isotope with the radioactive Sr isotope, those studies did not receive sufficient attention, and the clinical use of Sr nearly ceased in the 1980s [41]. Currently, therapy for osteoporosis includes dietary supplementation of Ca and Vitamin D, in addition to treatment with oestrogen, pharmaceutical products, or fluoride [42]. Strontium ranelate (Sr2+ and ranelic acid) was licenced and introduced to the European market for treatment of osteoporosis in 2004 [39].

The typical adult body burden of Sr is 0.3–0.4 g (99% in the skeleton), and the primary exposure sources are DW, grains, leafy vegetables, and dairy products [43]. Watts and Howe [43] estimated that the total daily intake of Sr in many parts of the world is up to 4 mg/day, with DW contributing 0.7–2 mg/day (based on 2 L daily consumption of DW with Sr concentrations of 0.34–1.1 mg/L). However, they noted that intakes may be higher in areas where DW concentrations are higher. Watts and Howe [43] and Agency for Toxic Substances and Disease Registry (ATSDR) [44] found that there was not enough evidence for Sr toxicity to humans and that human data were inadequate for setting a tolerable intake and a tolerable concentration of Sr.

The positive effects of Sr supplementation on bone have been examined in rats, monkeys, laying hens, and humans in various studies (a few examples are given in [41,42,45]). Very few studies of lifelong Sr exposure effects exist, as reviewed by ATSDR [44]. Dawson *et al.* [46] measured Sr in DW and urine (*n* = 2187) in families that had been residing within their respective communities for at least 10 years, and found a statistically significant product-moment correlation for decreased community mortality rate (in people over 45 years old) for hypertension with heart disease. Polyakova [47] proposed a classification dividing the Arkhangelsk region (Russia) into three zones with different probabilities of Kashin-Beck or similar bone disease manifestation based on the hypothesis that areas with DW characterised by Ca/Sr < 100 are coinciding with Kashin-Beck endemic regions. Curzon [48] found an association between caries prevalence and Sr in DW for lifelong residents in four Ohio cities, with minimum caries prevalence at DW concentrations around 5–6 mg Sr/L. There are few indications that Sr, likely in combination with other trace elements, such as F<sup>í</sup> or Ba, could be beneficial to enhancing remineralisation of teeth, and hence protection against caries (for more references see [49]). To our best knowledge, there are no published reports on the potential public health effects (beneficial or adverse) of long-term exposure to different levels of naturally occurring Sr in DW in combination with Ca, F, or other DW constituents.

#### *1.3. Study Objectives*

The general objective of this study is to evaluate the potential for future epidemiological investigations of long-term (lifelong, chronic) exposure to low doses of three naturally occurring compounds (I, Li, and Sr) from DW. The specific study aims are accordingly (1) to characterise the nationwide spatial patterns of I, Li, and Sr concentrations in Danish DW and (2) to quantify the exposure to I, Li, and Sr from Danish DW.

#### **2. Experimental Section**

#### *2.1. Danish Public Drinking Water Supply*

Denmark is a relatively small country (approximately 43,000 km2 ) with about 5.6 million inhabitants. The Danish DW supply is entirely of groundwater origin. The major part of the population is supplied by public waterworks. The reported groundwater abstraction for DW purposes for 2010 was 397 million m3 (by 2585 public waterworks) [50]. Next to the public waterworks, there are single wells and small waterworks (supplying <10 households), which were estimated to supply about 0.4 million people (7%) [51].

The DW supply is highly decentralised. Sørensen and Møller [52] reported that about 72% of the active waterworks have annual abstractions of <0.1 million m3 , whereas only about 3% are abstracting >1 million m3 .

Bottled water consumption in Denmark was amongst the lowest in Europe in 2013 (22.8 L/cap or 0.127 million m3 [53]) and below the global average (30 L/cap [54]). Thus, the major source of potable water in Denmark is delivered by public waterworks.

Generally, Danish groundwater requires simple treatment with aeration and sand filtration only. In the aeration step, naturally occurring gasses such as methane are removed and substituted by oxygen. During filtration, oxidised iron and manganese are removed. A few other components are detained in the sand filters in various magnitudes. Only 74 waterworks, producing about 50.47 million m3 , use some sort of more advanced water treatment [55]. However, neither chlorination nor ozone treatment was used in Denmark by 2012 [55].

The aquifers used for DW abstraction in Denmark are mainly unconsolidated Quaternary glacial sand, Tertiary marine and fluvial sand or Cretaceous limestone and chalk.

#### *2.2. Water Chemistry Data (I, Li, and Sr)*

The chemical data used here is from a DW sampling campaign conducted from April to June 2013 and reported in detail in [3,56]. The samples represent treated DW from groundwater origin, ready to be supplied to consumers (sampling point: exit waterworks). The treatment consists of

Iodine data were collected for all of the waterworks included in the study (*n* = 144); however, where there were no total I measurements (*n* = 5), iodide (I<sup>í</sup> ) determined by Ion-exchange chromatography (IC) was used instead. Lithium and Sr data were obtained for 139 of the locations. The 144 waterworks abstract about 175 million m3 annually, which accounts for 45% of the total groundwater abstraction by all public water supplies (excluding small waterworks or wells supplying fewer than 10 households) [3]. The samples were filtered in the lab (0.45 ȝm pore size Q-max syringe filter, Frisenette Aps, Ebeltoft, Denmark). Inductively coupled plasma mass spectrometry (ICP-MS) was used for determining I, Li, and Sr concentrations. A short summary of I, Li, and Sr data is presented in Table 1. Further details on the laboratory methods, sampling design and execution, hydrogeochemical characterisation, and water treatment are provided in [3,56].

#### *2.3. Water Supply Areas*

Schullehner and Hansen [57] recently compiled a map with the water supply areas of all 2852 public waterworks in Denmark but excluding a number of very small waterworks and wells supplying <10 households. For the purposes of our study, we have used only the areas supplied by the 144 included waterworks (Figure 1). It should be noted that: some of the selected areas are supplied by more than one waterworks (Figure 1) and it is possible that some of the residents in these areas are not connected to the public water supply but get their water from a privately owned well [57].


**Table 1.** Summary of the iodine, lithium, and strontium datasets (concentrations in drinking water) used in this study.

Notes: \* Here I<sup>í</sup> was used where total iodine was not measured (*n* = 5); IC is the method for I<sup>í</sup>; because these Ií measurements are included, the mean and median calculated here differ slightly from [3].

**Figure 1.** Water supply areas of the waterworks included in the study (*n* = 144); the areas in red, blue, green, and orange are supplied by more than one of the included waterworks (source: map on supply areas of 2852 Danish waterworks [57]).

All selected water supply areas were assigned the water quality measurements (I, Li, and Sr) of the waterworks by which they are supplied. The areas supplied by more than one waterworks were assigned the average concentrations measured at the waterworks supplying the specific area (Figure 1). The two largest cities, Copenhagen and Aarhus, are supplied with water treated by at least 15 and nine waterworks, respectively. An effort was made to pinpoint specific parts of the cities (neighbourhoods), which are preferentially supplied with water from one/some of these waterworks. Previously collected data from the water supply companies (as part of two studies [3,57]) was used for this purpose. However, for parts of the cities (blue and red areas from Figure 1) this was not possible, as the water from all these waterworks is mixed together in the water distribution system. More precise estimations of I, Li, and Sr concentrations are possible only if water samples are obtained at carefully selected points in the distribution system, which was not feasible for this study.

#### *2.4. Estimation of the Population Living in the Selected Water Supply Areas*

Two datasets were used for estimating the population living in the selected areas:

1st dataset: a population density map based on the population counts in the smallest census unit (parishes) for 2008 (further details can be found in [57]). This method yields "number of residents" in the selected supply areas.

2nd dataset: a database including geocoded addresses with at least one registered resident from the Danish Civil Registration System (DCRS), provided by the Centre for Integrated Register-Based Research at Aarhus University (CIRRAU). This database contains one record for each specific address (municipality, road, house number, and, if relevant, door number) used as a residence in DCRS [58]. The DCRS was established in 1968 and has since recorded current and historical information not only on the place of residence, but also on vital status, gender, place and date of birth, parents, spouses, and siblings and twins for all persons living in Denmark [59]. This information is regarded as being of very high quality and yields an important and rare asset which can be used for epidemiological research [59]. A subset of this database has been used here. It consists of the geocoded addresses for 2012 only (*n* = 2,092,090), which are further referred to as "households".

The number of residents (1st dataset) and the number of household addresses (2nd dataset) within each water supply area were calculated using the geographical information system ArcMap 10.0 (Esri, Redlands, CA, USA).

#### *2.5. Inverse Distance Weighted Interpolation and Cluster Analysis*

#### 2.5.1. Data Pretreatment

Due to skewed distributions of I and Sr concentrations, a square root transformation of I and a logarithmic transformation of Sr were applied prior to analysis. The transformations were selected by comparing the distribution of the transformed measurements with a normal distribution.

#### 2.5.2. Inverse distance weighted interpolation

Inverse distance weighting (IDW) was used to estimate a density surface for each of the elements I, Li, and Sr. This method assigns a weighted average of the neighbouring values to each unmeasured grid cell on the map. The weight given to each observation is a function of the distance between that observation's location ݏ and the grid point ݏ at which the interpolation is desired. Generally, the inverse distance interpolator is given as in Equation (1):

$$Z(\mathbf{s}\_0) = \frac{\sum\_{l=1}^{n} \omega(\mathbf{s}\_l) Z(\mathbf{s}\_l)}{\sum\_{l=1}^{n} \omega(\mathbf{s}\_l)} \tag{l}$$

where ܼመሺݏሻ is the predicted value at the unsampled location ݏ and ܼሺݏሻ is the observed value at the *i*th location within a given maximum distance for *i* = 1, …, *n*, with *n* being the number of locations in the study. The weights ߱ attributed to the observations were computed as in Equation (2):

$$\mathfrak{w}(\mathfrak{s}\_l) = ||\mathfrak{s}\_l - \mathfrak{s}\_0||^{-p} \tag{2}$$

where ȁȁݏ െ ݏȁȁ is the Euclidian distance between locations ݏ and ݏ and *p* is an inverse distance weighting power. The weighting power is selected in order to determine how fast the weights tend towards zero as the distance from the grid point increases [60–62].

Grid cells of 1 × 1 km, a power of *p* = 2, and a maximum distance of 75 km were applied. The density maps of I and Sr were derived using a square root and a logarithm transformation, respectively. Back-transformed interpolated values were calculated and mapped.

#### 2.5.3. Cluster Analysis

A local cluster analysis was performed to investigate areas with significantly higher or lower levels of I, Li, or Sr in the DW. I and Sr were transformed prior to the analysis. The presence, significance, and approximate location of clusters were evaluated using spatial scan statistics implemented by Kulldorff [63] in the software SaTScan (v9.3.1, http://www.satscan.org/). The spatial scan statistic searches for clusters by using a search window of varying shape and size. For each location, a test is performed, evaluating whether the mean value is significantly higher (or lower) within the search window compared to outside. Given the continuous data, a normal distribution was used as the probability model in which the null hypothesis was that all observations come from the same distribution, whereas the alternative hypothesis was that there was one cluster location where the measurements have either a larger or smaller mean than outside that cluster. A central feature of this method is that the statistical inference is still valid, even if the true distribution is not normal [64].

The significance of identified clusters was tested using a likelihood ratio test with a *p*-value obtained using Monte Carlo simulations (999 permutations). The likelihood function is maximized over all window locations and sizes, and the one with the maximum likelihood constitutes the most likely cluster. An elliptic search window with a centre at the location of each waterworks was used, allowing no geographical overlap between clusters. The maximum percentage of the measurements to be included in a cluster was varied at 10%, 20%, 25%, 30%, and 50%, respectively. Changing the maximum percentage of the measurements included in a cluster did not change the location and number of clusters identified. A maximum percentage of 20% of the measurements included in a cluster was used.

#### *2.6. Exposure Analysis*

To estimate the exposure to I, Li, and Sr from DW the following data were used: (1) water chemistry data, which was assigned to each of the supply areas and (2) the 1st and 2nd datasets with the number of residents and the number of households in the selected supply areas, respectively. A schematic visualisation of the different datasets (and subsets) and the links between them is provided in Figure 2.

It was estimated that 2,479,976 residents (about 45.3% of all residents in 2008) and 892,725 households (42.7% of all households in 2012) are supplied with DW by the 144 waterworks analysed in this study. For the 139 waterworks where Li and Sr data are present, the number of residents is 2,442,705 (about 44.6%) and the number of households is 874,375 (41.8%). Thus, the exposure analysis covers close to half the population of Denmark (see Figure 1 for spatial reference).

The calculation of residents and households exposed to different levels of I, Li, and Sr are given as a percentage of the households and population included in this study, respectively.

**Figure 2.** Scheme presenting the datasets, their subsets, and the links between them used in this study. Some of these datasets and links were prepared in previous studies (colour coded, references: A [3], B [57], and C [58]).

#### **3. Results and Discussion**

#### *3.1. Spatial Distribution of Drinking Water I, Li and Sr*

The spatial distribution of I, Li, and Sr concentrations in treated DW of groundwater origin was examined using IDW interpolation and cluster analysis. The IDW interpolated maps are presented in Figure 3a,c,e. The cluster analysis identified areas with significantly high concentrations and significantly low concentrations of each of the elements I, Li, and Sr. The number of measurements (*i.e.*, waterworks) in a cluster, mean concentrations inside and outside the clusters, and *p*-values for the three elements are shown in Table 2. The ellipses of the significant hot and cold spots are presented overlaying the maps with the supply areas (see Figure 3b,d,f). However, it should be kept in mind that the cluster analyses are based on point data.

The lowest concentrations for all three elements are observed in the western part of Jylland, where the significant cold spots of I, Li, and Sr also are located. Another similarity in the spatial patterns is the general east-west trend: relatively higher concentrations are found in the eastern part and lower in the western part of the country. Despite that, the spatial distribution of I, Li, and Sr also show differences.

**Figure 3.** Iodine, lithium, and strontium in drinking water in Denmark; (**a**,**c**,**e**) IDW interpolation; (**b**,**d**,**f**) drinking water concentrations in the selected supply areas and hot and cold spot clusters. Note: each category represents ~20% of the observations; the cluster analysis is based on the point data. See Section 2 for details.

An east-west trend for I concentrations in DW was reported by [25,65] and is thereafter largely used by others. However, based on a cluster analysis of I data characterised by higher spatial sampling resolution, Voutchkova *et al.* [3] concluded that there is complex spatial variation and the east-west trend is an oversimplification. The same data as in [3] are used in the current study, while the different analyses employed here add to our previous work; e.g., the IDW interpolated maps (Figure 3a) visually confirm our statement about the complexity of the spatial variation. Northern Jylland is found to be a significant hot spot of DW-I by both the present and the previous analysis [3]. However, due to different data pretreatment (here: square root transformation *vs.* normal score transformation in [3]) and analytical method (here: spatial scan statistics *vs.* Local Moran's I in [3]), a second hot spot (Sjælland according to [3]) was not found here. The governing factors for this complex spatial pattern have been attributed to both the geology and the groundwater treatment procedures [3].


**Table 2.** Geographical clusters of high (hot spot) and low concentrations (cold spot) of iodine, lithium, and strontium in drinking water.

Of all three elements, Li is the one with the most clearly manifested east-west trend: a smooth transition between the low (Li < 8 ȝg/L) and the high concentrations (Li > 17 ȝg/L) is characteristic. A significant hotspot is covers parts of Sjælland and the islands to the south (Figure 3c).

The highest Sr concentrations (>2 mg/L) are observed in the eastern part of Sjælland (covered by the significant hot spot, too), as well as in a few locations in Jylland. The hot spot for Sr is slightly smaller than the one for Li. Strontium is also the only element from the three which has a second significant cold spot, located on the raised Holocene seabed in northern Jylland.

#### *3.2. Exposure to I, Li, and Sr via Drinking Water*

The exposure to different concentrations of I, Li, and Sr from treated DW (groundwater origin) is given as percentage of exposed consumers or households from the ones included in this study, *i.e.*, the residents and households within each water supply area where I, Li, and Sr measurements were available (Figure 4; see Table 3 for absolute numbers).

The largest proportion of households (*h*) and residents (*r*) in this study are exposed to I concentrations in the range of 14–20 ȝg/L (*h*: 44%, *r*: 50%), Li concentrations in the highest range of 17–30.7 ȝg/L (*h*: 33%, *r*: 38%), and Sr concentrations in the range of 2–14 mg/L (*h*: 37%, *r*: 42%). However, only a small proportion of the population is exposed to the highest levels of the observed concentrations: 0.8% (*h*) or 0.4% (*r*) are exposed to Sr > 10 mg/L, which is the current provisional DW standard in Denmark; 1.7% (*h*) or 1.8% (*r*) are exposed to Li > 25 ȝg/L; and 0.5% (*h*) or 0.6% (*r*) are exposed to I > 50 ȝg/L. This exposure calculation takes into account the population density for 2008 (*r*) or all geocoded residential addresses in 2012 (*h*) in the areas supplied by the selected waterworks. Thus, it provides information on the differences in exposure based on the spatial variation of I, Li, and Sr in DW. The spatial distribution of DW supply areas exposed to different levels of I, Li, and Sr is provided in Figure 3b,d,f. From the results presented in Figure 4 (and Table 2), it can be concluded that there is a contrast in the exposure of the Danish population to I, Li, and Sr from DW. Possible health effects of these exposure contrasts could be studied by combining these results with data from the Danish health registers [66].

**Figure 4.** Proportion of households and residents exposed to different concentrations of (**a**) iodine, (**b**) lithium, and (**c**) strontium. Note: the number of households and number of residents are estimated using two different datasets (see Section 2); see Table 3 for absolute numbers.


**Table 3.** Households and residents exposed to different concentrations of iodine, lithium, and strontium in absolute numbers (*n*) and in percentages (%; see also Figure 4).

#### *3.3. Discussion*

The spatial variation of I concentrations in Danish groundwater is characterised both by a large-scale east-west trend [4] and by small-scale variation [4,5]. This is also clearly reflected in the geographical trend of the treated DW-I, which was shown here and in [3]. The elevated I concentrations in Danish DW and groundwater were found to be associated mainly with the Palaeocene to Cretaceous limestone or chalk and postglacial marine-sand aquifers [3–5].

There is limited information about the Li content in Quaternary and Tertiary deposits [7], and especially about the geochemical processes governing the release of Li to groundwater. Lithium is strongly enriched in ocean water compared to freshwater, and in brines. This may lead to the hypothesis that Danish aquifers of marine origin are Li enriched compared to those of glacial and fluvial origin. Moreover, an association between I and Li pointing at saline water influence was found based on Danish historical groundwater data [4].

Strontium concentrations in Danish DW seem to be similar to average concentrations in U.S. streams (between 0.5 and 1.5 mg/L) but higher than average Sr concentrations in U.S. groundwater (<0.5 mg/L) as reported by ATSDR [44]. The hot-spot location supports the regional findings by Ramsay [9] that local differences in the pre-Quaternary chalk and limestone aquifer geology and hydrogeology are responsible for elevated Sr concentrations in present-day groundwater. However, the lower Sr concentrations in DW (western part of Denmark) are most probably governed by the relatively constant contributions from the weathering of Sr-poor silicates in the soil, which is strongly dependent on texture and mineralogy, and from the dissolution of Sr-poor carbonates at the acidification front.

The significant spatial difference in the concentrations of I, Li, and Sr in Danish DW results in varying human exposure to these elements. Therefore, there is great potential for future epidemiological investigations of the long-term (lifelong, chronic) exposure to low doses of the three selected naturally occurring compounds from DW. In addition to exposure to a single element, there is a potential for studying the effects related to a combination of these and additional elements.

For example, in a project from Norway, nationwide data on municipal DW was combined with data on all registered treated hip fractures to study whether Ca and Mg have a protective function [67]. Corresponding studies based on Sr in a single exposure or in combination with Ca, Mg, or F<sup>í</sup> in Danish DW, residential history and the various nationwide registers on health and social issues [66] can be conducted. The spatial distribution of Sr in Danish DW resembles somewhat the spatial distribution of F<sup>í</sup> [68] and Ca and Mg [69]. Hence, this first prospective data analysis reveals a large potential for future nationwide public health studies, especially if Sr in DW is combined with Ca, Mg, and F. Findings relating Sr in DW to life-long health, as reported by Curzon [48] and Dawson *et al.* [46] on hypertension and caries, could most likely be improved considerably with such a multi-element approach, as Sr is closely related to Ca incorporation in human bones [42].

Similarly, an epidemiological investigation can elucidate whether the observed spatial differentiation in exposure to I from DW (see also [3]) influences the health status of the Danish population. The DCRS yields the unique possibility to connect past exposure of mothers (e.g., during pregnancy) to I from DW with the health status of their children in order to explore, whether the observed spatial differences in DW-I affect the children's performance (e.g., physical and mental development).

With respect to Li, there is currently an ongoing nationwide study conducted at the National Institute of Public Health (University of Southern Denmark, Copenhagen, Denmark) using geospatial methods to investigate whether long-term intake of naturally occurring low doses of Li in DW is protective against suicide, when accounting for socioeconomic as well as other factors.

The presented exposure analysis is based on water chemistry data from a single point in time. There is only limited data on the temporal variability of the studied geogenic elements in both treated DW and groundwater in Denmark. A comparison between the Li data used here and other, previously unreported Li measurements can be seen in Figure 5a, showing that the Li levels are similar even though the analytical methods (ICP-MS *vs.* Atomic absorption spectroscopy (AAS)) and the sampling dates differ (2013 *vs.* 2009–2010).

**Figure 5.** (**a**) Lithium concentrations in untreated mixed groundwater from well sites (black dots) at seven waterworks in the Copenhagen area and in treated drinking water (data from two studies are used: <sup>a</sup> the ICP-MS data reported here and in [3] (red +), b unreported AAS data provided by the waterworks (blue +; see Supplementary materials for further detail). (**b**) Comparison between I concentrations in drinking water (red +) and untreated groundwater from wells (black dots) of five waterworks located in Jylland (<sup>c</sup> data from 2012 reported in [70]).

These results imply that the Li concentration in DW at the given waterworks can be assumed to be stable over time. Another interesting aspect is that most of the large waterworks extract water from many wells, often grouped in well sites. As there are differences in I, Li, and Sr concentrations in the abstracted groundwater (Figure 5a, black dots), abstraction volumes and pumping strategies will govern the resulting concentrations in the treated DW. An illustration of the differences in Li concentrations in mixed groundwater at/between the well sites of seven waterworks in the Copenhagen area and the Li concentrations in treated DW at these waterworks is also presented in Figure 5a. Further, the results imply that there is no Li removal or enrichment during the treatment. A similar comparison between groundwater and treated DW is made for I at five waterworks located in Jylland (Figure 5b) based on the 2012 data reported in [70]. This comparison implies that at least at some waterworks, I can be partially removed by treatment (DW-I is lower than the groundwater-I concentrations). The issue of the effect of treatment and the temporal variation of the I concentrations in DW is further discussed in [3]. To our best knowledge, no such analysis exists on Sr in Danish DW or groundwater.

Another limitation of this exposure analysis (or future epidemiological studies based on these data) is that some misclassification could have occurred, as some of the households/residents included here may be supplied by private wells. There is also uncertainty associated with the concentrations in the supply areas where more than one waterworks distributes DW to the consumers (see Figure 1). Yet another issue is connected to the fact that we used I, Li, and Sr concentrations in DW at the exit of the waterworks but not much is known about whether these elements are involved in chemical processes in the distribution system before the DW reaches the consumer. Further investigations are needed in order to evaluate the effect of these misclassifications and/or uncertainties.

Last but not least, some limitations with respect to the exposure levels are due to the data on geographical location of the residency addresses. For the purposes of carrying out an epidemiological study on possible health effects, it is important to take the residential history of each studied individual into account. This is possible using the DCRS database. However, the exposure estimation will still be associated with uncertainty due to e.g., individuals commuting to areas with different exposure characteristics. Data from Statistics Denmark for 2013 shows that 470,950 people or about 31% of the employed Danish population commute less than 5 km, whereas about 7.6% commute more than 50 km (202,289 people).

#### **4. Conclusions**

This study revealed significant spatial variations in I, Li, and Sr concentrations in Danish DW representing approximately 45% of the annual groundwater abstraction for DW purposes. A general east-west trend of relatively high concentrations in the eastern part and lower ones in the western part of the country was observed. However, there are element-specific smaller-scale differences, so the general trend should be interpreted cautiously. The exposure to different I (Li, Sr) levels covers about 45.3% (44.6%) of all residents (2008) and 42.7% (41.8%) of all households (2012) that were supplied with DW by the investigated waterworks (I: 144; Sr and Li: 139). The spatial distribution of I, Li, and Sr in DW results in an exposure contrast of these elements. The largest part of the population (about 40%) is exposed to 14–20 ȝg I/L, 17–30.7 ȝg Li/L, and 2–14 mg Sr/L. The rest of the population covered here is exposed to both higher and lower I concentrations from DW. For Li and Sr, this range is at the upper end of the observed concentrations; thus the rest of the population is exposed only to lower Li and Sr concentrations in DW. The results presented here show that there is a great potential for future epidemiological studies on the long-term (lifelong, chronic) effects of exposure to I, Li, and Sr from DW as single elements. Additionally, it is also possible to include other relevant elements.

#### **Acknowledgments**

We would like to thank Malene Thygesen and the Centre for Integrated Register-Based Research at Aarhus University (CIRRAU) for the provision of the geocoded Danish addresses, and, Ann-Katrin Pedersen from HOFOR (The Greater Copenhagen Water Supply Company) for performing the AAS data analyses on lithium (Figure 5a and Supplementary materials), and providing these data for the present study. The rest of the water chemistry data as well as some of the data on supply areas was collected as part of a GEOCENTER (Denmark) project (2011–2014) funded by the Geological Survey of Denmark and Greenland (GEUS) and Aarhus University (Denmark). The authors express their sincere gratitude to the rest of the GEOCENTER project participants.

#### **Author Contributions**

The linking of the different data sources and the exposure analyses was done by D.D.V. and J.S. The IDW and the cluster analyses were performed by N.N.K. and A.K.E. The data for Figure 5 on Li in the groundwater and drinking water, as well as interpretation was provided by L.F.J. B.H. and S.M.K. provided background information and supervised the project. D.D.V compiled the initial version of this manuscript, with all authors contributing by writing different sections of it. All authors have equally participated in the revisions and the final editing of this text prior to its submission.

#### **Supplementary Materials**

Supplementary materials which include additional information on the unpublished data from Figure 5a can be accessed at: http://www.mdpi.com/2076-3263/5/1/45/s1.

#### **Conflicts of Interest**

The authors declare no conflict of interest.

#### **References and Notes**


