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Special Issue "Environmental Tracers"

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A special issue of Water (ISSN 2073-4441).

Deadline for manuscript submissions: closed (30 November 2013)

Special Issue Editor

Guest Editor
Dr. Trevor Elliot

Environmental Tracers Laboratory (ETL), Environmental Engineering Research Centre (EERC), School of Planning, Architecture & Civil Engineering (SPACE), Queen's University Belfast, David Keir Building, Stranmillis Road, Belfast. BT9 5AG. Northern Ireland, UK
Website | E-Mail
Interests: intelligent tracers for environmental & engineering systems, eg natural, ambient environmental tracers (including stable isotopes, noble gases, CFC’s, SF6, environmental radioactivity) and/or applied tracers; geohydrology; isotope hydrology; groundwater dating; aquifer sustainability issues

Special Issue Information

Dear Colleagues,

Aquifer resources continue to be overexploited, leaving the world's most impoverished (or vulnerable) populations and/or the aquatic environment at an ever increasing risk from climate change.  Adaptation strategies demand detailed evaluation and management of water as a resource, requiring an understanding of the chemical, geological (hydrogeological/geohydrological) and biological interactions that waters effect or undergo in the hydrologic cycle. Environmental tracers are ambient natural or man-made compounds widely distributed in the Earth’s near-surface. They may be injected into the hydrological system from the atmosphere at recharge and/or are added/lost/exchanged inherently as waters flow over and through materials.  Variations in their chemical abundances and isotopic compositions can be used as tracers to determine sources (provenance), pathways (of reaction or interaction) and also timescales (dating) of environmental processes. Water dating may invoke their characteristic decay or accumulation functions, (cf. radioactive and radiogenic compounds and isotopes) in a system or the characteristic injection of sources. Environmental tracers in groundwater systems can give information both on current and past flow conditions independently of hydraulic analyses and modelling. Thus, environmental tracers generically are important tools for developing sustainable management policies for the protection of water resources and the aquatic environment.

Recent overviews have highlighted how most environmental tracer systematics have become well-established through proof-of-concept studies in geochemically and hydraulically simple aquifers. The challenge now lies in enhancing the way they are put to use by the hydrologic community and water resource managers in more complex systems (e.g. inter- and intra-aquifer mixing; aquifers as distributed water systems – water coming in at one point is going somewhere, and pumping of water represents an interception) and how they may be used to address issues of vulnerability, sustainability, and uncertainty in water resource systems (including resource, flooding, drought, climate justice, water and food security, water footprints,  etc.).

Therefore we would like to call for papers to disseminate and share findings especially on the robustness or fitness-for-purpose of the application and use of environmental tracers in water resource systems in addressing problems and opportunities scientifically. Papers are selected by a rigorous peer review procedure with the aim of rapid and wide dissemination of research results, development and application in the wide area of environmental tracers. Original research papers or critical reviews are invited.

Dr. Trevor Elliot
Guest Editor

Submission

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. Papers will be published continuously (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are refereed through a peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Water is an international peer-reviewed Open Access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1200 CHF (Swiss Francs).

 

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Keywords

  • environmental tracers
  • water dating, transit and residence times
  • complex systems
  • inter- and intra-aquifer mixing
  • distributed systems
  • vulnerability, sustainability, and uncertainty in water resource systems

Published Papers (12 papers)

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Editorial

Jump to: Research

Open AccessEditorial Environmental Tracers
Water 2014, 6(11), 3264-3269; doi:10.3390/w6113264
Received: 3 October 2014 / Revised: 21 October 2014 / Accepted: 23 October 2014 / Published: 30 October 2014
Cited by 1 | PDF Full-text (656 KB) | HTML Full-text | XML Full-text
Abstract
Environmental tracers continue to provide an important tool for understanding the source, flow and mixing dynamics of water resource systems through their imprint on the system or their sensitivity to alteration within it. However, 60 years or so after the first isotopic tracer
[...] Read more.
Environmental tracers continue to provide an important tool for understanding the source, flow and mixing dynamics of water resource systems through their imprint on the system or their sensitivity to alteration within it. However, 60 years or so after the first isotopic tracer studies were applied to hydrology, the use of isotopes and other environmental tracers are still not routinely necessarily applied in hydrogeological and water resources investigations where appropriate. There is therefore a continuing need to promote their use for developing sustainable management policies for the protection of water resources and the aquatic environment. This Special Issue focuses on the robustness or fitness-for-purpose of the application and use of environmental tracers in addressing problems and opportunities scientifically, to promote their wider use and to address substantive issues of vulnerability, sustainability, and uncertainty in (ground)water resources systems and their management. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available

Research

Jump to: Editorial

Open AccessArticle Identifying Seasonal Groundwater Recharge Using Environmental Stable Isotopes
Water 2014, 6(10), 2849-2861; doi:10.3390/w6102849
Received: 24 March 2014 / Revised: 12 September 2014 / Accepted: 19 September 2014 / Published: 26 September 2014
Cited by 4 | PDF Full-text (1800 KB) | HTML Full-text | XML Full-text
Abstract
In this study, the stable isotope values of oxygen and hydrogen were used to identify the seasonal contribution ratios of precipitation to groundwater recharge in the Hualien River basin of eastern Taiwan. The differences and correlations of isotopes in various water bodies were
[...] Read more.
In this study, the stable isotope values of oxygen and hydrogen were used to identify the seasonal contribution ratios of precipitation to groundwater recharge in the Hualien River basin of eastern Taiwan. The differences and correlations of isotopes in various water bodies were examined to evaluate the groundwater recharge sources for the Hualian River basin and the interrelations between groundwater and surface water. Proportions of recharge sources were calculated based on the results of the mass balance analysis of the isotope composition of hydrogen and oxygen in the basin. Mountain river water accounted for 83% and plain rainfall accounted for 17% of the groundwater recharge in the Huanlian River basin. Using the mean d-values, a comparison of d-values of precipitation and groundwater indicates the groundwater consists of 75.5% wet seasonal sources and 24.5% dry seasonal sources, representing a distinct seasonal variation of groundwater recharge in the study area. Comparisons between hydrogen and oxygen isotopes in rainwater showed that differences in the amount of rainfall resulted in depleted oxygen and hydrogen isotopes for precipitation in wet seasons as compared to dry seasons. The river water contained more depleted hydrogen and oxygen isotopes than was the case for precipitation, implying that the river water mainly came from the upstream catchment. In addition, the hydrogen and oxygen isotopes in the groundwater slightly deviated from the hydrogen and oxygen isotopic meteoric water line in Huanlian. Therefore, the groundwater in this basin might be a mixture of river water and precipitation, resulting in the effect of the river water recharge being greater than that of rainfall infiltration. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available
Open AccessArticle Water Isotopes as Environmental Tracers for Conceptual Understanding of Groundwater Flow: An Application for Fractured Aquifer Systems in the “Scansano-Magliano in Toscana” Area (Southern Tuscany, Italy)
Water 2014, 6(8), 2255-2277; doi:10.3390/w6082255
Received: 7 May 2014 / Revised: 2 July 2014 / Accepted: 10 July 2014 / Published: 31 July 2014
Cited by 3 | PDF Full-text (20979 KB) | HTML Full-text | XML Full-text
Abstract
TheScansano-Magliano in Toscana” area is characterized by a morpho-structure chiefly made-up by sandstone and shelly-calcareous lithologies. Generally, these complexes host minor aquifers in Tuscany, since they have medium to medium-low permeability. In the area under examination, a sandstone outcrop develops with
[...] Read more.
TheScansano-Magliano in Toscana” area is characterized by a morpho-structure chiefly made-up by sandstone and shelly-calcareous lithologies. Generally, these complexes host minor aquifers in Tuscany, since they have medium to medium-low permeability. In the area under examination, a sandstone outcrop develops with continuity along the ridge of the structure for several kilometers and above a shelly substratum. Consequently, this hydrostructural context suggested the possibility that a significant groundwater body was hosted in the sandstones. In order to verify this assumption, an isotopic study was carried out taking into account several wells and springs sited on the sandstone outcrop and its surrounding area; the samples collected over a period of two years were analyzed to obtain δ18O‰, δ2H‰ and 3H. A study of the hydrostructural and morphological condition was also performed, and minor springs were selected. The analyses of this spring-water resulted in the characterization of the isotopic features of the infiltration water in the studied area, which represents a fundamental base of work for the interpretation of the data of groundwater points which drain long flow paths. By means of this approach, the groundwater framework was defined and the presence of a significant and continuous groundwater body within the sandstone complex was verified. A preliminary conceptual hydrogeological model was also proposed. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available
Open AccessArticle Investigation of Groundwater Flow Variations near a Recharge Pond with Repeat Deliberate Tracer Experiments
Water 2014, 6(6), 1826-1839; doi:10.3390/w6061826
Received: 25 January 2014 / Revised: 30 May 2014 / Accepted: 3 June 2014 / Published: 17 June 2014
Cited by 4 | PDF Full-text (1263 KB) | HTML Full-text | XML Full-text
Abstract
Determining hydraulic connections and travel times between recharge facilities and production wells has become increasingly important for permitting and operating managed aquifer recharge (MAR) sites, a water supply strategy that transfers surface water into aquifers for storage and later extraction. This knowledge is
[...] Read more.
Determining hydraulic connections and travel times between recharge facilities and production wells has become increasingly important for permitting and operating managed aquifer recharge (MAR) sites, a water supply strategy that transfers surface water into aquifers for storage and later extraction. This knowledge is critical for examining water quality changes and assessing the potential for future contamination. Deliberate tracer experiments are the best method for determining travel times and identifying preferential flow paths between recharge sites over the time scales of weeks to a few years. This paper compares the results of two deliberate tracer experiments at Kraemer Basin, Orange County, CA, USA. Results from the first experiment, which was conducted in October 1998, showed that a region of highly transmissive sedimentary material extends down gradient from the basin for more than 3 km [1]. Mean groundwater velocities were determined to be approximately 2 km/year in this region based on the arrival time of the tracer center of mass. A second experiment was initiated in January 2008 to determine if travel times from this basin to monitoring and production wells changed during the past decade in response to new recharge conditions. Results indicate that flow near Kraemer Basin was stable, and travel times to most wells determined during both experiments agree within the experimental uncertainty. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available
Open AccessArticle Effect of Hydrograph Separation on Suspended Sediment Concentration Predictions in a Forested Headwater with Thick Soil and Weathered Gneiss Layers
Water 2014, 6(6), 1671-1684; doi:10.3390/w6061671
Received: 26 February 2014 / Revised: 16 April 2014 / Accepted: 20 April 2014 / Published: 11 June 2014
Cited by 1 | PDF Full-text (2565 KB) | HTML Full-text | XML Full-text
Abstract
Two-component hydrograph separation using oxygen-18 concentrations was conducted at a sediment runoff observation weir installed in a small subcatchment of a forested gneiss catchment in Japan. The mean soil thickness of this catchment is 7.27 m, which comprises 3.29 m of brown forest
[...] Read more.
Two-component hydrograph separation using oxygen-18 concentrations was conducted at a sediment runoff observation weir installed in a small subcatchment of a forested gneiss catchment in Japan. The mean soil thickness of this catchment is 7.27 m, which comprises 3.29 m of brown forest soil (A and B layers) and a 3.98-m layer of heavily weathered gneiss. Data were collected for a storm on 20–21 May 2003, and the percentage of event water separated by the stable isotope ratio in comparison with the total rainfall amount was about 1%. This value is within the ratio of a riparian zone in a drainage area. Temporal variation of suspended sediment concentration exhibited higher correlation with the event water component than with the total runoff or pre-event water component. This shows that the riparian zone causes rainwater to flow out quickly during a rain event, and that this is an important area of sediment production and transportation in a forested headwater with thick soil and weathered gneiss layers. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available
Figures

Open AccessArticle Quantifying Reaeration Rates in Alpine Streams Using Deliberate Gas Tracer Experiments
Water 2014, 6(4), 1013-1027; doi:10.3390/w6041013
Received: 20 January 2014 / Revised: 1 April 2014 / Accepted: 9 April 2014 / Published: 22 April 2014
Cited by 5 | PDF Full-text (1692 KB) | HTML Full-text | XML Full-text
Abstract
Gas exchange across the air-water interface is a critical process that maintains adequate dissolved oxygen (DO) in the water column to support life. Oxygen reaeration rates can be accurately measured using deliberate gas tracers, like sulfur hexafluoride (SF6) or xenon (Xe).
[...] Read more.
Gas exchange across the air-water interface is a critical process that maintains adequate dissolved oxygen (DO) in the water column to support life. Oxygen reaeration rates can be accurately measured using deliberate gas tracers, like sulfur hexafluoride (SF6) or xenon (Xe). Two continuous release experiments were conducted in different creeks in the Sierra Nevada of California: Sagehen Creek in September, 2009, using SF6 and Martis Creek in August, 2012, using both SF6 and Xe. Measuring gas loss along the creek, which was approximated with the one-dimensional advection-dispersion equation, allows for the estimation of the SF6 or Xe reaeration coefficient (KSF6, KXe), which is converted to DO reaeration (KDO or K2) using Schmidt numbers. Mean KSF6 for upper and lower Sagehen and Martis Creeks were, respectively, 34 day−1, 37 day−1 and 33 day−1, with corresponding KDOs of 61 day−1, 66 day−1 and 47 day−1. In Martis Creek, KXe was slightly higher (21%) than KSF6, but the calculated KDO from SF6 agreed with the calculated KDO from Xe within about 15%; this difference may be due to bubble-enhanced gas transfer. Established empirical equations of KDO using stream characteristics did a poor job predicting KDO for both creeks. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available
Open AccessArticle Use of Natural and Applied Tracers to Guide Targeted Remediation Efforts in an Acid Mine Drainage System, Colorado Rockies, USA
Water 2014, 6(4), 745-777; doi:10.3390/w6040745
Received: 2 December 2013 / Revised: 19 March 2014 / Accepted: 19 March 2014 / Published: 27 March 2014
Cited by 4 | PDF Full-text (1425 KB) | HTML Full-text | XML Full-text
Abstract
Stream water quality in areas of the western United States continues to be degraded by acid mine drainage (AMD), a legacy of hard-rock mining. The Rico-Argentine Mine in southwestern Colorado consists of complex multiple-level mine workings connected to a drainage tunnel discharging AMD
[...] Read more.
Stream water quality in areas of the western United States continues to be degraded by acid mine drainage (AMD), a legacy of hard-rock mining. The Rico-Argentine Mine in southwestern Colorado consists of complex multiple-level mine workings connected to a drainage tunnel discharging AMD to passive treatment ponds that discharge to the Dolores River. The mine workings are excavated into the hillslope on either side of a tributary stream with workings passing directly under the stream channel. There is a need to define hydrologic connections between surface water, groundwater, and mine workings to understand the source of both water and contaminants in the drainage tunnel discharge. Source identification will allow targeted remediation strategies to be developed. To identify hydrologic connections we employed a combination of natural and applied tracers including isotopes, ionic tracers, and fluorescent dyes. Stable water isotopes (δ18O/δD) show a well-mixed hydrological system, while tritium levels in mine waters indicate a fast flow-through system with mean residence times of years not decades or longer. Addition of multiple independent tracers indicated that water is traveling through mine workings with minimal obstructions. The results from a simultaneous salt and dye tracer application demonstrated that both tracer types can be successfully used in acidic mine water conditions. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available
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Open AccessArticle Submarine Groundwater Discharge at a Single Spot Location: Evaluation of Different Detection Approaches
Water 2014, 6(3), 584-601; doi:10.3390/w6030584
Received: 6 December 2013 / Revised: 28 February 2014 / Accepted: 4 March 2014 / Published: 24 March 2014
Cited by 7 | PDF Full-text (1191 KB) | HTML Full-text | XML Full-text
Abstract
Submarine groundwater discharge (SGD) into the ocean is of general interest because it acts as vehicle for the transport of dissolved contaminants and/or nutrients into the coastal sea and because it may be accompanied by the loss of significant volumes of freshwater. Due
[...] Read more.
Submarine groundwater discharge (SGD) into the ocean is of general interest because it acts as vehicle for the transport of dissolved contaminants and/or nutrients into the coastal sea and because it may be accompanied by the loss of significant volumes of freshwater. Due to the large-scale and long-term nature of the related hydrological processes, environmental tracers are required for SGD investigation. The water parameters of electrical conductivity and temperature, the naturally occurring radionuclides of radon and radium as well as the stable water isotopes 18O and 2H have proven in previous studies their general suitability for the detection and quantification of SGD. However, individual hydrogeological settings require a site-specific application of this “tool box”. This study evaluates and compares the applicability of the abovementioned tracers for investigating SGD from a distinct submarine source in a karst environment at Cabbé, southern France. The specific advantages and disadvantages of each individual parameter under the given hydrogeological conditions are discussed. Radon appeared to be the most suitable environmental tracer in the site specific context. The water temperature was less reliable due to the little temperature difference between seawater and groundwater and since the diurnal variation of the air temperature masks potential SGD signals. Radium isotopes are less applicable in the studied region due to the lack of a well-developed subterranean estuary. The stable water isotopes showed results consistent with the salinity and radon data; however, the significantly higher effort required for stable isotope analyses is disadvantageous. A multi-temporal thermal remote sensing approach proved to be a powerful tool for initial SGD surveying. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available
Open AccessArticle Isotopes as Tracers of Water Origin in and Near a Regional Carbonate Aquifer: The Southern Sacramento Mountains, New Mexico
Water 2014, 6(2), 301-323; doi:10.3390/w6020301
Received: 11 November 2013 / Revised: 16 January 2014 / Accepted: 24 January 2014 / Published: 28 January 2014
Cited by 7 | PDF Full-text (1582 KB) | HTML Full-text | XML Full-text
Abstract
High-elevation groundwater sampled in 2003 in the Sacramento Mountains defines a line resembling an evaporation trend in δD-δ18O space. The trend results from recharge of winter precipitation into fractured limestone, with evaporation prior to recharge in broad mountain valleys. The same
[...] Read more.
High-elevation groundwater sampled in 2003 in the Sacramento Mountains defines a line resembling an evaporation trend in δD-δ18O space. The trend results from recharge of winter precipitation into fractured limestone, with evaporation prior to recharge in broad mountain valleys. The same trend occurs in basin groundwater east and west of the range, indicating the high Sacramento Mountains as the principal regional water source, either direct from the limestone aquifers or from mountain-derived surface water. Tritium and carbon-14 indicate bulk residence times of a few decades in the high Sacramento Mountains and at Alamogordo, and of thousands of years south of Alamogordo and in the artesian aquifer near Artesia. Stable O, H isotope data fail to demonstrate the presence of Sacramento Mountains water in a saline aquifer of the Hueco Bolson (Texas). Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available
Open AccessArticle Diagnosing Atmospheric Influences on the Interannual 18O/16O Variations in Western U.S. Precipitation
Water 2013, 5(3), 1116-1140; doi:10.3390/w5031116
Received: 28 April 2013 / Revised: 6 June 2013 / Accepted: 9 July 2013 / Published: 25 July 2013
Cited by 10 | PDF Full-text (15387 KB) | HTML Full-text | XML Full-text
Abstract
Many climate proxies in geological archives are dependent on the isotopic content of precipitation (δ18Op), which over sub-annual timescales has been linked to temperature, condensation height, atmospheric circulation, and post-condensation exchanges in the western U.S. However, many proxies
[...] Read more.
Many climate proxies in geological archives are dependent on the isotopic content of precipitation (δ18Op), which over sub-annual timescales has been linked to temperature, condensation height, atmospheric circulation, and post-condensation exchanges in the western U.S. However, many proxies do not resolve temporal changes finer than interannual-scales. This study explores causes of the interannual variations in δ18Op within the western U.S. Simulations with the Isotope-incorporated Global Spectral Model (IsoGSM) revealed an amplifying influence of post-condensation exchanges (i.e., raindrop evaporation and vapor equilibration) on interannual δ18Op variations throughout the western U.S. Mid-latitude and subtropical vapor tagging simulations showed that the influence of moisture advection on δ18Op was relatively strong in the Pacific Northwest, but weak over the rest of the western U.S. The vapor tags correlated well with interannual variations in the 18O/16O composition of vapor, an indication that isotopes in vapor trace atmospheric circulation. However, vertical-tagging simulations revealed a strong influence of condensation height on δ18Op in California. In the interior of the western U.S., a strong temperature effect was found only after annual mean temperatures were weighted by monthly precipitation totals. These multiple influences on δ18Op complicate interpretations of western U.S. climate proxies that are derived from isotopes in precipitation. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available
Open AccessArticle Air Masses Origin and Isotopic Tracers: A Study Case of the Oceanic and Mediterranean Rainfall Southwest of France
Water 2013, 5(2), 617-628; doi:10.3390/w5020617
Received: 18 March 2013 / Revised: 8 May 2013 / Accepted: 8 May 2013 / Published: 17 May 2013
Cited by 2 | PDF Full-text (550 KB) | HTML Full-text | XML Full-text
Abstract
Aquifers recharge mainly by local rainfall, which depend on the air mass humidity and orographic lifting, causing rain. The stable isotopes of the water molecule, i.e., oxygen-18 and deuterium, are useful tracers to determine the water source origin. Moreover, the calculation of
[...] Read more.
Aquifers recharge mainly by local rainfall, which depend on the air mass humidity and orographic lifting, causing rain. The stable isotopes of the water molecule, i.e., oxygen-18 and deuterium, are useful tracers to determine the water source origin. Moreover, the calculation of the deuterium excess enables one to differentiate between the air masses from the Atlantic Ocean or the Mediterranean Sea. A transect from one coast to the other one and going through the city of Toulouse have been made to sample the groundwater and determine their isotopic characteristic. A monthly rainfall sampling has also been done over one year, close to the city Toulouse, to see how the d-excess values range over the season. The discussion replaces these results in available isotopic data. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available
Open AccessArticle A Combined Radio- and Stable-Isotopic Study of a California Coastal Aquifer System
Water 2013, 5(2), 480-504; doi:10.3390/w5020480
Received: 15 February 2013 / Revised: 3 April 2013 / Accepted: 3 April 2013 / Published: 19 April 2013
Cited by 3 | PDF Full-text (1637 KB) | HTML Full-text | XML Full-text
Abstract
Stable and radioactive tracers were utilized in concert to characterize geochemical processes in a complex coastal groundwater system and to provide constraints on the kinetics of rock/water interactions. Groundwater samples from wells within the Dominguez Gap region of Los Angeles County, California were
[...] Read more.
Stable and radioactive tracers were utilized in concert to characterize geochemical processes in a complex coastal groundwater system and to provide constraints on the kinetics of rock/water interactions. Groundwater samples from wells within the Dominguez Gap region of Los Angeles County, California were analyzed for a suite of major cations (Na+, K+, Mg2+, Ca2+) and anions (Cl, SO42−), silica, alkalinity, select trace elements (Ba, B, Sr), dissolved oxygen, stable isotopes of hydrogen (δD), oxygen (δ18O), dissolved inorganic carbon (δ13CDIC), and radioactive isotopes (3H, 222Rn and 223,224,226,228Ra). In the study area, groundwater may consist of a complex mixture of native groundwater, intruded seawater, non-native injected water, and oil-field brine water. In some wells, Cl concentrations attained seawater-like values and in conjunction with isotopically heavier δ18O values, these tracers provide information on the extent of seawater intrusion and/or mixing with oil-field brines. Groundwater 3H above 1 tritium unit (TU) was observed only in a few select wells close to the Dominguez Gap area and most other well groundwater was aged pre-1952. Based on an initial 14C value for the study site of 90 percent modern carbon (pmc), groundwater age estimates likely extend beyond 20 kyr before present and confirm deep circulation of some native groundwater through multiple aquifers. Enriched values of groundwater δ13CDIC in the absence of SO42− imply enhanced anaerobic microbial methanogenesis. While secular equilibrium was observed for 234U/238U (activity ratios ~1) in host matrices, strong isotopic fractionation in these groundwater samples can be used to obtain information of adsorption/desorption kinetics. Calculated Ra residence times are short, and the associated desorption rate constant is about three orders of magnitude slower than that of the adsorption rate constant. Combined stable- and radio-isotopic results provide unique insights into aquifer characteristics, such as geochemical cycling, rock/water interactions, and subsurface transport and mixing. Full article
(This article belongs to the Special Issue Environmental Tracers) Print Edition available

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