Soil and Urine Mercury Levels in Secocha: A Case Study of Artisanal and Small-Scale Gold Mining in Peru

: In recent decades, artisanal and small-scale gold mining (ASGM) has spurred population and economic growth in the small rural communities in which it is located, along with causing contamination of the soil, air, and water with grave effects on human health due to the uncontrolled use of mercury for gold processing activities. This study analyzes the levels of mercury in Secocha, an ASGM community in Arequipa, Peru. A total of 44 soil samples were taken from two ASGM commercial-extractive zones (n = 18) and non-processing urban zones (n = 26). Soil mercury analysis with atomic absorption spectrometry revealed average mercury levels of 86.11 mg/kg in commercial-extractive zone I, where ore processing has occurred for about 17 years, and mercury levels of 43.81 mg/kg in commercial-extractive zone II, where processing has occurred for about 7 years. In the urban zone, the average mercury level was 9.53 mg/kg. The average mercury concentrations for each zone exceed the relevant environmental quality standards established by the Peruvian Ministry of the Environment. In addition, urine samples were obtained from 15 miners and 15 people from the general urban population (non-miners). The average mercury level in the urine of miners was 7.04 µ g/L, and in the urine of non-miners it was 0.49 µ g/L. In both cases, the mercury urine level did not exceed the limits established by WHO and the Peruvian Ministry of Health, though miners’ elevated levels do raise concerns.


Introduction
Between 10 and 19 million people in more than 70 countries around the world are directly or indirectly involved in artisanal and small-scale gold mining (ASGM) [1].This economic activity is typically rudimentary, relying on manual labor and simple equipment, and is practiced by groups of people who mine gold-bearing deposits on a small scale; it is a livelihood that allows them to try to overcome poverty in villages and rural communities [2,3].This activity commonly results in the emission of mercury and other contaminants into the air, soil, and water, causing deterioration of the environment and the health of exposed human beings [4][5][6].
In our case study site of Secocha, an ASGM settlement in the Arequipa region of Southern Peru, ore is extracted from underground mining operations in the Miski and Posco zones located in a canyon above the town.The gold ore is extracted and sorted within the mines.Once classified, ore is transported to and processed by miners in rudimentary plants located in commercial-extractive zones (CEZ) I and II.An open-circuit amalgamation process is carried out using "quimbaletes," which are composed of a cement vat and a large granite crusher stone called a mortar (Figure 1).A person perched atop the mortar stone rocks it, creating a back-and-forth motion that mechanically grinds the ore.Water and mercury are applied directly to the ore in the quimbalete to form gold amalgam for subsequent refining and release of the gold [7,8].The tailings from the amalgamation process, which contain water and significant amounts of gold and mercury, are deposited in small pits to settle out the solids, which are then shoveled into polypropylene sacks and transported by miners or sold to other people who transport them to commercial treatment plants in other parts of southern Peru for further processing with sodium cyanide (NaCN).The reprocessing of mercury-laden tailings with cyanide creates further environmental harm and spreads the contamination to new sites [9].Mercury amalgamation in ASGM is known to involve the use of mercury in high quantities that cause a high degree of contamination in mining areas, leading to its recognition as the largest anthropogenic source of global mercury emissions [7,10,11].
the ore.Water and mercury are applied directly to the ore in the quimbalete to form gold amalgam for subsequent refining and release of the gold [7,8].The tailings from the amalgamation process, which contain water and significant amounts of gold and mercury, are deposited in small pits to settle out the solids, which are then shoveled into polypropylene sacks and transported by miners or sold to other people who transport them to commercial treatment plants in other parts of southern Peru for further processing with sodium cyanide (NaCN).The reprocessing of mercury-laden tailings with cyanide creates further environmental harm and spreads the contamination to new sites [9].Mercury amalgamation in ASGM is known to involve the use of mercury in high quantities that cause a high degree of contamination in mining areas, leading to its recognition as the largest anthropogenic source of global mercury emissions [7,10,11].A typical ASGM operation in Secocha.The person in the center-top is perched on a quimbalete, used for mechanical ore grinding and gold amalgamation with mercury.In the bottom right is a "posa" or settling pond for tailings separation.In the bottom left, miners are bagging filtered tailings that contain mercury and gold residues to be sold to a cyanide plant for reprocessing.Some miners and family members live on site, in the cinderblock building.(Photo by author, October 2019).
Mercury emitted by ASGM affects the soil, water, plants, and living beings.It is a toxic metal that, according to the time of exposure and the level of consumption, has an impact on human health [6,12], especially in the centers in which ASGM activities are developed.This impact extends to miners, employees and neighbors of gold-buying stores, and residents including children and women living in the environment.The mercury used for processing is inorganic, elemental mercury (Hg 0 ).Exposure to inorganic mercury can be effectively analyzed through urine testing, with the urine levels of mercury in people exposed occupationally often surpassing 50 μg/L, while in people free from exposure, the urine mercury levels rarely exceed 5 μg/L [13].These same trends have been identified in ASGM settings [14].The fate of mercury in the environment, including methylation and bioaccumulation, which can create additional exposure routes for humans, are not specifically addressed in this study.
Mercury pollution is often linked to a lack of state control and poor or unenforceable regulation [7].In Peru, the use of mercury is regulated by Legislative Decree No. 1103-2017-Ministry of the Environment, which establishes control and inspection measures in distribution, transport, and commerce through the national tax agency (SUNAT), but Figure 1.A typical ASGM operation in Secocha.The person in the center-top is perched on a quimbalete, used for mechanical ore grinding and gold amalgamation with mercury.In the bottom right is a "posa" or settling pond for tailings separation.In the bottom left, miners are bagging filtered tailings that contain mercury and gold residues to be sold to a cyanide plant for reprocessing.Some miners and family members live on site, in the cinderblock building.(Photo by author, October 2019).
Mercury emitted by ASGM affects the soil, water, plants, and living beings.It is a toxic metal that, according to the time of exposure and the level of consumption, has an impact on human health [6,12], especially in the centers in which ASGM activities are developed.This impact extends to miners, employees and neighbors of gold-buying stores, and residents including children and women living in the environment.The mercury used for processing is inorganic, elemental mercury (Hg 0 ).Exposure to inorganic mercury can be effectively analyzed through urine testing, with the urine levels of mercury in people exposed occupationally often surpassing 50 µg/L, while in people free from exposure, the urine mercury levels rarely exceed 5 µg/L [13].These same trends have been identified in ASGM settings [14].The fate of mercury in the environment, including methylation and bioaccumulation, which can create additional exposure routes for humans, are not specifically addressed in this study.
Mercury pollution is often linked to a lack of state control and poor or unenforceable regulation [7].In Peru, the use of mercury is regulated by Legislative Decree No. 1103-2017-Ministry of the Environment, which establishes control and inspection measures in distribution, transport, and commerce through the national tax agency (SUNAT), but these standards are rarely enforced.Key barriers to effective control in Peru include neglect in the supply chain and distribution by the state [15,16].In addition, Peru is a signatory to the Minamata Convention, which encourages global mercury reduction.Annex C of the Convention calls for addressing mercury in a progressive manner through a National Action Plan; however, this agreement is not widely complied with and is often unknown among ASGM miners [16,17].In 2016, the Peruvian Ministry of Environment addressed this situation by establishing a plan to reduce and progressively eliminate the use of Hg (DS 010-2016-MINAM) [16].
Peru is the sixth largest gold producer in the world, with a total production of 263 Tn per year [18].ASGM produces 149 Tn per year (57% of the total), of which an estimated 88% is processed using amalgamation and 12% is processed using cyanidation.It has been estimated that, given the volume of treatment through amalgamation with Hg, 362 Tn of Hg is lost to the environment [19].Previous research in our study community of Secocha estimated that the Hg loss rate is 3.7 Hg:1 Au, meaning that about 28 Tn of Hg is lost per year through the quimbalete processing method there [19].In the rest of the Arequipa region, the ASGM process is carried out in communities located in ravines, valleys, and riverbanks in the provinces of Camaná, Caravelí, and La Unión.
Mercury contamination of soils by ASGM activity is a problem that has been identified in many countries, as summarized in Table 1.The presence of Hg in the urine of ASGM workers has also been documented in different countries, as summarized in Table 2, with Hg levels often linked to direct occupational exposure.[33] Italy, Amansie 6.97 25.00 50.00 The objective of this study was to quantify the concentration of Hg in soils and its impact on humans in the municipality of Secocha.Direct exposure comes from miners' daily work in milling and processing, managing tailings, and transporting tailings, as well as the burning of amalgam in local gold shops to produce doré (semi-refined gold).All of these activities can contribute to mercury contamination that is distributed in the soils of treatment plants and the broader urban area.We complement soil contamination measurements with analysis of the Hg levels in the urine of locals directly or indirectly exposed to this activity.These results could be used by local authorities, community members, and miners to implement preventive and corrective measures to reduce environmental and health hazards and risks in mining processes and other activities carried out by this population.

Materials and Methods
The town of Secocha is located in the Arequipa region, in the province of Camaná, in the district of Nicolás Valcárcel, on the right bank of the Ocoña river.The population was estimated at 5119 people in 2017, according to the Peruvian Institute of Statistics and Informatics (INEI).The area geologically constitutes a metamorphic base of Proterozoic age, covered by marine sedimentary rocks of Jurassic-Cretaceous intrusive age.Ore is in hydrothermal deposits located in veins hosted in intrusive rocks of the Incahuasi Super unit and the Bella Union Complex of the Upper Cretaceous age of the Nazca-Ocoña metallogenic belt.The gold mineralization is associated with quartz, pyrite, chalcopyrite, galena, pyrrhotite and native gold [34].Geomorphologically, the town is in an alluvial area of 42 hectares at an altitude of between 570 and 460 m above sea level in an arid environment.The alluvial area is formed at the mouth of the Posco-Secocha intermittent stream that descends from north to south to the Ocoña River.The stream channel separates two asymmetrically terraced banks, with the higher bank being on the east side or left bank of the alluvial van and the lower terrace being on the west side or right bank.The Secocha community is spread across both sides, on the upper and intermediate parts of the alluvial fan (Figure 2).The commercial-extractive zones (CEZ I and II), where artisanal milling plants operate on a daily basis, are located in the highest portions of the alluvial fan, upslope from the urban areas.CEZ I is older, with approximately 17 years of operation at the time of the study, and is on the left bank of the ravine; CEZ II is newer, with approximately 7 years of operation at the time of the study, and is on the right bank.The urban zone (UZ) is located on the lower portions of the alluvial fan, with avenues, streets, parks, commercial activity, offices, schools, markets, and houses located in a rough grid that radiates out toward the Ocoña River. is on the lower, wider section of the alluvial fan, with streets and avenues that radiate towards the Ocoña valley.Sampling point locations are shown; isolines of Hg concentration were calculated using ArcGIS.The 6.6 mg/kg isoline (blue) represents Peru's limit for urban soils, while the 24 mg/kg (brown) line represents the limit for industrial soils.

Soil Sampling
Soil samples were collected in August 2019.Soil sampling in the CEZs was non-random, depending on access being granted by ASGM operators to sample within their working areas.The first author built trust with miners and the local mining association to gain access for sampling in 19 processing sites within CEZ I and CEZ II; one sample returned implausible mercury values and was discarded, resulting in a total of 18 analyzed samples.Soil sampling in the UZ was random, based on a grid overlaid on the full urban area and sampling one point within each square.Most samples were taken from dirt is on the lower, wider section of the alluvial fan, with streets and avenues that radiate towards the Ocoña valley.Sampling point locations are shown; isolines of Hg concentration were calculated using ArcGIS.The 6.6 mg/kg isoline (blue) represents Peru's limit for urban soils, while the 24 mg/kg (brown) line represents the limit for industrial soils.

Soil Sampling
Soil samples were collected in August 2019.Soil sampling in the CEZs was nonrandom, depending on access being granted by ASGM operators to sample within their working areas.The first author built trust with miners and the local mining association to gain access for sampling in 19 processing sites within CEZ I and CEZ II; one sample returned implausible mercury values and was discarded, resulting in a total of 18 analyzed samples.Soil sampling in the UZ was random, based on a grid overlaid on the full urban area and sampling one point within each square.Most samples were taken from dirt streets or public areas nearest to the identified sampling point.Twenty-six samples were collected in the UZ.

Urine Sampling
Urine samples were collected from volunteers in two groups: miners who work in underground extraction as well as processing their ore in quimbaletes, and urban residents who do not participate in ASGM activities.Fifteen samples were collected in each study group by trained personnel from the Secocha health center, who obtained informed consent, gave instructions to avoid sample contamination, and collected urine in sterilized closed polyethylene bottles.Samples were stored in refrigerated conditions at temperatures between 2 and 8 • C for up to 24 h.The samples were coded and transported for analysis to the environmental laboratory of Certificaciones del Peru S.A.-CERPER, with accreditation NTP-ISO/IEC 17025:2017, registration code LE-003.

Sample Preparation and Analysis
For soil sampling, quality control was performed during field sampling, following the procedures of the SGC-INS-P-EHS.4-SGS of Peru, as indicated in the guide for soil sampling [35], recording for each sample its code, type of sample (CEZ I and II, UZ), date and UTM coordinates.First, any organic material or debris was cleaned away in a radius of 40 cm at each sampling point, then 30 cm deep holes were excavated with shovels and rods and the material to be analyzed was obtained from the side walls of the cavity with the help of a spatula.Then, the material was immediately mixed to homogenization, formed into a flattened cone, and divided into 4 equal portions (quarters), and opposite sides were selected until the required amount to be placed in sterile Ziploc bags with 500 g capacity was reached.To maintain their preservation temperature, the samples were placed in a cooler with gel ice packs, avoiding direct exposure to sunlight.For the control and contrast of results at each sampling point, 2 samples were collected; one of the samples was sent to the laboratory after a maximum of 48 h and the other sample was retained according to the protocols of the aforementioned guide.
Secondly, quality control was carried out in the laboratory analysis of the soil samples in each of the stages: pretreatment, homogenization, milling, 60 mesh sieving, and separation.For separation, the total volume of each sample was divided into four parts, "like a pizza", and two opposing parts were taken and then put together again to obtain a smaller sample weight; this procedure was repeated until the sample weight required for analysis was obtained.The resulting sample was subjected to a drying process at room temperature, following the standard procedure, EPA 7471B: Rev 2, 2007, Procedures related to the precision of the analysis results were also followed to certify whether the values were close to each other.Regarding the veracity of the results, additional certified samples were used to assess the probable interferences in the results of the analysis of the samples, and blanks in this process acted as a control for the results as an indication that the analysis was not affected by contamination.
Hg concentration was determined by means of the cold vapor atomic absorption spectrometry method (Spectrometry and Cold Vapor Atomic Absorption) [36,37], which is applicable for soil samples; samples were analyzed by the laboratory SGS del Peru SAC., which is certified by the National Institute of Quality (INACAL-DA with registration No. LE-002) according to the Peruvian Technical Standard NTP-ISO/IEC 17025-2017.The two mercury limits, the limit of detection of 0.0109 mg/kg and the limit of quantification of 0.0348 mg/kg, were calculated and established in EPA Method 7471B.The measurements corresponded to total mercury, without speciation to differentiate between organic and inorganic mercury, which limited the interpretation of health risks.
Soil samples were dried at a temperature <20 • C and then were ground, sieved, and homogenized.The samples were analyzed in duplicate, and during sample treatment, approximately 0.5 g of each sample was weighed out, to which 5 mL of distilled water and 5 mL of aqua regia were added, and then the mixture was heated at a temperature of 95 • C for 2 min to dissolve the mercury and keep it in the solution.Then, it was handled at room temperature, adding 50 mL of distilled water and 15 mL of potassium permanganate solution (to eliminate the possible interference of sulfide by exceeding); additional portions of permanganate solution were added until the purple color persisted for at least 15 min.Once homogenized, the sample was heated for 30 min at 95 • C ± 3 • C. Once cooled, 6 mL of sodium sulfate-hydroxylamine chloride solution was added, then 55 mL of distilled water was added to reduce the excess permanganate.The product was stirred and 5 mL of tin sulfate was added, after which the container was immediately connected to the aeration system, in order to ultimately perform the reading of the mercury value in the atomic absorption spectrophotometer at 253.7 nm.
For the analysis of the statistical data of the soil and urine samples, the Origin and Statgraphics 19 analysis and graphing software was used, and box plots were used for the design of the statistical results.According to the results, Hg concentrations did not follow a normal distribution, so non-parametric tests were used to observe that the three groups considered in this work were different in terms of Hg presence, with commercial-extractive zones I and II (CEZ I-II) containing a higher Hg concentration, followed by the avenues and streets in the UZ having a lower Hg concentration.For all cases, statistical significance was defined as p < 0.05.

Standards
For the comparative analysis and discussion of the results of soil samples in our research and the information obtained from other countries, we used Peru's Environmental Quality Standards for soils, which establish the maximum permissible limits for urban residential soil (6.6 mg/kg), and commercial-extractive soil (24 mg/kg) [38].This Peruvian standard was adopted from the Canadian soil quality standards [39].In the case of urine results, we used the World Health Organization (WHO) guidelines to identify populations at risk of exposure to mercury [13], and the clinical practice guide for the diagnosis and treatment of mercury poisoning, RM N • 757 of the Peruvian Ministry of Health [14].

Hg Levels in Soils of the Commercial Extractive Zones (CEZ I and II)
In CEZ I and II, 18 soil samples were taken and analyzed from yards of houses where quimbaletes are installed and gold processing occurs.Table 3 summarizes the results.Ten samples correspond to CEZ-I and display Hg concentration values from 38.35 to 128.39 mg/kg, with an average of 86.11 mg/kg.The other 8 samples correspond to CEZ-II, with Hg concentration values from 6.30 to 92.48 mg/kg, with an average value of 43.81 mg/kg.Overall, the CEZ Hg content values vary from 6.30 mg/kg to 128.0 mg/kg, with an overall average of 67.31 mg/kg.Overall, 89% of samples (16 of 18) exceeded the commercial-extractive soil standard of 24 mg/kg.Table 1 compares these results to those from similar studies in other countries.

Hg Levels in Soils of the Urban Zone (UZ)
In the UZ, 26 soil samples were taken from streets and avenues, revealing that mercury contamination was widespread in the urban zone and not limited only to the CEZs.The Hg concentrations in the UZ samples range from 1.12 mg/kg to 32.28 mg/kg, with an average of 9.53 mg/kg (Table 3).Overall, 54% of samples (14 of 26) exceeded the urban residential soil standard of 6.6 mg/kg.Table 1 compares these results to those from similar studies in other countries.The highest values correspond to the areas closest to the CEZs, as would be expected.Notably, various gold shops are located in the upper portions of the urban area and do not use filters or condensers (retorts) to capture the mercury evaporated when refining amalgam to produce doré.

Analysis of Urinary Hg Content Data
Urine sample analyses from 15 miners and 15 non-miners are summarized in Table 4.The Hg concentrations in the urine of miners averaged 7.04 µg/L, with a minimum value of 1.21 µg/L and a maximum of 20.72 µg/L.The corresponding values in non-miners averaged 0.49 µg/L with a minimum value of 0.11 µg/L and maximum value of 1.58 µg/L.All values are below the WHO alert level of 25 µg/L and well below the Peruvian clinical definition for mercury poisoning, 50 µg/L.However, average Hg concentration of 7.04 µg/L in the miners raises concerns that continued, chronic exposure could have long-term health implications.Table 2 compares these results to those from similar studies in other countries.

Discussion
Mercury is an important problem in Secocha, where concentrations in the soils commonly exceed applicable standards and urine sampling revealed alarming levels in miners, though not yet exceeding WHO alert levels or Peru's definition for mercury poisoning.The distribution of ASGM activity is an important determinant of exposure.Ore processing, amalgamation, and tailings handling and storage are often co-located with living quarters for some workers in the CEZs.The CEZs are the main source of Hg emissions and contamination, which also affect the UZ.The soil sampling confirms that the CEZs are mercury hotspots.Mercury levels decrease with distance from the CEZs.There is also a clear cumulative effect, with average mercury levels twice as high in CEZ I, which has been operating for about 17 years, than in CEZ II, which has been operating for about 7 years.Across both CEZs, 89% of samples exceed the maximum permissible standard for commercial-extractive soils, indicating severe and widespread contamination in these zones.In the UZ, 54% of samples exceeded the maximum permissible standard for urban residential soils.
Hg contamination is distributed from the two uncontrolled sources, CEZ I and II, which are determined to be the main Hg emission zones in the study area.It is important to recognize that these are relatively new operations, no more than 17 years old, and levels could continue to increase over time.CEZ I's levels are on par with La Rinconada [21], one of Peru's most notorious ASGM sites, but still much lower than the levels reported in Mantey et al.'s study in Ghana [20].
A recent survey of miners and residents in Secocha revealed the widespread recognition that mercury is most prevalent in the processing zones, but also showed that most residents underestimated its dispersion in other parts of the community and assumed that only those who directly handle mercury would face health impacts [20].Identification of mercury contamination is often not sufficient to motivate changes to limit emissions.Studies have found limited concern from miners to control the use of mercury, exposure is underestimated, and health care is not prioritized, as they seek greater profitability, speed, and ease of use to obtain gold [7,16,40].These conditions predominate in Secocha as well.
The lowest observed mercury levels correspond to the lower parts of town, farthest from the CEZs, which is also where the population concentration is higher.Still, the levels are not negligible, and a cumulative impact is likely over time.We identify four important vectors for mercury contamination in the UZ.First, the transfer of amalgamation tailings to other locations for cyanidation treatment leads to contamination with mercury-laden liquid.The tailings are transported through the streets without any type of control, often dripping contaminated water onto the streets during transport.Second, gold shops in the community burn amalgam without using retorts, releasing vaporized mercury into the urban area.Third, mercury can be dispersed by the traffic of people and vehicles entering and leaving the concentrator plants where the amalgam is processed.Fourth, the dry, windy environment in Secocha leads to constant wind movement of dust, which likely redistributes mercury pollution.
The mean urinary Hg concentrations in both study groups are below the clinical definition of mercury poisoning [14], as well as the WHO alert level for exposure [13].Still, the mean Hg concentration of 7.04 µg/L in the study group of miners who process their ore in quimbaletes with amalgamation methods is concerning.This is a dangerous situation, considering that the exposure identified today could, over time, lead to greater adverse effects due to chronic occupational exposure.Furthermore, in the abovementioned survey in Secocha, miners who self-reported handling mercury had on average only worked in mining for 5 years [20].This suggests that cumulative effects could worsen with time.On the other hand, the same survey found that the residents of Secocha had lived there for average of 12 years, indicating that the population primarily comprises migrants from other parts of Peru.High turnover in the community and the ASGM sector could reduce cumulative exposure, as individuals enter and exit the sector or rotate seasonally between mining and agriculture.The low levels measured in the urine samples from the non-mining population suggest that despite indirect and environmental exposure, most participants have not yet been severely impacted.While encouraging, this finding does not minimize the importance of controlling mercury use and limiting emissions.For example, even low levels of exposure are concerning for high-risk populations like children and pregnant people, both of which are present in Secocha.

Conclusions
This research identifies the distribution and levels of mercury in soils and urine in a Peruvian community where ASGM has been practiced intensively for about 17 years.There are numerous other ASGM communities in Peru's southern coastal deserts, where similar conditions and practices exist.In Secocha, 89% of sampled commercial-extractive soils exceeded standards, as did 54% of urban residential soils.Overall, this equates to 69% of all samples exceeding the relevant standards.Future research could examine mercury levels in the air and water to extend insights on the overall environmental impact.
Regarding mercury levels in urine, non-miners had low levels, although these were non-zero.Miners had elevated urine levels of Hg, although these did not reach the alert status established by the WHO.Chronic direct and indirect exposure can be expected to lead to increasing levels over time for both groups, raising potential concerns even though the current levels are not above the standards.Future research could examine mercury levels in the blood and hair to provide greater insights on organic mercury exposure.
In addition, a direct relationship is established in which the highest concentrations of mercury contained in the soil are in CEZ I and II, where ore processing occurs.These areas represent focal points for the contamination and dispersion of Hg content into the streets and avenues of the community.
People who work directly in gold mining through amalgamation are the most exposed to the risk; our results indicate that miners have mercury in their urine at elevated levels.Further studies are needed to determine whether such contents are associated with adverse health outcomes at a longer exposure time; the present study will help decision makers to address this problem.
ASGM activities in Secocha continue to grow.It will be important to organize the mining sector and the population to improve mercury management and offer alternatives for gold recovery that could be less harmful, more efficient, and more profitable [41].However, it is also important to take into account surveys showing that mercury contamination is not a high priority concern for Secocha residents, which could complicate remediation efforts [17,40].Mercury reduction in ASGM has been a challenge in varied contexts around the globe, clearly demonstrating that there are no simple solutions [42].

Figure 1 .
Figure1.A typical ASGM operation in Secocha.The person in the center-top is perched on a quimbalete, used for mechanical ore grinding and gold amalgamation with mercury.In the bottom right is a "posa" or settling pond for tailings separation.In the bottom left, miners are bagging filtered tailings that contain mercury and gold residues to be sold to a cyanide plant for reprocessing.Some miners and family members live on site, in the cinderblock building.(Photo by author, October 2019).

Figure 2 .
Figure 2. Soil mercury concentrations.Secocha is located on an alluvial fan formed at the mouth of the Posco-Secocha ravine near its confluence with the Ocoña River.The upper areas of the alluvial fan have a higher elevation and steeper slope, where the gold processing and extraction plants are located.Commercial-extractive zone I (CEZ I) is the older processing zone, located on the left bank of the ravine; CEZ II is the newer processing zone, located on the right bank.The urban zone (UZ)is on the lower, wider section of the alluvial fan, with streets and avenues that radiate towards the Ocoña valley.Sampling point locations are shown; isolines of Hg concentration were calculated using ArcGIS.The 6.6 mg/kg isoline (blue) represents Peru's limit for urban soils, while the 24 mg/kg (brown) line represents the limit for industrial soils.

Figure 2 .
Figure 2. Soil mercury concentrations.Secocha is located on an alluvial fan formed at the mouth of the Posco-Secocha ravine near its confluence with the Ocoña River.The upper areas of the alluvial fan have a higher elevation and steeper slope, where the gold processing and extraction plants are located.Commercial-extractive zone I (CEZ I) is the older processing zone, located on the left bank of the ravine; CEZ II is the newer processing zone, located on the right bank.The urban zone (UZ)is on the lower, wider section of the alluvial fan, with streets and avenues that radiate towards the Ocoña valley.Sampling point locations are shown; isolines of Hg concentration were calculated using ArcGIS.The 6.6 mg/kg isoline (blue) represents Peru's limit for urban soils, while the 24 mg/kg (brown) line represents the limit for industrial soils.

Table 1 .
Comparison of Hg content in soils at the international level and in this study, in descending order, in relation to the maximum permissible levels.

Table 2 .
Comparison of Hg content in urine at the international level and in this study, in descending order, in relation to the maximum permissible levels.

Table 3 .
Descriptive statistics of mercury levels in CEZ I and II and UZ.

Table 4 .
Descriptive statistics of mercury levels in urine samples from miners and non-miners.