Mercury Mine Research Articles

Mortalidad por enfermedades genitourinarias en los mineros de mercurio

ObjectivesTo study the mortality due to genitourinary diseases in mercury miners. Population and methods3.998 workers exposed to mercury in Minas de Almadén y Arrayanes S.A. were studied. The follow-up period was a century, since 1,895 to 1,994. It was completed assessing the vital status and the basic cause of death, in case of fatalities. Standardized Mortality Ratios by age, sex and calendar period were calculated. Expected deaths were obtained from age, sex and calendar period specific rates for the Spanish and Castilla-La Mancha populations. ResultsA significant increase in mortality due to genitourinary diseases was found (CIE-9 codes 580 to 629), being significant with respect to the total population for nephritis, nephrotic syndrome and nephrosis, with SMR of 1.69 an 95% CI 1.18 to 2.34. Mortality excesses due to nephritis, nephrotic syndrome and nephrosis were higher in the metallurgy workers than in miners. A Poisson multivariate regression detected a positive trend in the mortality due to nephritis and nephrosis associated to the exposure length, with the risk increasing fivefold after 30 years of exposure to mercury. ConclusionsThis paper shows an excess in the mortality due to genitourinary diseases, specially in nephritis, nephrotic syndrome and nephrosis, whose estimation and significance increases when compared to the population of Castilla-La Mancha.

Mercury Methylation from Unexpected Sources: Molybdate-Inhibited Freshwater Sediments and an Iron-Reducing Bacterium

Methylmercury has been thought to be produced predominantly by sulfate-reducing bacteria in anoxic sediments. Here we show that in circumneutral pH sediments (Clear Lake, CA) application of a specific inhibitor of sulfate-reducing bacteria at appropriate concentrations typically inhibited less than one-half of all anaerobic methylation of added divalent mercury. This suggests that one or more additional groups of microbes are active methylators in these sediments impacted by a nearby abandoned mercury mine. From Clear Lake sediments, we isolated the iron-reducing bacterium Geobacter sp. strain CLFeRB, which can methylate mercury at a rate comparable to Desulfobulbus propionicus strain 1pr3, a sulfate-reducing bacterium known to be an active methylator. This is the first time that an iron-reducing bacterium has been shown to methylate mercury at environmentally significant rates. We suggest that mercury methylation by iron-reducing bacteria represents a previously unidentified and potentially significant source of this environmental toxin in iron-rich freshwater sediments.

Calculation of the mercury accumulation in the Idrijca River alluvial plain sediments

From the historic literature on the Idrija mercury mine, it is evident that part of the smelting and mining waste was dumped into the Idrijca River. This waste was transported downstream during floods. The amount of mercury which has accumulated in the alluvial sediments of the Idrijca River until the present was studied. Mapping of Holocene river terraces of the Idrijca River was performed in order to estimate the volume of the alluvial sediment. For the purpose of the assessment of the mercury concentration, we sampled the alluvial sediments on different levels and performed an analysis of variance. The greatest variability is between the floodplain and terraces inside the same alluvial plain. Considering this fact, which determined the methodology employed for calculation, we estimated that about 2029 tons of mercury is stored in the Idrijca River alluvial sediments.

Modelling of mercury transport and transformation processes in the Idrijca and Soča river system

In the town of Idrija, Slovenia, the world's second largest mercury mine was active for 500 years and about 37,000 tons of mercury has been lost in the environment. Mercury is still drained from soil, riverbed and floodplains and transported with the Idrijca and Soča Rivers to the Gulf of Trieste. A part of inorganic mercury is methylated either in the river system, or later in the coastal area, and, due to its bioaccumulation and biomagnification represents potential danger to human health. A 1-D aquatic model MeRiMod was used to simulate hydrodynamics and sediment transport in the river system from Idrija to the Soča River mouth. Transport of particle bound and dissolved mercury as well as potential net methylation of mercury in the river system was simulated. The simulation of an observed flood wave with 20-year recurrence period was performed in order to validate the model. Methylation was simulated at lower discharges, as higher methylation rates occur in such conditions. The measurement data and the MeRiMod model were also used to establish a historical mercury mass balance of the Idrijca and Soča Rivers catchment. Sediment core data from the Gulf of Trieste and the measured concentrations from floodplains were used to verify and calibrate the model. Simulations of different high discharges were performed as most of the transport of particulate mercury occurs within flood wave conditions. Compared to the measurements, the results of the model showed an agreement within an order of magnitude, for the transport of total mercury mostly within a factor of 4, and for the methylation within a factor of 5. However, proper trends of the phenomena were obtained by simulations. The combination of modelling and measurements has resulted in some interesting conclusions about the phenomenon of the transport and transformations of mercury in the observed river system.

Current and past mercury distribution in air over the Idrija Hg mine region, Slovenia

Mercury in air over the Idrija region, where the world's second largest mercury (Hg) mine is located, decreased significantly in the last decade, from more than 20,000 ng m −3 in the early 1970s to values below 100 ng m −3 in the 1980s, and finally reached a level of 10 ng m −3 or even lower at the summer of the year 2004. The air concentration of Hg was continuously monitored after closure of the Hg mine. Hg 0 in air was mapped in November 2003 at over 100 locations in the Idrija region during a 3-day period under different weather conditions, and the concentrations found were between 2.5 to over 2000 ng m −3. The Hg concentration in air was mostly below 10 ng m −3. The highest values were observed in the near vicinity of the former smelting plant, as well under its chimney. Elevated concentrations were also observed at some other locations in Idrija town. Mercury evaporation from topsoil was measured continuously for a 24 h period at two heavily polluted locations in Idrija and 50 km downstream the River Idrijca at Bača pri Modreju. The average Hg concentration in air at Bača pri Modreju was 5.5 ng m −3, with an average Hg flux from soil to atmosphere of 34 ng m −2 h −1. At the site in Idrija the average Hg concentration in air was 11 ng m −3 with an average Hg flux from soil to the atmosphere of 84.4 ng m −2 h −1.

Size fractionation in mercury-bearing airborne particles (HgPM 10) at Almadén, Spain: Implications for inhalation hazards around old mines

Almadén has a >2000y mining history and an unprecedented legacy of mercury contamination. Resuspended airborne particles were extracted from mine waste (Las Cuevas), retort site soil (Almadenejos), and urban car park dust (Almadén), separated into fine (PM 10) and coarse (PM >10 μm ) fractions, analysed for mercury using ICP-MS, and individual HgPM characterised using SEM. Cold extractable mercury concentrations in PM 10 range from 100 to 150 μg g −1 (car parks), to nearly 6000 μg g −1 (mine waste), reaching a world record of 95,000 μg g −1 above the abandoned retort at Almadenejos where ultrafine HgPM have pervaded the brickwork and soil and entered the food chain: edible wild asparagus stem material from here contains 35–65 μg g −1 Hg, and pig hair from animals living, inhaling and ingesting HgPM 10 at the site yielded 8–10 μg g −1. The PM 10 fraction (dusts easily wind transported and deeply inhaled) contains much more mercury than the coarser fraction. The contribution of HgPM 10 to ecosystem contamination and potential human health effects around old mercury mines has been underestimated.

Impact of mercury mine and smelter St. Ana – Podljubelj on spatial distribution of chemical elements in soil

The objective of the research project was to establish the extension of Hg pollution as a consequence of mining and smelting activities in a narrow Alpine valley. The St. Ana mine was first exploited as early as in 1557 and was finally abandoned in 1902. The entire operating period yielded about 110.000 tons of ore, from which 360 tons of Hg was produced. By soil sampling it was established that on about 9 ha the Hg contents in soil exceed the Slovenian critical values for soil (10 mg/kg). The estimated mercury mean for the studied area is 1.3 mg/kg (0.17 - 718 mg/kg). The highest contents of mercury in soils were found in the area of the mercury smelter. That is a consequence of former atmospheric emissions and technological losses. High values of Hg were found also in soil on the mine and smelter waste dump. The highest determined contents of Hg (108 mg/kg) in this area are almost 7-times lower than the contents of Hg in the area of the smelter. Mercury in soils generally decrease with depth and distance from the mine and smelter. Apart from the area around the former mine and smelter, mercury appear in higher concentrations also along the road that runs along the valley, which is due to the use of Hg bearing mine tailings in road construction.

What is the amount of mercury accumulated in the Idrijca River overbank sediments?

Part of mine tailings and roasted ore remains of the Idrija mercury mine was deposited in the bed of the Idrijca River that transported the load downstream at high waters. It was deposited in alluvial sediments of the Idrijca and Soča Rivers, and in the Trieste Bay, essentially contributing to mercury pollution of environment. The goal of the reported research was to estimate the mercury amount accumulated in the Idrijca alluvial deposits. This was attained by multiplying the mercury contents in sediments by their weight. For calculation of this amount the Idrijca Holocene sediments in the terrace systems from Idrija to confluence of Bača river into it were mapped. For assessing the spatial distribution of mercury contents in sediments the alluvial floodplains were sampled according to an analysis-of-variance sampling design. This was important for establishing the amounts of mercury also in alluvial deposits that were not sampled. The analysis of variance indicated the mercury contents in alluvium to be the most dependent upon the position of the material within the alluvial plain, i.e. either on the most polluted floodplain, on the first or on some higher terrace. By considering this indication and the mapping results we estimated the amount of mercury accumulated in the Idrijca river alluvial sediments from Idrija to the confluence of Bača at 2029 tons.

Mercury emissions from the Idrija mercury mine measured by differential absorption lidar techniques and a point monitoring absorption spectrometer

Mercury emission measurements from the Idrija mercury mine in Slovenia were performed during an early November 2003 campaign, where the differential lidar technique was used to map mercury concentrations and an attempt was made to quantify the total mercury flux from the most contaminated area, the abandoned cinnabar roasting oven complex. Lidar concentration data were compared with data recorded with a Zeeman modulated atomic absorption instrument, operated from a vehicle equipped with a GPS localization system. Concentrations and fluxes were comparatively low due to low temperature and rainfall. The average flux from the distillation plant was measured to approximately 2 g h −1 .

Long-Term Effects of Elemental Mercury on Renal Function in Miners of the Idrija Mercury Mine

The kidneys are one of the main target organs for elemental mercury (Hg(0)). The influence of Hg(0) on kidneys has been extensively studied but the long-term effects on this organ have not yet been determined with certainty. The basic aim of this research was to study the effects of a long-term exposure to Hg(0) vapours on the renal function in miners in the post-exposure period. The population studied comprised 53 miners (33 active and 20 retired) from the Idrija Mercury Mine as the exposed miners group and 53 unexposed workers as the control group. On the basis of mine exposure records (air and biological monitoring), the environmental and biological indicators of the past exposure to Hg(0) were calculated for each miner. Kidney function was determined in both groups, i.e. in the exposed miners as well as in the controls. Glomerular kidney function was evaluated by a quantitative analysis of albumin and IgG in urine. Tubular kidney function, however, was determined by a quantitative analysis of alpha(1)-microglobulin in urine and by the enzymatic activity of N-acetyl-beta-d-glucosaminidase (NAG). The mean exposure time in miners was 15 years. The total number of cycles of exposure ranged from 13 to 119. The mean annual time-weighted exposure was 0.29 mg m(-3) and the mean integrated exposure intensity (IEI) was 1413 mg m(-3)-h. Throughout the period of exposure the average urine mercury concentration in miners was 68.24 microg l(-1) and the average sum of peak urine mercury concentrations was 3901 microg l(-1). Albumin, IgG and alpha(1)-microglobulin in urine were significantly elevated in the exposed miners compared with the unexposed controls (t = 2.17, P = 0.03; t = 2.81, P < 0.01; and t = 2.07, P = 0.04). No significant differences were found in the urine NAG activity when the exposed miners and the unexposed workers were compared. Among the indicators of renal function only alpha(1)-microglobulin in the urine correlated significantly with the IEI (r = 0.73; P <or= 0.01) and with the sum of peak mercury urine concentrations (r = 0.67; P <or= 0.01) in the group of miners who still worked, while no significant correlations were found between these parameters in the group of the retired miners. The results of the differences in albumin, IgG and alpha(1)-microglobulin concentrations in urine between the exposed miners and unexposed controls suggest that a long-term occupational exposure to Hg(0) could cause renal dysfunction. A high correlation between alpha(1)-microglobulin in urine and the IEI as well as between alpha(1)-microglobulin in urine and the sum of peak urine mercury concentrations in the group of active but no longer exposed miners indicates that a long-term occupational exposure to Hg(0) may cause a non-permanent tubular dysfunction.

Oxidative stress on domestic ducks ( Shaoxing duck) chronically exposed in a Mercury–Selenium coexisting mining area in China

The Wanshan mercury mine is the largest mercury deposit in Guizhou Province, China. Few attempts have been made to study the toxic effects of mercury on biota in this mining area. This study was the first to investigate the oxidative stress on domestic ducks (Shaoxing duck) chronically exposed to mercury in the Wanshan mining area. Chemical analyses revealed higher concentrations of both, mercury and selenium in samples from the Wanshan area. Total mercury and selenium concentrations in duck tissues varied from 0.073 to 4.465 mg/kg and from 1.073 to 6.35 mg/kg, respectively. Analysis of covariance revealed that there were significant effects of zone on accumulation of mercury and selenium in all duck tissues ( P < 0.01 ). Moreover, analysis of covariance indicated that mercury content significantly affected the accumulation of selenium in duck muscle, brain, and liver ( P < 0.01 ). Mercury and selenium were also highly correlated in Wanshan duck liver, muscle, brain, and lung. The statistical analysis suggested that selenium might be an interactive factor in mercury toxicity. As for the biochemical analyses, it was observed that selenium-dependent glutathione peroxidase and superoxide dismutase activities and glutathione (GSH) content were significantly increased in the livers and brains of Wanshan ducks ( P < 0.01 , 0.05). However, no significant changes were observed in malondialdehyde content ( P > 0.05 ). Although the results indicated that adaptive responses of the redox-defense system are associated with the increased enzyme activities and GSH content, the most likely explanation is that selenium plays a critical role. Therefore, the effects of the interaction environmentally occurring selenium and mercury on public health in the Wanshan area should be examined in further studies.

Mercury Levels and Relationships in Water, Sediment, and Fish Tissue in the Willamette Basin, Oregon

In Oregon's Willamette River Basin (the Basin), health advisories currently limit consumption of fish that have accumulated methylmercury (MeHg) to levels posing a significant human health risk. These advisories created the requirement for a mercury total maximum daily load for the Basin, which required a greater understanding of the behavior, distribution, and levels of mercury and MeHg in the Basin. In 2002, the Oregon Department of Environmental Quality initiated a study to measure (using ultraclean techniques) mercury and MeHg levels in water, sediment, and fish samples collected throughout the Basin. Results from the Middle Fork (nominal background) suggested that naturally occurring surface-water concentrations of mercury and MeHg would on an annual average basis be expected in the range of 0.5 to 1.0 and 0.04 to 0.06 ng L(-1), respectively. Concentrations in the Coast Fork (Cottage Grove), which were markedly higher, are likely the result of historical mining discharges. The possibility exists that wetlands alone could contribute the dissolved MeHg levels (approximately 0.04 ng L(-1)) observed in the Main Stem. Mercury levels in sediment were similar, and near background, in the Main Stem, Coast Fork (Row River), and Middle Fork but significantly increased in the Coast Fork (Cottage Grove). Fish tissue mercury levels were typically highest in piscivorous and lowest in invertivorous species but highest in the Coast Fork (Cottage Grove). In the Coast Fork and Cottage Grove Reservoir, discharges from historical mercury mining activities appear to have significantly impacted water, sediment, and fish tissue levels; however these impacts do not appear to extend into the Main Stem. Basinwide mercury data are at present too spottily distributed to determine whether significant mercury point sources exist along the Main Stem.

Distribution and mobility of mercury in soils from an old mining area in Mieres, Asturias (Spain)

Mercury content in soils near abandoned mine wastes in the mercury mining area of Mieres (Asturias, Spain) is highly elevated as a result of the long period of mining and abundant Hg production. In this work, an evaluation of Hg concentration, distribution, and mobility in three soil samples from the immediate vicinity of a chimney used for vapour evacuation during pyrometallurgical treatment of the ore was carried out. For that purpose, total Hg contents were determined for the original samples and their grain-size subsamples. The study of mercury mobility in the original samples and in the different particle-size subsamples was made by the application of a sequential extraction method. Results showed that Hg concentration in soils decreases directly with the distance from the chimney and the dispersion of Hg is not influenced by the topographic height of the site tested. The sample collected in the base of the chimney exhibited appreciable amounts of mobile Hg. In general, a higher Hg concentration was found for the finest particle-size subsamples. Hg mobility was found to be higher for quite developed soils. The sample collected downstream from the chimney showed a significant Hg mobile content as a result of a more intensive weathering. An increase of Hg mobility at decreasing particle size was found in all three analysed samples.

Analysis of the relationship between pineal hormone melatonin level and occupational mercury exposure in ex-miners with machine learning methods

The aim of this study was an analysis of the relationship between pineal hormone melatonin level and long-term occupational exposure to elemental mercury vapour (Hg°) of ex-mercury miners. Melatonin (MEL) is a hormonal product of the pineal gland. Hg accumulation in the pineal gland in ex-miners could modify the synthesis of melatonin and there are no data available in the scientific literature on the possible effects of Hg on melatonin excretion. Regression/model trees were used to perform the analyses and investigate the interactions between melatonin secretion and excretion on one hand, and various indicators of occupational Hg exposure and some other medical variables on the other. The results point us towards some previously unknown factors that influence the excretion of melatonin and support some assumptions that melatonin also has antioxidative effect. In the process of antioxidation activity melatonin decomposes and we assume this is the reason for decreased excretion of melatonin in mercury miners.

Characterisation of Almadén mercury mine environment by XAS techniques

This manuscript describes the analysis and evaluation of mercury species present in three different types of samples (ore, slag and soil) belonging to the abandoned mining area of Almadén (Spain), by means of X-ray Absorption Spectroscopy (XAS) techniques. The applied methodology includes the analysis of possible interfering compounds by ICP-OES and the characterisation of solid samples by identification of heterogeneities using SEM-EDS technique prior to measurements at the synchrotron facilities (ESRF in Grenoble, France and HASYLAB in Hamburg, Germany) and subsequent analysis of data. Results show that cinnabar is the main species both in ore and soil samples, its concentration ranging from 41 to 77% of the total mercury content. On the other hand, metacinnabar (a polymorph of cinnabar) is shown to be the main species in slag samples (42-88%). Other mercury forms have been found in minor proportions (<30%), such as slightly soluble mercury salts (HgCl(2) and HgSO(4)) and HgO. This is the first time that Almadén-type mercury ores have been characterised by a synchrotron-based spectroscopic technique for a direct determination of mercury species. The conclusions presented in this report show the important similarities between Almadén and hot-spring type mercury mineral deposits, despite its different geological origin.

Release of toxic metals and metalloids from Los Rueldos mercury mine (Asturias, Spain)

The abandoned mercury mining works of “Los Rueldos” are located 20 km from Oviedo, along the northwestern border of the Asturian Central Coal Basin, in an area with intense tectonization. Hg mainly appears as cinnabar, but occasionally metacinnabar and native Hg are present; associated with Hg ore appear As-rich minerals (arsenopyrite, As-rich pyrite, realgar and scorodite). In the spoil heap, Hg content ranges from 14 to 2224 mg kg −1, and As from 4746 to 62,196 mg kg −1. Mine drainage and spoil heap leachates show acidic conditions (pH: 2.43–2.50), 2900–4600 mg l −1 sulphate, 1.4–9.2 mg l −1 As, 0.03–0.48 mg l −1 Pb, and 3.6–14 μg l −1 Hg. According to the analytical data and characteristics of the site, the application of corrective measurements to avoid the dispersion of contaminants in the environment must be considered.

Mercury and antimony in soils and non-vascular plants near two past-producing mercury mines, British Columbia, Canada

Mercury and Sb in soils (humus, B horizon, C horizon) and environmental biomonitors (moss and epiphytic lichen) were examined near two past-producing Hg mines located in British Columbia, Canada: Pinchi Lake and Bralorne Takla mines. Sequential extraction analyses and scanning electron microscopy were used to determine the chemical associations of Hg and Sb and the mineralogical forms of Hg in soils. Mercury and Sb in the C horizon are derived mostly from natural sources because their concentrations are controlled by the bedrock geology, glacial transport, and sediment types. Distribution of Sb in the B horizon is similar to that of the C horizon suggesting a common geogenic source. Mercury in the B horizon at Pinchi Lake mine could be derived in part from anthropogenic sources, because (1) only at that site, and not at any other sites of cinnabar occurrences in bedrock, is there a labile Hg enrichment in the B compared to the C horizon, and (2) there is a weak correlation between non-labile Hg concentrations in the B horizon and distance from the mine which occurs independently of the natural concentrations of the C horizon. Mercury and Sb concentrations in humus near Pinchi Lake mine (&lt;10 km) appear to be derived in part from anthropogenic sources because there are strong correlations between labile and non-labile Hg and Sb concentrations with distance from the mine which occurs independently of natural concentrations in the C horizon. Near Pinchi Lake mine, biogeochemical cycling and gaseous Hg derived from the substrate are thought to be minimal pathways and sources of Hg to the humus compared to anthropogenic sources, otherwise the strong Hg enrichment in the humus compared to the C horizon only observed near Pinchi Lake mine would also be present at several sites of cinnabar occurrences in bedrock along Pinchi Fault. Anthropogenic Hg and Sb enrichment in the soils near Bralorne Takla mine is indistinguishable from natural enrichment. Mercury and Sb levels in the sampled moss and lichen species reflect locally derived wind-borne soil and rock dust.

7th International Conference on Mercury as a Global Pollutant

The 7th International Conference on Mercury as a Global Pollutant (ICMGP) was held from 28th of June to 2nd of July 2004 in Ljubljana, the capital of Slovenia, where the Idrija mercury mine is located. The mercury mine is no longer in operation and the mine shafts are being closed in order to avoid any further releases of mercury to the environment. A professional organization of the Conference was provided by the Jo~ef Stefan Institute, with Dr. Milena Horvat as Chairperson. Besides the aim of the conference of exchanging scientific developments, reporting of scientific breakthroughs, and promoting a sound scientific understanding of the distribution, pathways and effects of mercury on our society and the environment, the scientific programme of the 7th ICMGP comprised 4 parallel sessions, poster sessions and round table discussions centred around the following four components of the worldwide mercury problematics:

Macroscopic and Microscopic Observations of Particle-Facilitated Mercury Transport from New Idria and Sulphur Bank Mercury Mine Tailings

Mercury (Hg) release from inoperative Hg mines in the California Coast Range has been documented, but little is known about the release and transport mechanisms. In this study, tailings from Hg mines located in different geologic settings--New Idria (NI), a Si-carbonate Hg deposit, and Sulphur Bank (SB), a hot-spring Hg deposit--were characterized, and particle release from these wastes was studied in column experiments to (1) investigate the mechanisms of Hg release from NI and SB mine wastes, (2) determine the speciation of particle-bound Hg released from the mine wastes, and (3) determine the effect of calcinations on Hg release processes. The physical and chemical properties of tailings and the colloids released from them were determined using chemical analyses, selective chemical extractions, XRD, SEM, TEM, and X-ray absorption spectroscopy techniques. The total Hg concentration in tailings increased with decreasing particle size in NI and SB calcines (roasted ore), but reached a maximum at an intermediate particle size in the SB waste rock (unroasted ore). Hg in the tailings exists predominantly as low-solubility HgS (cinnabar and metacinnabar), with NI calcines having >50% HgS, SB calcines having >89% HgS, and SB waste rock having approximately 100% HgS. Leaching experiments with a high-ionic-strength solution (0.1 M NaCl) resulted in a rapid but brief release of soluble and particulate Hg. Lowering the ionic strength of the leach solution (0.005 M NaCI) resulted in the release of colloidal Hg from two of the three mine wastes studied (NI calcines and SB waste rock). Colloid-associated Hg accounts for as much as 95% of the Hg released during episodic particle release. Colloids generated from the NI calcines are produced by a breakup and release mechanism and consist of hematite, jarosite/alunite, and Al-Si gel with particle sizes of 10-200 nm. ATEM and XAFS analyses indicate that the majority (approximately 78%) of the mercury is present in the form of HgS. SB calcines also produced HgS colloids. The colloids generated from the SB waste rockwere heterogeneous and varied in composition according to the column influent composition. ATEM and XAFS results indicate that Hg is entirely in the HgS form. Data from this study identify colloidal HgS as the dominant transported form of Hg from these mine waste materials.

Mercury deposition in a tidal marsh of south San Francisco Bay downstream of the historic New Almaden mining district, California

A record of mercury deposition was provided by sediment recovered from piston cores of a south San Francisco Bay tidal marsh that is 30 km downstream of the New Almaden mining district, formerly the largest mercury mining district in North America. Pre-mining sediment mercury concentrations were 0.40±0.15 nmol g −1, which are similar to pre-mining concentrations in cores taken from other parts of San Francisco Estuary. Concentrations in the core increase to a maximum of about 6.0 nmol g −1, corresponding to a period in the mid-20th century, nearly 50 years after the peak in mercury production at the mines. The extent of contamination from upstream mining activity appears to reflect the amount of processed ore disposed of at the surface, and also periods when mercury was recovered from reworking these surface ore dumps and open cuts. Transport of this contaminant mercury to the tidal marsh appears to be influenced by hydrologic modifications in the watershed, including dam building and subsidence related to groundwater withdrawal. Although San Francisco Estuary is contaminated with mercury from numerous historic mining sources, including late 19th century hydraulic gold mining in the Sierra Nevada, there is little evidence of pre-mining contamination from natural mercury sources in the southern reach of San Francisco Estuary.