Paul J. Lechler

Elevated mercury concentrations in soils, sediments, water, and fish of the Madeira River basin, Brazilian Amazon: a function of natural enrichments?

Previous site-specific investigations have found that mercury concentrations in water, sediments, and biota of the Brazilian Amazon are elevated above global averages, and that these concentrations are a direct result of widespread mercury amalgamation mining operations conducted by non-organized prospectors. In order to assess the regional impacts of Hg contamination from these non-organized gold mining activities, water, sediments, and fish were systematically collected in 1997 along a 900-km reach of the Madeira River. The sampling program extended from the Amazon River upstream to Porto Velho, the site of historic and ongoing mercury amalgamation mining. Mercury concentrations were found to be elevated above global averages in all sampled media. However, the geochemical data suggest that the high mercury levels are due largely to natural sources and natural biogeochemical processes, and that the impacts of anthropogenically released mercury from mine sites is relatively localized.

Sources, distribution and storage of heavy metals in the Rio Pilcomayo, Bolivia

Hudson-Edwards, K. A., Macklin, M. G., Miller, J. R., Lechler, P. J. (2001). Sources, distribution and storage of heavy metals in the Rio Pilcomayo, Bolivia. Journal of Geochemical Exploration, 72(3), 229-250.

Mercury mobility at the Carson River Superfund Site, west-central Nevada, USA: Interpretation of mercury speciation data in mill tailings, soils, and sediments

Abstract The Carson River Superfund Site in west-central Nevada is an area of Hg-contaminated soil, sediment, water, air, and biola resulting from the amalgamation milling of Ag-Au ores of the Comstock lode worked approximately a century ago. In order to develop an understanding of the behavior, transport, and fate of Hg at this site, a technique was developed to estimate the proportions of total, elemental, exchangeable, organic, and sulfide Hg in soils, sediments, and tailings. Results of this analysis performed on active Carson River sediments indicate that Hg is selectively dissolved out of Hg-Au amalgam particles and subsequently adsorbed to fine-grained sediments which are then deposited in downstream, low-energy reaches of the Carson River and Labontan Reservoir. In the relatively more-reducing environment of the reservoir Hg appears to be converted, in large part, to relatively-insoluble HgS. The original elemental form of Hg released to the environment is the chemical form which is still dominant in most highly-contaminated soils, sediments, and tailings. Deeper, more-reducing soil horizons, however, appear to fix a significant portion of the Hg as HgS, analogous to the Lahontan Reservoir example described above. This fixation as HgS is documented to be largely limited to higher-sulfur areas where sulfide minerals from the Comstock ores increase the total sulfur concentrations of contaminated soils, sediments, and tailings.

DISPERSAL OF MERCURY-CONTAMINATED SEDIMENTS BY GEOMORPHIC PROCESSES, SIXMILE CANYON, NEVADA, USA: IMPLICATIONS TO SITE CHARACTERIZATION AND REMEDIATION OF FLUVIAL ENVIRONMENTS

The discovery of the Comstock Lode near Virginia City, Nevada, in 1859 resulted in the construction of more than 30 stamp-mills along Sixmile Canyon and its tributaries to process the gold and silver ore. Extraction of the precious metals from the ore was accomplished using a crude mercury (Hg) amalgamation process. As a result, a substantial quantity of Hg was released along with tailings materials to this tributary of the Carson River. During the past 134 years, Hg-contaminated sediments have been eroded from the mill sites and transported downstream by fluvial processes, thereby expanding the influence of Hg pollution. Geomorphic and geochemical data have been combined in this study to document the distribution, quantity, and physical dispersal of Hg-contaminated materials from Sixmile Canyon to the Carson River. These data show that the influx of Hg to the Carson River has varied through time as a function of the erosional and depositional processes operating on the Sixmile Canyon Alluvial Fan located between the canyon and the Carson River channel; relatively high influx rates to the river occurred immediately after mining began and from approximately 1933 to 1948. Hg-polluted sediments are located within discrete areas of the fan and comprise about 21 % of the total active and relict fan surface. Mass balance calculations estimate that about 31,500 kg of Hg, 18,200 oz of An, and 1,205,800 oz of Ag are contained within 710,700 m3 of contaminated materials. If site remediation is conducted, extraction of An and Ag, which is worth about

A review of the use of loss on ignition as a measurement of total volatiles in whole-rock analysis

12 million at current market prices, would greatly defray the costs of clean-up activities. The study also illustrates that the effects of Hg may be temporally and spatially displaced from the period and location of milling activities. Thus, we conclude that to accurately assess the site for remediation, an understanding of the spatial and temporal variations in geomorphic dispersal processes is required.

Effects of the 1997 Flood on the Transport and Storage of Sediment and Mercury within the Carson River Valley, West‐Central Nevada

The use of loss on ignition (LOI) in whole-rock analysis is evaluated in the modern context — as a measurement of total volatiles, mainly added to the other oxides to assess analytical accuracy by testing whether or not the total is 100.0 ± − 1.0%. By measuring LOI- and FeO-decrease with time for several igneous rocks, it is demonstrated that the simple determination of weight loss after ignition at 1000°C, as a measurement of the volatile content, is interfered with by ferrous-iron oxidation. It is shown that ferrous-iron oxidation is virtually complete, and that the LOI measurement can be corrected for this interference if the initial ferrous/ferric ratio is known. If the ferrous/ferric ratio is not measured, total Fe must be reported as Fe2O3 in which case, when the oxides are summed, an accurate total is obtained (although an accurate measurement of total volatiles is not). It is also shown that ignition should be carried out at greater than 900°C, preferably at 1000°C, and that, in practice, 1 hr. is probably sufficient time to achieve virtually complete expulsion of volatiles and conversion of ferrous to ferric oxide in most igneous rocks.

Lead isotopic fingerprinting of heavy metal contamination,Rio Pilcomayo basin, Bolivia

Intense, warm rains falling on a heavy snowpack in the Sierra Nevada at the end of December 1996 produced some of the largest floods on record in west‐central Nevada. Within the Carson River basin, a peak discharge of 632 cm was recorded at the Fort Churchill gaging station on January 3, 1997, a flow exceeding the 100‐yr event. Geomorphic impacts of the event, and the redistribution of mercury (Hg) released to the Carson River valley by Comstock mining operations during the mid‐ to late‐1800s, were assessed by combining field data with the interpretation of aerial photographs. Geomorphic impacts included significant increases in channel width, measuring up to 280% of preflood conditions, and large‐scale shifts in channel position, ranging from <10 to 110 m. Both changes in channel width and position vary as a function of valley morphometry (width and slope) and differ from the long‐term trends measured from 1965 to 1991. The 1997 flood also produced widespread overbank deposits that vary morphologically and sedimentologically according to distance from the channel and the nature of the vegetation on the valley floor. Within the overbank deposits, Hg is primarily associated with the fine‐grained (<63 μm) sediment fraction, which makes up a larger percentage of the deposits immediately adjacent to the channel and at the extremities of overbank deposition. Mass balance calculations demonstrate that, along reaches with narrow valleys (<450 m), approximately 10%–65% of the sediment eroded from the channel banks was stored in overbank deposits, whereas more than 90% of the sediment eroded along reaches with wider valleys was stored on the valley floor. Locally, however, storage exceeded 650% where meander cutoff was extensive. The above data indicate that the erosion, redeposition, and storage of sediment and sediment‐bound Hg were greater along reaches characterized by low gradients and wide valley floors. Downstream trends in Hg concentration within the channel bed did not change following the 1997 flood and are presumably controlled by the overall structure of the system, including valley morphometry, the location of tributaries that deliver “clean” sediment to the channel, and the distribution of Hg within the valley fill.

Geochemical modeling approach to predicting arsenic concentrations in a mine pit lake

Waste materials from mining of the Cerro Rico de Potosí precious metal-polymetallic tin deposits of southern Bolivia have been released to the headwaters of the Rio Pilcomayo for the past 450 years, resulting in extensive contamination of water, sediments and soils along the upper reaches of the river. This study uses isotopic data to identify the primary sources of Pb to the aquatic environment, and the relative contributions of each source to pre and post-mining alluvial deposits. Prior to the onset of mining activities in 1545, alluvial sediments along the Rio Pilcomayo were dominated by Pb from the underlying bedrock and from mineralized rocks exposed at the surface of Cerro Rico. Mining and milling operations at Cerro Rico released a new source of Pb to the river that can be traced downstream for at least 200 km. Simple mixing models suggest that Pb from the mines comprise between 30 and 89% of the Pb in the modern channel bed sediment. However, these estimates may be low because the isotopic composition of the contaminant source was based on samples of ore deposits rather than mill tailings, the latter of which contain fragments of both ore and host rock.

Mercury contamination in the Carson River, Nevada: A preliminary study of the impact of mining wastes

Abstract Between 1968 and 1983, the North pit at the Getchell Mine, Humboldt County, NV, filled with water to form a lake. In 1983, water quality data were collected with the following results: As concentrations of 0.29 to 0.59 mg/L, pH of 7.1 to 7.9, SO 4 concentrations of 1490 to 1640 mg/L, and TDS of 2394 to 2500 mg/L. Using geochemical modeling techniques presented here, pit lake waters have been theoretically allowed to react for 8.5 a, the approximate time that the North pit had been completely full by 1983. Modeling results predict pH of 7.9 to 8.2, SO 4 concentrations of 1503 to 1644 mg/L, TDS of 2054 to 2366 mg/L, and As concentrations ranging from 0.57 in the hypolimnion to 96 mg/L in the epilimnion. In the epilimnion, model results do not match observed As concentrations, suggesting that mechanisms, such as precipitation of arsenate salts or adsorption to mineral surfaces, may control As levels in an actual pit lake system. Adsorption to Fe oxyhydroxide surfaces is questioned by the authors because of the low Fe content in the Getchell system, but adsorption to Al(OH) 3 (gibbsite) and clay mineral surfaces may be important in controlling natural As concentrations.

Mercury Contamination of Alluvial Sediments within the Essequibo and Mazaruni River Basins, Guyana

From 1860 to 1890, approximately 7 100 metric tons of metallic mercury (Hg) were released into the Carson River-Lahontan Reservoir watershed as a by-product of Comstock Lode silver (Ag) and gold (Au) ore refining. Present-day Hg contamination is most severe in mine tailings, where total Hg concentrations can exceed several hundred µg/g. Hg-laden tailings were also dumped directly into the Carson River, and were subsequently transported downstream into the Lahontan Reservoir and the Stillwater Wildlife Management Area. The Hg, Ag and Au contents of sediments from the Carson River and the Lahontan Reservoir are well above local background levels, and both Ag and Au contents are positively correlated to Hg. Thus, tailings-derived Hg has been redistributed throughout the entire Carson-Lahontan watershed over the last century. Total Hg concentrations in water samples from the Carson River at seven localities show that: 1) elevated (e.g., >20 ng/L) Hg levels in Carson River waters first appear downstream from accumulations of mill tailings, 2) total Hg concentrations in unfiltered and filtered water from the Carson River increase downstream (i.e. away from the tailings piles), and 3) Hg concentrations in both the Carson River (downstream from the tailings piles) and the Lahontan Reservoir are among the highest known worldwide (100 to 1000 ng/L). Filtered water samples from the Carson-Lahontan system also have high Hg contents (up to 113 ng/L), and suggest that the >0.4 µm particle fraction constitutes over 60% of the total water-borne Hg.