CHEMICAL SIGNATURE OF GROUNDWATER IN COVER OVERLYING DULUTH COMPLEX NI-CU-PGE DEPOSITS, NE MINNESOTA

The U.S. Geological Survey initiated a project in 2015 aimed at evaluating geochemical exploration methods for covered deposits in the northern Midcontintent Rift, employing both site-scale studies and regional geochemical databases. A first group of groundwater samples was collected from unconsolidated material overlying the Spruce Road, Wyman Creek, and Skibo deposits in the Duluth Complex to determine effective sampling methods, characterize the groundwater chemical signature of these deposits, and determine chemical evolution along flow paths. Twenty-seven samples were collected from mini-piezometers at depths <5 m and analyzed for major and trace element chemistry and stable isotopes of water. Ten samples were also analyzed for groundwater age tracers, including noble gases, 3H, He isotopes, and chlorofluorocarbons. Site conditions presented challenges for deriving well-constrained specific ages. However, samples could be sorted into the following age categories by employing multiple tracers: <0.5 yr; 0.5 to 2 yr; 2 to 10 yr; and 15 to 30 yr. Cu and Ni concentrations over the deposits range from <0.5 to 150 μg/L and from <1 to 348 μg/L, respectively, and are commonly elevated above background. Cu and Ni are negatively correlated with pH (range of 5.7 to 8.6), probably due to progressively more adsorption on negatively charged mineral surfaces at higher pH. The pH also increases with groundwater age, likely due to weathering of abundant mafic minerals. As a result, Cu and Ni concentrations generally decrease with increasing age. These results suggest that pH provides an important limit on Cu and Ni mobility in the groundwater system, which must be taken into account in designing geochemical exploration approaches. In addition to site-scale work, a regional groundwater chemical database was compiled from available USGS (NWIS) and state databases. Initial examination reveals both geogenic and anthropogenic metal anomalies, and spatial analyses are ongoing

Natural attenuation can lead to environmental resilience in mine environment

Four streams flowing in the Iglesiente and Arburese mine districts (SW Sardinia, Italy), exploited for zinc (Zn) and lead (Pb) extraction from sulphides and secondary non-sulphide mineralization (calamine ores), have been studied combining investigations from the macroscale (hydrologic tracer techniques) to the microscale (X-ray powder diffraction, scanning electron microscopy, X-ray absorption spectroscopy). In the investigated area, concerns arise from release of metals to water during weathering of ore minerals and mine-waste. Specifically, Zn is observed at extremely high concentrations (10s of mg/L or more) in waters in some of the investigated catchments. The results from synoptic sampling campaigns showed marked differences of Zn loads, from 6.3 kg/day (Rio San Giorgio) to 2000 kg/day (Rio Irvi). Moreover, natural attenuation of metals was found to occur i) through precipitation of Fe compounds (Fe oxy/hydroxides and “green rust”), ii) by means of the authigenic formation of metal sulphides promoted by microbial sulphate reduction, iii) by metal intake in roots and stems of plants (Phragmites australis and Juncus acutus) and by immobilization in the rhizosphere, and iv) by cyanobacterial biomineralization processes that lead to formation of Zn-rich phases (hydrozincite and amorphous Zn-silicate). The biologically mediated natural processes that lead to significant abatement and/or reduction of metal loads, are the response of environmental systems to perturbations caused from mine activities, and can be considered part of the resilience of the system itself. The aim of this study is to understand the effect of these processes on the evolution of the studied systems towards more stable and, likely, resilient conditions, e.g. by limiting metal mobility and favouring the improvement of the overall quality of water. The understanding of how ecosystems adapt and respond to contamination, and which chemical and physical factors control these natural biogeochemical barriers, can help to plan effective remediation actions

Assessment of origin and fate of contaminants along mining-affected Rio Montevecchio (SW Sardinia, Italy): A hydrologic-tracer and environmental mineralogy study

Hydrologic tracer techniques were applied to Rio Montevecchio (SW Sardinia, Italy), a stream affected by mine drainage, allowing the calculation of discharge and contaminant loads. Discharge along the stream showed a constant increase throughout the 2.7 km-long study reach, up to 13.6 l/s at the last synoptic point. Calculated loads of mine-related constituents were large, reaching values of 1780 kg/day for, 340 kg/day for Zn, 47 kg/day for Fe, and 50 kg/day for Mn. The difference of the cumulative instream metal loads between the first and the last synoptic sampling points indicated gains of 421 kg/day for Zn, 2080 kg/day for, 56 kg/day for Mn, and 50 kg/day for Fe. The source areas critical for contaminants loading were almost all concentrated in the first 800 meters of the stream, with the exception of Pb, whose loading occurs evenly along the whole study reach. Precipitation of secondary minerals along the streambed was responsible for a very high attenuation of Al and Fe loads (66% and 77%) and affected also and Zn loads, though less effectively. Rio Montevecchio has the second highest metal load among the rivers investigated with tracer techniques in SW Sardinia. In comparison with Rio Irvi, which has one order of magnitude higher metal loads, natural attenuation processes limit the loads in Rio Montevecchio. Results are useful to clarify the hydrogeochemical paths involved in the release and attenuation of pollutants, improving our understanding of stream responses to contamination and aiding development of site-specific remediation actions

Results of Chemical and Stable Isotopic Analyses of Water Samples Collected in the Patagonia Mountains, Southern Arizona

Water samples were collected in the Patagonia Mountains in February, 1997. Most of the samples were collected from portals of abandoned mines, or from stream drainages immediately downstream from abandoned mines. The data are presented in tabular and graphical formats, with descriptions of data quality and brief descriptions of results

Aquatic pollution increases use of terrestrial prey subsidies by stream fish

1. Stream food webs are connected with their riparian zones through cross-ecosystem movements of energy and nutrients. The use and impact of terrestrial subsidies on aquatic consumers is often determined by in situ biomass of aquatic prey. Thus, stressors such as aquatic pollutants that greatly reduce aquatic secondary production could increase the need for and reliance of stream consumers on terrestrial resource subsidies. 2. To test this hypothesis, we surveyed stream fish, their diets, and resource availability in 16 subalpine streams over a regional gradient of trace metals known to strongly impact aquatic insect communities (i.e. fish prey) in the Colorado Rocky Mountains, USA. 3. Fish increased their reliance on terrestrial insect prey as stream metals increased. Relative biomass of terrestrial insects in stomach contents of Brook and Brown Trout increased with respect to aquatic insect biomass and total stomach contents. Drifting insect biomass showed a declining trend for aquatic, but not terrestrial insects, over the metals gradient. Trout densities were unrelated to metal concentrations in streams where we found fish. 4. Synthesis and applications. Our results indicate that diets of aquatic consumers can become more terrestrial as aquatic stressors that limit in situ food production increase and that these subsidies may compensate for loss of aquatic resources. This work implies an important connection between preserving aquatic–terrestrial linkages and management of fish populations in stressed watersheds. Specifically, intact riparian zones and aquatic–terrestrial linkages are likely to be important for maintaining trout production in streams with moderate meta

Zinc Isotope Investigation of Surface and Pore Waters in a Mountain Watershed Impacted by Acid Rock Drainage

The pollution of natural waters with metals derived from the oxidation of sulfide minerals like pyrite is a global environmental problem. However, the metal loading pathways and transport mechanisms associated with acid rock drainage reactions are often difficult to characterize using bulk chemical data alone. In this study, we evaluated the use of zinc (Zn) isotopes to complement traditional geochemical tools in the investigation of contaminated waters at the former Waldorf mining site in the Rocky Mountains, Colorado, U.S.A. Geochemical signatures and statistical analysis helped in identifying two primary metal loading pathways at the Waldorf site. The first was characterized by a circumneutral pH, high alkalinity, and high Zn/Cd ratios. The second was characterized by acidic pHs and low Zn/Cd ratios. Zinc isotope signatures in surface water samples collected across the site were remarkably similar (the δ66Zn, relative to JMC 3-0749-L, for most samples ranged from 0.20 to 0.30‰ ± 0.09‰ 2σ). This probably suggests that the ultimate source of Zn is consistent across the Waldorf site, regardless of the metal loading pathway. The δ66Zn of pore water samples collected within a nearby metal-impacted wetland area, however, were more variable, ranging from 0.20 to 0.80‰ ± 0.09‰ 2σ. Here the Zn isotopes seemed to reflect differences in groundwater flow pathways. However, a host of secondary processes might also have impacted Zn isotopes, including adsorption of Zn onto soil components, complexation of Zn with dissolved organic matter, uptake of Zn into plants, and the precipitation of Zn during the formation of reduced sulfur species. Zinc isotope analysis proved useful in this study; however, the utility of this isotopic tool would improve considerably with the addition of a comprehensive experimental foundation for interpreting the complex isotopic relationships found in soil pore waters

Regional occurrence of aqueous tungsten and relations with antimony, arsenic and molybdenum concentrations (Sardinia, Italy)

Tungsten (W) is rarely found in natural waters, yet it can be introduced into the food chain and cause potentially toxic effects. Uptake of W by plants and vegetables, or trace presence of W in drinking water are possible vectors for ingestion of W by humans. The latter is recognized as a possible cause of lymphatic leukemia. Increased uses of W might result in a degradation of water resources, with attendant adverse effects on biota and human health. Therefore, this study was aimed at investigating regional occurrence and speciation of W in aquatic systems in Sardinia, Italy, factors affecting W mobility and possible relations with other oxyanion-forming trace elements such as Sb, As and Mo. Although our results are specifically from Sardinia, the implications are broader and should prompt future studies in other areas with known high W concentrations. A total of 350 sample sites are reported here, including surface waters, groundwaters, mine drainages, thermal waters and local seawater. The waters were analyzed for major and trace components, including W, Sb, As and Mo. The waters showed a variety of major chemical compositions and W concentrations. High concentrations of W were found in some mine waters and drainages from slag heaps, with W, Sb and As up to 140, 5000 and 800 μg L 1, respectively. The highest concentrations of W occurred under slightly alkaline pH and oxygenated conditions, and were likely due to the dissolution of scheelite [CaWO4] hosted in materials with which the water came into contact. High W concentrations also were observed in thermal waters, under alkaline pH and reducing conditions, and sometimes coincided with relatively high concentrations either of As or Mo. Previous studies of W geochemistry have focused on WO42 as the major dissolved form of W. For this study, we have augmented the thermodynamic database in PHREEQC to include possible formation of many other Wbearing complexes gleaned from the literature. The results of the speciation calculations with the newly added complexation reactions shows that the neutral species CaWO4◦ and MgWO4◦ are particularly dominant in most W-bearing waters and lead to undersaturation with respect to scheelite and other W-bearing minerals. Assessing W contamination in water systems and establishing W limits in drinking water may prevent potential adverse effects of W on human and ecosystem health

Influence of Sulfur-bearing Polyatomic Species on High Precision Measurements of Cu Isotopic Composition

An increased interest in high precision Cu isotope ratio measurements using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has developed recently for various natural geologic systems and environmental applications, these typically contain high concentrations of sulfur, particularly in the form of sulfate (SO42-) and sulfide (S). For example, Cu, Fe, and Zn concentrations in acid mine drainage (AMD) can range from 100 μg/L to greater than 50 mg/L with sulfur species concentrations reaching greater than 1000 mg/L. Routine separation of Cu, Fe and Zn from AMD, Cu-sulfide minerals and other geological matrices usually incorporates single anion exchange resin column chromatography for metal separation. During chromatographic separation, variable breakthrough of SO42- during anion exchange resin column chromatography into the Cu fractions was observed as a function of the initial sulfur to Cu ratio, column properties, and the sample matrix. SO42- present in the Cu fraction can form a polyatomic 32S-14N-16O-1H species causing a direct mass interference with 63Cu and producing artificially light δ65Cu values. Here we report the extent of the mass interference caused by SO42- breakthrough when measuring δ65Cu on natural samples and NIST SRM 976 Cu isotope spiked with SO42- after both single anion column chromatography and double anion column chromatography. A set of five 100 μg/L Cu SRM 976 samples spiked with 500 mg/L SO42- resulted in an average δ65Cu of - 3.50‰ ± 5.42‰ following single anion column separation with variable SO42- breakthrough but an average concentration of 770 μg/L. Following double anion column separation, the average SO42-concentration of 13 μg/L resulted in better precision and accuracy for the measured δ65Cu value of 0.01‰ ± 0.02‰ relative to the expected 0‰ for SRM 976. We conclude that attention to SO42- breakthrough on sulfur-rich samples is necessary for accurate and precise measurements of δ65Cu and may require the use of a double ion exchange column procedure