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USGS2006

Mercury in coal and the impact of coal quality on mercury emissions from combustion systems

The proportion of Hg in coal feedstock that is emitted by stack gases of utility power stations is a complex function of coal chemistry and properties, combustion conditions, and the positioning and type of air pollution control devices employed. Mercury in bituminous coal is found primarily within Fe-sulfides, whereas lower rank coal tends to have a greater proportion of organic-bound Hg. Preparation of bituminous coal to reduce S generally reduces input Hg relative to in-ground concentrations, but the amount of this reduction varies according to the fraction of Hg in sulfides and the efficiency of sulfide removal. The mode of occurrence of Hg in coal does not directly affect the speciation of Hg in the combustion flue gas. However, other constituents in the coal, notably Cl and S, and the combustion characteristics of the coal, influence the species of Hg that are formed in the flue gas and enter air pollution control devices. The formation of gaseous oxidized Hg or particulate-bound Hg occurs post-combustion; these forms of Hg can be in part captured in the air pollution control devices that exist on coal-fired boilers, without modification. For a given coal type, the capture efficiency of Hg by pollution control systems varies according to type of device and the conditions of its deployment. For bituminous coal, on average, more than 60% of Hg in flue gas is captured by fabric filter (FF) and flue-gas desulfurization (FGD) systems. Key variables affecting performance for Hg control include Cl and S content of the coal, the positioning (hot side vs. cold side) of the system, and the amount of unburned C in coal ash. Knowledge of coal quality parameters and their effect on the performance of air pollution control devices allows optimization of Hg capture co-benefit.

Applied Geochemistry↗

USGS2008

Thermal history of the unsaturated zone at Yucca Mountain, Nevada, USA

Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry, temperatures, and timing of past fluid movement in the unsaturated zone at Yucca Mountain, Nevada, the proposed site of a high-level radioactive waste repository. The distribution and geochemistry of these deposits are consistent with low-temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone. However, the discovery of fluid inclusions in calcite with homogenization temperatures (Th) up to ???80 ??C was construed by some scientists as strong evidence for hydrothermal deposition. This paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition. Mineral precipitation temperatures in the unsaturated zone are estimated from calcite- and fluorite-hosted fluid inclusions and calcite ??18O values, and depositional timing is constrained by the 207Pb/235U ages of chalcedony or opal in the deposits. Fluid inclusion Th from 50 samples of calcite and four samples of fluorite range from ???35 to ???90 ??C. Calcite ??18O values range from ???0 to ???22??? (SMOW) but most fall between 12 and 20???. The highest Th and the lowest ??18O values are found in the older calcite. Calcite Th and ??18O values indicate that most calcite precipitated from water with ??18O values between -13 and -7???, similar to modern meteoric waters. Twenty-two 207Pb/235U ages of chalcedony or opal that generally postdate elevated depositional temperatures range from ???9.5 to 1.9 Ma. New and published 207Pb/235U and 230Th/Uages coupled with the Th values and estimates of temperature from calcite ??18O values indicate that maximum unsaturated zone temperatures probably predate ???10 Ma and that the unsaturated zone had cooled to near-present-day temperatures (24-26 ??C at a depth of 250 m) by 2-4 Ma. The evidence of elevated temperatures persisting in ash flow tuffs adjacent to parent calderas for as much as ???8 Ma is a new finding, but consistent with thermal modeling. Simulations using the HEAT code demonstrate that prolonged cooling of the unsaturated zone is consistent with magmatic heat inputs and deep-seated (sub-water table) hydrothermal activity generated by the large magma body ???8 km to the north that produced the 15-11 Ma ash flows and ash falls that make up Yucca Mountain. The evidence discussed in this and preceding papers strongly supports unsaturated zone deposition of the secondary minerals from descending meteoric waters. Although depositional temperatures reflect conductive (and possibly vapor-phase convective) heating of the unsaturated zone related to regional magmatic sources until perhaps 6 Ma, depositional conditions similar to the present-day unsaturated zone have prevailed for at least the past 2-4 Ma.

Applied Geochemistry↗

USGS2008

Environmental geochemistry of a Kuroko-type massive sulfide deposit at the abandoned Valzinco mine, Virginia, USA

The abandoned Valzinco mine, which worked a steeply dipping Kuroko-type massive sulfide deposit in the Virginia Au-pyrite belt, contributed significant metal-laden acid-mine drainage to the Knight's Branch watershed. The host rocks were dominated by metamorphosed felsic volcanic rocks, which offered limited acid-neutralizing potential. The ores were dominated by pyrite, sphalerite, galena, and chalcopyrite, which represented significant acid-generating potential. Acid-base accounting and leaching studies of flotation tailings - the dominant mine waste at the site - indicated that they were acid generating and therefore, should have liberated significant quantities of metals to solution. Field studies of mine drainage from the site confirmed that mine drainage and the impacted stream waters had pH values from 1.1 to 6.4 and exceeded aquatic ecosystem toxicity limits for Fe, Al, Cd, Cu, Pb and Zn. Stable isotope studies of water, dissolved SO42 -, and primary and secondary sulfate and sulfide minerals indicated that two distinct sulfide oxidation pathways were operative at the site: one dominated by Fe(III) as the oxidant, and another by molecular O2 as the oxidant. Reaction-path modeling suggested that geochemical interactions between tailings and waters approached a steady state within about a year. Both leaching studies and geochemical reaction-path modeling provided reasonable predictions of the mine-drainage chemistry.

Applied Geochemistry↗

USGS2008

Temporal variation and the effect of rainfall on metals flux from the historic Beatson mine, Prince William Sound, Alaska, USA

Several abandoned Cu mines are located along the shore of Prince William Sound, AK, where the effect of mining-related discharge upon shoreline ecosystems is unknown. To determine the magnitude of this effect at the former Beatson mine, the largest Cu mine in the region and a Besshi-type massive sulfide ore deposit, trace metal concentration and flux were measured in surface run-off from remnant, mineralized workings and waste. Samples were collected from seepage waters; a remnant glory hole which is now a pit lake; a braided stream draining an area of mineralized rock, underground mine workings, and waste piles; and a background location upstream of the mine workings and mineralized rock. In the background stream pH averaged ???7.3, specific conductivity (SC) was ???40 ??S/cm, and the aqueous components indicative of sulfide mineral weathering, SO4 and trace metals, were at detection limits or lower. In the braided stream below the mine workings and waste piles, pH usually varied from 6.7 to 7.1, SC varied from 40 to 120 ??S/cm, SO4 had maximum concentrations of 32 mg/L, and the trace metals Cu, Ni, Pb, and Zn showed maximum total acid extractable concentrations of 186, 5.9, 6.2 and 343 ??g/L, respectively. With an annual rainfall of ???340 cm (estimated from the 2006 water year) it was expected that rain water would have a large effect on the chemistry of the braided stream draining the mine site. A linear mixing model with two end members, seepage water from mineralized rock and background water, estimated that the braided stream contained 10-35% mine drainage. After rain events the braided stream showed a decrease in pH, SC, Ca + Mg, SO4, and alkalinity, due to dilution. The trace metals Ni and Zn followed this same pattern. Sodium + K and Cl did not vary between the background and braided stream, nor did they vary with rainfall. At approximately 2 and 3 mg/L, respectively, these concentrations are similar to concentrations found in rainfall on the coasts of North America. High concentrations of total acid extractable Al and Fe were found at near-neutral pH in most of the waters collected at the site. Equilibrium solubility simulations, performed with PHREEQC, show that the stream waters are saturated with respect to Al, Fe and SiO2 solid phases. Because the "dissolved" sample fractions (acid preserved and filtered to 0.45 ??m) show significant concentrations of Al and Fe it is presumed that these are present as colloids. The relationship between concentrations of Al and Fe, and rainfall was the opposite of that observed for the major ions Ca + Mg, SO4, and alkalinity, in that Al and Fe concentrations increased with increasing rainfall. Concentrations of Cu and Pb followed the same pattern. Adsorption calculations were performed with Visual MINTEQ, using the diffuse double layer electrostatic model and surface complexation constants for the ferrihydrite surface. These results suggest that 30-93% of Cu and 58-97% of Pb was adsorbed to ferrihydrite precipitates in the stream waters. Ni and Zn showed little adsorption in this pH range. Flux calculations show that the total mass of trace metals transported from the mine site, during the 60 day study period, was ranked as Zn (196 kg) > Cu (87 kg) > Pb(1.9 kg) ??? Ni(1.9 kg). Nickel and Zn were transported mostly as dissolved species while Cu and Pb were transported mostly as adsorbed species. pH control on adsorption was evident when Cu and Pb isotherms were normalized by ferrihydrite flux. Decreased stream water pH due to periods of frequent and high volume rain events would cause desorption of Cu and Pb from the ferrihydrite surface, thus changing not only their speciation in solution but also their mechanism of transport. ?? 2007 Elsevier Ltd. All rights reserved.

Applied Geochemistry↗

USGS2008

In situ arsenic removal in an alkaline clastic aquifer

In situ removal of As from ground water used for water supply has been accomplished elsewhere in circum-neutral ground water containing high dissolved Fe(II) concentrations. The objective of this study was to evaluate in situ As ground-water treatment approaches in alkaline ground-water (pH > 8) that contains low dissolved Fe (<a few tens of μg/L). The low dissolved Fe content limits development of significant Fe-oxide and the high-pH limits As adsorption onto Fe-oxide. The chemistries of ground water in the two aquifers studied are similar except for the inorganic As species. Although total inorganic As concentrations were similar, one aquifer has dominantly aqueous As(III) and the other has mostly As(V). Dissolved O 2 , Fe(II), and HCl were added to water and injected into the two aquifers to form Fe-oxide and lower the pH to remove As. Cycles of injection and withdrawal involved varying Fe(II) concentrations in the injectate. The As concentrations in water withdrawn from the two aquifers were as low as 1 and 6 μg/L, with greater As removal from the aquifer containing As(V). However, Fe and Mn concentrations increased to levels greater than US drinking water standards during some of the withdrawal periods. A balance between As removal and maintenance of low Fe and Mn concentrations may be a design consideration if this approach is used for public-supply systems. The ability to lower As concentrations in situ in high-pH ground water should have broad applicability because similar high-As ground water is present in many parts of the world.

Applied Geochemistry↗

USGS2009

The Drenchwater deposit, Alaska: An example of a natural low pH environment resulting from weathering of an undisturbed shale-hosted Zn-Pb-Ag deposit

The Drenchwater shale-hosted Zn-Pb-Ag deposit and the immediate vicinity, on the northern flank of the Brooks Range in north-central Alaska, is an ideal example of a naturally low pH system. The two drainages, Drenchwater and False Wager Creeks, which bound the deposit, differ in their acidity and metal contents. Moderately acidic waters with elevated concentrations of metals (pH ??? 4.3, Zn ??? 1400 ??g/L) in the Drenchwater Creek drainage basin are attributed to weathering of an exposed base-metal-rich massive sulfide occurrence. Stream sediment and water chemistry data collected from False Wager Creek suggest that an unexposed base-metal sulfide occurrence may account for the lower pH (2.7-3.1) and very metal-rich waters (up to 2600 ??g/L Zn, ??? 260 ??g/L Cu and ???89 ??g/L Tl) collected at least 2 km upstream of known mineralized exposures. These more acidic conditions produce jarosite, schwertmannite and Fe-hydroxides commonly associated with acid-mine drainage. The high metal concentrations in some water samples from both streams naturally exceed Alaska state regulatory limits for freshwater aquatic life, affirming the importance of establishing base-line conditions in the event of human land development. The studies at the Drenchwater deposit demonstrate that poor water quality can be generated through entirely natural weathering of base-metal occurrences, and, possibly unmineralized black shale.

Applied Geochemistry↗

USGS2009

Sulfur geochemistry of hydrothermal waters in Yellowstone National Park: IV Acid-sulfate waters

Many waters sampled in Yellowstone National Park, both high-temperature (30-94 ??C) and low-temperature (0-30 ??C), are acid-sulfate type with pH values of 1-5. Sulfuric acid is the dominant component, especially as pH values decrease below 3, and it forms from the oxidation of elemental S whose origin is H2S in hot gases derived from boiling of hydrothermal waters at depth. Four determinations of pH were obtained: (1) field pH at field temperature, (2) laboratory pH at laboratory temperature, (3) pH based on acidity titration, and (4) pH based on charge imbalance (at both laboratory and field temperatures). Laboratory pH, charge imbalance pH (at laboratory temperature), and acidity pH were in close agreement for pH < 2.7. Field pH measurements were predominantly used because the charge imbalance was <??10%. When the charge imbalance was generally >??10%, a selection process was used to compare acidity, laboratory, and charge balance pH to arrive at the best estimate. Differences between laboratory and field pH can be explained based on Fe oxidation, H2S or S2O3 oxidation, CO2 degassing, and the temperature-dependence of pK2 for H2SO4. Charge imbalances are shown to be dependent on a speciation model for pH values <3. The highest SO4 concentrations, in the thousands of mg/L, result from evaporative concentration at elevated temperatures as shown by the consistently high ??18O values (-10??? to -3???) and a ??D vs. ??18O slope of 3, reflecting kinetic fractionation. Low SO4 concentrations (<100 mg/L) for thermal waters (>350 mg/L Cl) decrease as the Cl- concentration increases from boiling which appears inconsistent with the hypothesis of H2S oxidation as a source of hydrothermal SO4. This trend is consistent with the alternate hypothesis of anhydrite solubility equilibrium. Acid-sulfate water analyses are occasionally high in As, Hg, and NH3 concentrations but in contrast to acid mine waters they are low to below detection in Cu, Zn, Cd, and Pb concentrations. Even concentrations of SO4, Fe, and Al are much lower in thermal waters than acid mine waters of the same pH. This difference in water chemistry may explain why certain species of fly larvae live comfortably in Yellowstone's acid waters but have not been observed in acid rock drainage of the same pH.

Applied Geochemistry↗

USGS2009

Diffuse gas emissions at the Ukinrek Maars, Alaska: Implications for magmatic degassing and volcanic monitoring

Diffuse CO2 efflux near the Ukinrek Maars, two small volcanic craters that formed in 1977 in a remote part of the Alaska Peninsula, was investigated using accumulation chamber measurements. High CO2 efflux, in many places exceeding 1000 g m−2 d−1, was found in conspicuous zones of plant damage or kill that cover 30,000–50,000 m2 in area. Total diffuse CO2 emission was estimated at 21–44 t d−1. Gas vents 3-km away at The Gas Rocks produce 0.5 t d−1 of CO2 that probably derives from the Ukinrek Maars basalt based on similar δ13C values (∼−6‰), 3He/4He ratios (5.9–7.2 RA), and CO2/3He ratios (1–2 × 109) in the two areas. A lower 3He/4He ratio (2.7 RA) and much higher CO2/3He ratio (9 × 1010) in gas from the nearest arc-front volcanic center (Mount Peulik/Ugashik) provide a useful comparison. The large diffuse CO2 emission at Ukinrek has important implications for magmatic degassing, subsurface gas transport, and local toxicity hazards. Gas–water–rock interactions play a major role in the location, magnitude and chemistry of the emissions.

Alaska↗

USGS2009

Geochemistry of surface water in alpine catchments in central Colorado, USA: Resolving host-rock effects at different spatial scales

The US Geological Survey is conducting a study of surface-water quality in the Rocky Mountains of central Colorado, an area of approximately 55,000 km 2 . Using new and existing geologic maps, the more than 200 rock formations represented in the area were arranged into 17 groups based on lithologic similarity. The dominant regional geologic feature affecting water quality in central Colorado is the Colorado mineral belt (CMB), a NE-trending zone hosting many polymetallic vein or replacement deposits, and porphyry Mo deposits, many of which have been mined historically. The influence of the CMB is seen in lower surface-water pH (<5), and higher concentrations of SO 4 2 - (>100 mg/L) and chalcophile metals such as Cu (>10 ??g/L), Zn (>100 ??g/L), and Cd (>1 ??g/L) relative to surface water outside the CMB. Not all streams within the CMB have been affected by mineralization, as there are numerous catchments within the CMB that have no mineralization or alteration exposed at the surface. At the regional-scale, and away from sites affected by mineralization, hydrothermal alteration, or mining, the effects of lithology on water quality can be distinguished using geochemical reaction modeling and principal components analysis. At local scales (100 s of km 2 ), effects of individual rock units on water chemistry are subtle but discernible, as shown by variations in concentrations of major lithophile elements or ratios between them. These results demonstrate the usefulness of regional geochemical sampling of surface waters and process-based interpretations incorporating geologic and geochemical understanding to establish geochemical baselines.

Applied Geochemistry↗

USGS2009

A survey of the occurrence of Bacillus anthracis in North American soils over two long-range transects and within post-Katrina New Orleans

Soil samples were collected along a north-south transect extending from Manitoba, Canada, to the US-Mexico border near El Paso, Texas in 2004 (104 samples), a group of sites within New Orleans, Louisiana following Hurricane Katrina in 2005 (19 samples), and a Gulf Coast transect extending from Sulphur, Louisiana, to DeFuniak Springs, Florida, in 2007 (38 samples). Samples were collected from the top 40 cm of soil and were screened for the presence of total Bacillus species and Bacillus anthracis (anthrax), specifically using multiplex-polymerase chain reaction (PCR). Using an assay with a sensitivity of ???170 equivalent colony-forming units (CFU) g -1 field moist soil, the prevalence rate of Bacillus sp./B. anthracis in the north-south transect and the 2005 New Orleans post-Katrina sample set were 20/5% and 26/26%, respectively. Prevalence in the 2007 Gulf Coast sample set using an assay with a sensitivity of ???4 CFU g -1 of soil was 63/0%. Individual transect-set data indicate a positive relation between occurrences of species and soil moisture or soil constituents (i.e., Zn and Cu content). The 2005 New Orleans post-Katrina data indicated that B. anthracis is readily detectable in Gulf Coast soils following flood events. The data also indicated that occurrence, as it relates to soil chemistry, may be confounded by flood-induced dissemination of germinated cells and the mixing of soil constituents for short temporal periods following an event.

Applied Geochemistry↗

USGS2009

Elevated naturally occurring arsenic in a semiarid oxidizing system, Southern High Plains aquifer, Texas, USA

High groundwater As concentrations in oxidizing systems are generally associated with As adsorption onto hydrous metal (Al, Fe or Mn) oxides and mobilization with increased pH. The objective of this study was to evaluate the distribution, sources and mobilization mechanisms of As in the Southern High Plains (SHP) aquifer, Texas, relative to those in other semiarid, oxidizing systems. Elevated groundwater As levels are widespread in the southern part of the SHP (SHP-S) aquifer, with 47% of wells exceeding the current EPA maximum contaminant level (MCL) of 10 μg/L (range 0.3–164 μg/L), whereas As levels are much lower in the north (SHP-N: 9% ⩾ As MCL of 10 μg/L; range 0.2–43 μg/L). The sharp contrast in As levels between the north and south coincides with a change in total dissolved solids (TDS) from 395 mg/L (median north) to 885 mg/L (median south). Arsenic is present as arsenate (As V) in this oxidizing system and is correlated with groundwater TDS (Spearman’s ρ = 0.57). The most likely current source of As is sorbed As onto hydrous metal oxides based on correlations between As and other oxyanion-forming elements (V, ρ = 0.88; Se, ρ = 0.54; B, ρ = 0.51 and Mo, ρ = 0.46). This source is similar to that in other oxidizing systems and constitutes a secondary source; the most likely primary source being volcanic ashes in the SHP aquifer or original source rocks in the Rockies, based on co-occurrence of As and F ( ρ = 0.56), oxyanion-forming elements and SiO 2 ( ρ = 0.41), which are found in volcanic ashes. High groundwater As concentrations in some semiarid oxidizing systems are related to high evaporation. Although correlation of As with TDS in the SHP aquifer may suggest evaporative concentration, unenriched stable isotopes (δ 2 H: −65 to −27; δ 18 O: −9.1 to −4.2) in the SHP aquifer do not support evaporation. High TDS in the SHP aquifer is most likely related to upward movement of saline water from the underlying Triassic Dockum aquifer. Mobilization of As in other semiarid oxidizing systems is caused by increased pH; however, pH in the SHP aquifer is near neutral (10–90 percentiles, 7.0–7.6). Although many processes, such as competitive desorption with SiO 2 , VO 4 , or PO 4 , could be responsible for local mobilization of As in the SHP aquifer, the most plausible explanation for the regional As distribution and correlation with TDS is the counterion effect caused by a change from Ca- to Na-rich, water as shown by the high correlation between As and Na/(Ca) 0.5 ratios ( ρ = 0.57). This change in chemistry is related to mixing with saline water that moves upward from the underlying Dockum aquifer. This counterion effect may mobilize other anions and oxyanion-forming elements that are correlated with As (F, V, Se, B, Mo and SiO 2 ). Competition among the oxyanions for sorption sites may enhance As mobilization. The SHP case study has similar As sources to those of other semiarid, oxidizing systems (original volcanic ash source followed by sorption onto hydrous metal oxides) but contrasts with these systems by showing lack of evaporative concentration and pH mobilization of As but counterion mobilization of As instead in the SHP-S aquifer.

Texas↗

USGS2010

Aquifer geochemistry at potential aquifer storage and recovery sites in coastal plain aquifers in the New York city area, USA

The effects of injecting oxic water from the New York city (NYC) drinking-water supply and distribution system into a nearby anoxic coastal plain aquifer for later recovery during periods of water shortage (aquifer storage and recovery, or ASR) were simulated by a 3-dimensional, reactive-solute transport model. The Cretaceous aquifer system in the NYC area of New York and New Jersey, USA contains pyrite, goethite, locally occurring siderite, lignite, and locally varying amounts of dissolved Fe and salinity. Sediment from cores drilled on Staten Island and western Long Island had high extractable concentrations of Fe, Mn, and acid volatile sulfides (AVS) plus chromium-reducible sulfides (CRS) and low concentrations of As, Pb, Cd, Cr, Cu and U. Similarly, water samples from the Lloyd aquifer (Cretaceous) in western Long Island generally contained high concentrations of Fe and Mn and low concentrations of other trace elements such as As, Pb, Cd, Cr, Cu and U, all of which were below US Environmental Protection Agency (USEPA) and NY maximum contaminant levels (MCLs). In such aquifer settings, ASR operations can be complicated by the oxidative dissolution of pyrite, low pH, and high concentrations of dissolved Fe in extracted water.The simulated injection of buffered, oxic city water into a hypothetical ASR well increased the hydraulic head at the well, displaced the ambient groundwater, and formed a spheroid of injected water with lower concentrations of Fe, Mn and major ions in water surrounding the ASR well, than in ambient water. Both the dissolved O2 concentrations and the pH of water near the well generally increased in magnitude during the simulated 5-a injection phase. The resultant oxidation of Fe2+ and attendant precipitation of goethite during injection provided a substrate for sorption of dissolved Fe during the 8-a extraction phase. The baseline scenario with a low (0.001M) concentration of pyrite in aquifer sediments, indicated that nearly 190% more water with acceptably low concentrations of dissolved Fe could be extracted than was injected. Scenarios with larger amounts of pyrite in aquifer sediments generally resulted in less goethite precipitation, increased acidity, and increased concentrations of dissolved Fe in extracted water. In these pyritic scenarios, the lower amounts of goethite precipitated and the lower pH during the extraction phase resulted in decreased sorption of Fe2+ and a decreased amount of extractable water with acceptably low concentrations of dissolved Fe (5.4??10-6M). A linear decrease in recovery efficiency with respect to dissolved Fe concentrations is caused by pyrite dissolution and the associated depletion of dissolved O2 (DO) and increase in acidity. Simulations with more than 0.0037M of pyrite, which is the maximum amount dissolved in the baseline scenario, had just over a 50% recovery efficiency. The precipitation of ferric hydroxide minerals (goethite) at the well screen, and a possible associated decrease in specific capacity of the ASR well, was not apparent during the extraction phase of ASR simulations, but the model does not incorporate the microbial effects and biofouling associated with ferric hydroxide precipitation.The host groundwater chemistry in calcite-poor Cretaceous aquifers of the NYC area consists of low alkalinity and moderate to low pH. The dissolution of goethite in scenarios with unbuffered injectate indicates that corrosion of the well could occur if the injectate is not buffered. Simulations with buffered injectate resulted in greater precipitation of goethite, and lower concentrations of dissolved Fe, in the extracted water. Dissolved Fe concentrations in extracted water were highest in simulations of aquifers (1) in which pyrite and siderite in the aquifer were in equilibrium, and (2) in coastal areas affected by saltwater intrusion, where high dissolved-cation concentrations provide a greater exchange of Fe2+ (FeX2). Results indicate that ASR in pyrite-beari

Applied Geochemistry↗

USGS2011

The role of critical zone processes in the evolution of the Prairie Pothole Region wetlands

The Prairie Pothole Region, which occupies 900,000 km2 of the north central USA and south central Canada, is one of the most important ecosystems in North America. It is characterized by millions of small wetlands whose chemistry is highly variable over short distances. The study involved the geochemistry of surface sediments, wetland water, and groundwater in the Cottonwood Lakes area of North Dakota, USA, whose 92 ha includes the dominant wetland hydrologic settings. The data show that oxygenated groundwater interacting with pyrite resident in a component of surficial glacial till derived from the marine Pierre Shale Formation has, over long periods of time, focused SO 4 2 - -bearing fluids from upland areas to topographically low areas. In these low areas, SO 4 2 - -enriched groundwater and wetlands have evolved, as has the CaSO4 mineral gypsum. Sulfur isotope data support the conclusion that isotopically light pyrite from marine shale is the source of SO 4 2 - . Literature data on wetland water composition suggests that this process has taken place over a large area in North Dakota.

North Dakota↗

USGS2011

Carbon and nitrogen biogeochemistry of a Prairie Pothole Wetland, Stutsman County, North Dakota, USA

The concentration and form of dissolved organic C (DOC) and N species (NH 4 + and NO 3 - ) were investigated as part of a larger hydrogeochemical study of the Cottonwood Lake Study Area within the Prairie Potholes region. Groundwater, pore water and surface wetland water data were used to help characterize the relationships between surface and groundwater with respect to nutrient dynamics. Photosynthesis and subsequent decomposition of vegetation in these hydrologically dynamic wetlands generates a large amount of dissolved C and N, although the subsurface till, derived in part from organic matter rich Pierre Shale, is a likely secondary source of nutrients in deeper groundwater. While surface water DOC concentrations ranged from 2.2 to 4.6 mM, groundwater values were 0.15 mM to 3.7 mM. Greater specific UV absorbance (SUVA 254 ) in the wetland water column and in soil pore waters relative to groundwater indicate more reactive DOC in the surface to near-surface waters. Circumneutral wetlands had greater SUVA254, possibly because of variations in vegetation communities. The dominant inorganic nitrogen species was NH 4 + in both wetland water and most ground water samples. The exceptions were 3 wells with NO 3 - ranging from 38 to 115 μM. Shallow groundwater wells (Well 28 and Well 13S) with greater connection to wetland surface water had greater NH 4 + concentrations (1.1 mM and 120 μM) than other well samples (3–90 μM). Pore water nutrient chemistry was more similar to surface water than ground water. Nitrogen results suggest reducing conditions in both groundwater and surface water, possibly due to the microbial uptake of O 2 by decaying vegetation in the wetland water column, labile organic C available in shallow groundwater, or the oxidation of pyrite associated with the subsurface.

North Dakota↗

USGS2011

Effects of slow recovery rates on water column geochemistry in aquitard wells

Monitoring wells are often installed in aquitards to verify effectiveness for preventing migration of surface contaminants to underlying aquifers. However, water sampling of aquitard wells presents a challenge due to the slow recovery times for water recharging the wells, which can take as long as weeks, months or years to recharge depending on the sample volume needed. In this study, downhole profiling and sampling of aquitard wells was used to assess geochemical changes that occur in aquitard wells during water level recovery. Wells were sampled on three occasions spanning 11 years, 1 year and 1 week after they were purged and casing water showed substantial water chemistry variations. Temperature decreased with depth, whereas pH and specific conductance increased with depth in the water column after 11 years of water level recovery. Less stable parameters such as dissolved O 2 (DO) and Eh showed strong zonation in the well column, with DO stratification occurring as the groundwater slowly entered the well. Oxidation of reduced till groundwater along with degassing of CO 2 from till pore water affects mineral solubility and dissolved solid concentrations. Recommendations for sampling slowly recovering aquitard wells include identifying the zone of DO and Eh stratification in the well column and collecting water samples from below the boundary to better measure unstable geochemical parameters.

Iowa↗

USGS2011

Hydrogeochemical processes governing the origin, transport and fate of major and trace elements from mine wastes and mineralized rock to surface waters

The formation of acid mine drainage from metals extraction or natural acid rock drainage and its mixing with surface waters is a complex process that depends on petrology and mineralogy, structural geology, geomorphology, surface-water hydrology, hydrogeology, climatology, microbiology, chemistry, and mining and mineral processing history. The concentrations of metals, metalloids, acidity, alkalinity, Cl - , F - and SO 4 2- found in receiving streams, rivers, and lakes are affected by all of these factors and their interactions. Remediation of mine sites is an engineering concern but to design a remediation plan without understanding the hydrogeochemical processes of contaminant mobilization can lead to ineffective and excessively costly remediation. Furthermore, remediation needs a goal commensurate with natural background conditions rather than water-quality standards that might bear little relation to conditions of a highly mineralized terrain. This paper reviews hydrogeochemical generalizations, primarily from US Geological Survey research, that enhance our understanding of the origin, transport, and fate of contaminants released from mined and mineralized areas. Mobility of potential or actual contaminants from mining and mineral processing activities depends on (1) occurrence : is the mineral source of the contaminant actually present? (2) abundance : is the mineral present in sufficient quantity to make a difference? (3) reactivity : what are the energetics, rates, and mechanisms of sorption and mineral dissolution and precipitation relative to the flow rate of the water? and (4) hydrology : what are the main flow paths for contaminated water? Estimates of relative proportions of minerals dissolved and precipitated can be made with mass-balance calculations if minerals and water compositions along a flow path are known. Combined with discharge, these mass-balance estimates quantify the actual weathering rate of pyrite mineralization in the environment and compare reasonably well with laboratory rates of pyrite oxidation except when large quantities of soluble salts and evaporated mine waters have accumulated underground. Quantitative mineralogy with trace-element compositions can substantially improve the identification of source minerals for specific trace elements through mass balances. Post-dissolution sorption and precipitation (attenuation) reactions depend on the chemical behavior of each element, solution composition and pH, aqueous speciation, temperature, and contact-time with mineral surfaces. For example, little metal attenuation occurs in waters of low pH (<3.5) and metals tend to maintain element ratios indicative of the main mineral or group of minerals from which they dissolved, except Fe, SiO 2 , and redox-sensitive oxyanions (As, Sb, Se, Mo, Cr, V). Once dissolved, metal and metalloid concentrations are strongly affected by redox conditions and pH. Iron is the most reactive because it is rapidly oxidized by bacteria and archaea and Fe(III) hydrolyzes and precipitates at low pH (1–3) which is related directly to its first hydrolysis constant, pK 1 = 2.2. Several insoluble sulfate minerals precipitate at low pH including anglesite, barite, jarosite, alunite and basaluminite. Aluminum hydrolyzes near pH 5 (pK 1 = 5.0) and provides buffering and removal of Al by mineral precipitation from pH 4–5.5. Dissolved sulfate behaves conservatively because the amount removed from solution by precipitation is usually too small relative to the high concentrations in the water column and relative to the flow rate of the water.

Applied Geochemistry↗

USGS2012

Surface heat flow and CO2 emissions within the Ohaaki hydrothermal field, Taupo Volcanic Zone, New Zealand

Carbon dioxide emissions and heat flow have been determined from the Ohaaki hydrothermal field, Taupo Volcanic Zone (TVZ), New Zealand following 20 a of production (116 MW e ). Soil CO 2 degassing was quantified with 2663 CO 2 flux measurements using the accumulation chamber method, and 2563 soil temperatures were measured and converted to equivalent heat flow (W m −2 ) using published soil temperature heat flow functions. Both CO 2 flux and heat flow were analysed statistically and then modelled using 500 sequential Gaussian simulations. Forty subsoil CO 2 gas samples were also analysed for stable C isotopes. Following 20 a of production, current CO 2 emissions equated to 111 ± 6.7 T/d. Observed heat flow was 70 ± 6.4 MW, compared with a pre-production value of 122 MW. This 52 MW reduction in surface heat flow is due to production-induced drying up of all alkali–Cl outflows (61.5 MW) and steam-heated pools (8.6 MW) within the Ohaaki West thermal area (OHW). The drying up of all alkali–Cl outflows at Ohaaki means that the soil zone is now the major natural pathway of heat release from the high-temperature reservoir. On the other hand, a net gain in thermal ground heat flow of 18 MW (from 25 MW to 43.3 ± 5 MW) at OHW is associated with permeability increases resulting from surface unit fracturing by production-induced ground subsidence. The Ohaaki East (OHE) thermal area showed no change in distribution of shallow and deep soil temperature contours despite 20 a of production, with an observed heat flow of 26.7 ± 3 MW and a CO 2 emission rate of 39 ± 3 T/d. The negligible change in the thermal status of the OHE thermal area is attributed to the low permeability of the reservoir beneath this area, which has limited production (mass extraction) and sheltered the area from the pressure decline within the main reservoir. Chemistry suggests that although alkali–Cl outflows once contributed significantly to the natural surface heat flow (∼50%) they contributed little (<1%) to pre-production CO 2 emissions due to the loss of >99% of the original CO 2 content due to depressurisation and boiling as the fluids ascended to the surface. Consequently, the soil has persisted as the major (99%) pathway of CO 2 release to the atmosphere from the high temperature reservoir at Ohaaki. The CO 2 flux and heat flow surveys indicate that despite 20 a of production the variability in location, spatial extent and magnitude of CO 2 flux remains consistent with established geochemical and geophysical models of the Ohaaki Field. At both OHW and OHE carbon isotopic analyses of soil gas indicate a two-stage fractionation process for moderate-flux (>60 g m −2 d −1 ) sites; boiling during fluid ascent within the underlying reservoir and isotopic enrichment as CO 2 diffuses through porous media of the soil zone. For high-flux sites (>300 g m −2 d −1 ), the δ 13 CO 2 signature (−7.4 ± 0.3‰ OHW and −6.5 ± 0.6‰ OHE) is unaffected by near-surface (soil zone) fractionation processes and reflects the composition of the boiled magmatic CO 2 source for each respective upflow. Flux thresholds of <30 g m −2 d −1 for purely diffusive gas transport, between 30 and 300 g m −2 d −1 for combined diffusive–advective transport, and ⩾300 g m −2 d −1 for purely advective gas transport at Ohaaki were assigned. δ 13 CO 2 values and cumulative probability plots of CO 2 flux data both identified a threshold of ∼15 g m −2 d −1 by which background (atmospheric and soil respired) CO 2 may be differentiated from hydrothermal CO 2 .

Taupo Volcanic Zone↗

USGS2013

Hydrothermal contamination of public supply wells in Napa and Sonoma Valleys, California

Groundwater chemistry and isotope data from 44 public supply wells in the Napa and Sonoma Valleys, California were determined to investigate mixing of relatively shallow groundwater with deeper hydrothermal fluids. Multivariate analyses including Cluster Analyses, Multidimensional Scaling (MDS), Principal Components Analyses (PCA), Analysis of Similarities (ANOSIM), and Similarity Percentage Analyses (SIMPER) were used to elucidate constituent distribution patterns, determine which constituents are significantly associated with these hydrothermal systems, and investigate hydrothermal contamination of local groundwater used for drinking water. Multivariate statistical analyses were essential to this study because traditional methods, such as mixing tests involving single species (e.g. Cl or SiO 2 ) were incapable of quantifying component proportions due to mixing of multiple water types. Based on these analyses, water samples collected from the wells were broadly classified as fresh groundwater, saline waters, hydrothermal fluids, or mixed hydrothermal fluids/meteoric water wells. The Multivariate Mixing and Mass-balance (M3) model was applied in order to determine the proportion of hydrothermal fluids, saline water, and fresh groundwater in each sample. Major ions, isotopes, and physical parameters of the waters were used to characterize the hydrothermal fluids as Na–Cl type, with significant enrichment in the trace elements As, B, F and Li. Five of the wells from this study were classified as hydrothermal, 28 as fresh groundwater, two as saline water, and nine as mixed hydrothermal fluids/meteoric water wells. The M3 mixing-model results indicated that the nine mixed wells contained between 14% and 30% hydrothermal fluids. Further, the chemical analyses show that several of these mixed-water wells have concentrations of As, F and B that exceed drinking-water standards or notification levels due to contamination by hydrothermal fluids.

California↗