Inorganic geochemistry and redox dynamics in bank filtration settings

Bank filtration induces flow of surface water through a hydraulically connected aquifer by excess pumping from a production well in the aquifer. This review presents the four main geochemical processes relevant for inorganic geochemistry, with a focus on iron (Fe) and manganese (Mn), during bank filtration: reduction near the bank, oxidation near the production well, carbonate dissolution, and sorption to aquifer materials. Physical and transport processes affect these geochemical processes and influence the redox state of the infiltrate. The presence of Fe and Mn in bank infiltrate is directly related to its redox status and can necessitate drinking water treatment after extraction. Long-term, in situ sequestration of Fe and Mn requires precipitation of oxide or carbonate solids, since a sorption front can breakthrough at the production well.     

Manganese oxidation induced by water table fluctuations in a sand column

On-off cycles of production wells, especially in bank filtration settings, cause oscillations in the local water table, which can deliver significant amounts of dissolved oxygen (DO) to the shallow groundwater. The potential for DO introduced in this manner to oxidize manganese(II) (Mn(II)), mediated by the obligate aerobe Pseudomonas putida GB-1, was tested in a column of quartz sand fed with anoxic influent solution and subject to 1.3 m water table changes every 30-50 h. After a period of filter ripening, 100 μM Mn was rapidly removed during periods of low water table and high dissolved oxygen concentrations. The accumulation of Mn in the column was confirmed by XRF analysis of the sand at the conclusion of the study, and both measured net oxidation rates and XAS analysis suggest microbial oxidation as the dominant process. The addition of Zn, which inhibited GB-1 Mn oxidation but not its growth, interrupted the Mn removal process, but Mn oxidation recovered within one water table fluctuation. Thus transient DO conditions could support microbially mediated Mn oxidation, and this process could be more relevant in shallow groundwater than previously thought.     

Assessing the impact of animal waste lagoon seepage on the geochemistry of an underlying shallow aquifer

Evidence of seepage from animal waste holding lagoons at a dairy facility in the San Joaquin Valley of California is assessed in the context of a process geochemical model that addresses reactions associated with the formation of the lagoon water as well as reactions occurring upon the mixture of lagoon water with underlying aquifer material. Comparison of model results with observed concentrations of NH4+, K+, PO4(3-), dissolved inorganic carbon, pH, Ca2+, Mg2+, SO4(2-), Cl-, and dissolved Ar in lagoon water samples and groundwater samples suggests three key geochemical processes: (i) off-gassing of significant quantities of CO2 and CH4 during mineralization of manure in the lagoon water, (ii) ion exchange reactions that remove K+ and NH4+ from seepage water as it migrates into the underlying anaerobic aquifer material, and (iii) mineral precipitation reactions involving phosphate and carbonate minerals in the lagoon water in response to an increase in pH as well as in the underlying aquifer from elevated Ca2+ and Mg2+ levels generated by ion exchange. Substantial off-gassing from the lagoons is further indicated by dissolved argon concentrations in lagoon water samples that are below atmospheric equilibrium. As such, Ar may serve as a unique tracer for lagoon water seepage since under-saturated Ar concentrations in groundwater are unlikely to be influenced by any processes other than mechanical mixing.     

Trace metal levels in uncontaminated groundwater of a coastal watershed: importance of colloidal forms

Groundwater and surface water were collected using trace metal clean techniques from the upper glacial aquifer of West Neck Bay (Shelter Island) in eastern Long Island, NY, during the late spring and summer of 1999. The collection sites on Shelter Island are located in an area that is primarily residential and believed to have uncontaminated groundwater. Ultrafiltration was used to size-fractionate the dissolved (<0.45 microm) fraction into colloidal (1 kDa - 0.45 microm) and low molecular weight (<1 kDa) size pools. These fractions were analyzed for trace metals (Al, Ag, Cd, Cu, Mn, Pb, and Zn), organic carbon, and inorganic nutrients (NH4, NO3, PO4). The levels of metals and organic carbon in the groundwater were as low as those found in the open ocean, far removed from anthropogenic inputs. These findings corroborate the need to apply trace metal clean techniques in the determination of metal levels in uncontaminated groundwater. A significant fraction of dissolved metals (22-96%) and organic carbon (approximately 40%) in the groundwater and in surface waters of the Bay was found to be associated with colloids. The significance of the metal association with the colloidal fraction decreased in the order of Al > Cu > Ag > Zn = Cd = Mn and appeared to be dependent on the affinities of these metals for humic substances. In contrast, NO3 and NH4 were found to be almost entirely (approximately 98-99%) in the low molecular weight size fraction. Metal/aluminum and metal/carbon ratios measured in the colloids were similar to those reported for humic substances and significantly different from those of soils. This suggests that colloidal particles might originate from humic materials as opposed to purely inorganic minerals. These results indicate the need to consider the colloidal fraction in the fate and mobility of metals in groundwater and that, despite the low levels of organic matter (<50 microM of DOC) measured in groundwater, some groundwater colloids appear to be organic in nature.     

Evaluating behavior of oxygen,nitrate, and sulfate during recharge and quantifying reduction rates in a contaminated aquifer

This study evaluates the biogeochemical changes that occur when recharge water comes in contact with a reduced aquifer. It specifically addresses (1) which reactions occur in situ, (2) the order in which these reactions will occur if terminal electron acceptors (TEAs) are introduced simultaneously, (3) the rates of these reactions, and (4) the roles of the aqueous and solid-phase portions of the aquifer. Recharge events of waters containing various combinations of O2, NO3, and SO4 were simulated at a shallow sandy aquifer contaminated with waste fuels and chlorinated solvents using modified push-pull tests to quantify rates. In situ rate constants for aerobic respiration (14.4 day(-1)), denitrification (5.04-7.44 day(-1)), and sulfate reduction (4.32-6.48 day(-1)) were estimated. Results show that when introduced together, NO3 and SO4 can be consumed simultaneously at similar rates. To distinguish the role of aqueous phase from that of the solid phase of the aquifer, groundwater was extracted, amended with NO3 and SO4, and monitored overtime. Results indicate that neither NO3 nor SO4 was reduced during the course of the aqueous-phase study, suggesting that NO3 and SO4 can behave conservatively in highly reduced water. It is clear that sediments and their associated microbial communities are important in driving redox reactions.     

Influence of net groundwater discharge on the chemical composition of a coastal environment: Flanders Bay, Long Island, New York

Seasonal (October 1997 and May 1998) concentrations of dissolved (< 0.45 micron) trace metals (Ag, Al, Cd, Cu, Fe, Mn), inorganic nutrients (NO3, PO4, Si), DOC and DON were measured at seven wells during periods of low and high groundwater flow, in the aquifer around Flanders Bay in Eastern Long Island, New York. Similar measurements were made in surface waters of Flanders Bay, a shallow coastal embayment with restricted water flushing and river input. Dissolved constituents in the groundwater were classified according to their behavior under different flow conditions as follows: (1) peak during high flow (DOC, pH, Si, NO3, Al and Cu); (2) peak during low flow (salinity, DON, Ag, Cd, Mn); and (3) concentrations independent of flow conditions (PO4 and Fe). The primarily urban and agricultural land use on the North Fork of Long Island was reflected in higher concentrations of nutrients, Cu and Cd in groundwater, compared to samples from the South Fork which is mostly open parkland. Principal component analysis indicated that groundwater seepage could influence the chemical composition of Flanders Bay with respect to the major geochemical carriers (e.g. Fe and Mn). However, mass balance estimates for Cu indicated that, during low flow conditions, net groundwater Cu input was about 10% of the total input. In contrast, during high flow, net groundwater flow could account for up to 58% of all Cu inputs. Nevertheless, a large imbalance, which accounted for up to 70% of the outflux during low aquifer recharge, suggested that the Cu budget of the Bay was not adequately described by the inputs considered (river, net groundwater flow, atmospheric deposition, and tidal exchange). Important missing components of the Cu mass balance in Flanders Bay may include groundwater circulation driven by tides and waves as well as diffusive benthic fluxes.     

Sedimentological control on Mn, and other trace elements, in groundwater of the Bengal delta

To reveal what controls the concentration and distribution of possibly hazardous (Mn, U, Se, Cd, Bi, Pb) and nonhazardous (Fe, V, Mo, PO(4)) trace elements in groundwater of the Bengal delta, we mapped their concentrations in shallow groundwater (<60 mbgl) across 102 km(2) of West Bengal. Only Mn is a potential threat to health, with 55% of well water exceeding 0.3 mg/L, the current Indian limit for drinking water in the absence of an alternate source, and 75% exceeding the desirable limit of 0.1 mg/L. Concentrations of V are <3 μg/L. Concentrations of U, Se, Pb, Ni, Bi, and Cd, are below WHO guideline values. The distributions of Fe, Mn, As, V, Mo, U, PO(4), and δ(18)O in groundwater reflect subsurface sedimentology and sources of water. Areas of less negative δ(18)O reveal recharge by sources of evaporated water. Concentrations of Fe, As, Mo, and PO(4) are high in palaeo-channel groundwaters and low in palaeo-interfluvial groundwaters. Concentrations of U, V, and Mn, are low in palaeo-channel groundwaters and high in palaeo-interfluvial groundwaters. Concentrations of Fe and Mn are highest (18 and 6 mg/L respectively) at dual reduction-fronts that form strip interfaces at depth around the edges of palaeo-interfluvial aquifers. The fronts form as focused recharge carries dissolved organic carbon into the aquifer margins, which comprise brown, iron-oxide bearing, sand. At the Mn-reduction front, concentrations of V and Mo reach peak concentrations of 3 μg/L. At the Fe-reduction front, concentrations of PO(4) and As reach concentrations 3 mg/L and 150 μg/L respectively. Many groundwaters contain >10 mg/L of Cl, showing that they are contaminated by Cl of anthropogenic origin and that organic matter from in situ sanitation may contribute to driving reduction.     

N2O emissions from streams in the Neuse river watershed, North Carolina

We present N2O emission data from 11 sites in the Neuse River watershed. Emissions were measured using a static surface enclosure technique deployed on eight sites on the main river channel and three tributary sites. Ancillary data collected included dissolved oxygen, nitrate, total nitrogen, ammonium, dissolved organic carbon, total phosphorus, and temperature. Analysis using standard linear models, and classification and regression trees (CART), indicated nitrate to be the primary driving variable associated with N2O emission, although dissolved organic carbon concentration and water temperature were positively related with N2O emission as well. Relationships between nitrate concentration and N2O emission were consistent with those found in previous studies, although the data presented here represent the lower end of the range for both variables among published studies. Using our measured N2O emission rates along with literature values for the ratio of nitrogen gas to N2O produced during denitrification, we estimate N loss via denitrification in the Neuse River is approximately 17% of the annual N load delivered to the estuary.     

Has submarine groundwater discharge been overlooked as a source of mercury to coastal waters?

We measured the mercury (Hg) in groundwater, aquifer sediments, and surface water in Waquoit Bay (Massachusetts) and found that this toxic metal (range: <3.2-262 pM) was being released within the subterranean estuary, with similarly high levels (range: 18-256 pM) found in the surface waters of the bay. None of the dissolved species (DOC, chloride, and Fe) normally observed to influence Hg partitioning correlated well with the observed Hg concentrations. It was hypothesized that this was in part due to the variable loading in time and space of Hg onto the aquifer sands in combination with the seasonality of groundwater flow through the aquifer. Aquifer sediment samples from the study site ranged from <1 to 12.5 pmol of Hg/g of sediment, suggesting log Kd values on the order of 1. We hypothesize that this was due to the low organic carbon content typical of the aquifer sediments. Last, itwas estimated that submarine groundwater discharge supplied 0.47-1.9 nmol of Hg m(-2) day(-1) to the bay, which is an order of magnitude higher than the atmospheric deposition rate for the northeastern U.S.     

GeoChip-based analysis of microbial functional gene diversity in a landfill leachate-contaminated aquifer

The functional gene diversity and structure of microbial communities in a shallow landfill leachate-contaminated aquifer were assessed using a comprehensive functional gene array (GeoChip 3.0). Water samples were obtained from eight wells at the same aquifer depth immediately below a municipal landfill or along the predominant downgradient groundwater flowpath. Functional gene richness and diversity immediately below the landfill and the closest well were considerably lower than those in downgradient wells. Mantel tests and canonical correspondence analysis (CCA) suggested that various geochemical parameters had a significant impact on the subsurface microbial community structure. That is, leachate from the unlined landfill impacted the diversity, composition, structure, and functional potential of groundwater microbial communities as a function of groundwater pH, and concentrations of sulfate, ammonia, and dissolved organic carbon (DOC). Historical geochemical records indicate that all sampled wells chronically received leachate, and the increase in microbial diversity as a function of distance from the landfill is consistent with mitigation of the impact of leachate on the groundwater system by natural attenuation mechanisms.     

Distribution and reactivity of O2-reducing components in sediments from a layered aquifer

The redox status of subsurface aqueous systems is controlled by the reactivity of solid redox-sensitive species and by the inflow of such species dissolved in groundwater. The reactivity toward molecular oxygen (O2) of solid reductants present in three particle size fractions of sediments from a pristine aquifer was characterized during 54 days. The stoichiometric relationships between carbon dioxide (CO2) production and O2 consumption was used in combination with sulfate production to discriminate between the contributions of sedimentary organic matter (0-87%), pyrite (6-100%), and siderite (0-43%) as the dominant reductants. The observed simultaneous oxidation of these reductants indicates that they are reactive on the same time scales. The measured reduction capacity 18-84 micromol O2/g) ranged from 8 to 42% of the total reduction capacity present as pyrite and organic carbon in the total sediment fraction (<2 mm). Fine fractions (<63 microm) were 10-250 times more reactive than their corresponding total fractions. Oxygen consumption rates decreased continuously during carbonate buffered conditions, due to a decreasing reactivity of reductants. Acidification accelerated pyrite oxidation but impeded SOM respiration. Our findings indicate that the geological history of aquifer sediments affects the amounts of organic matter, pyrite and siderite present, while environmental conditions, such as pH and microbial activity, are important in controlling the reactivity of these reductants. These controls should be considered when assessing the natural reduction activity of aquifer sediments in either natural or polluted systems.     

Redox processes and release of organic matter after thermal treatment of a TCE-contaminated aquifer

Redox conditions in heated and unheated microcosm experiments were studied to evaluate the effect of thermal remediation treatment on biogeochemical processes in subsurface environments. The results were compared to field-scale observations from thermal treatments of contaminated sites. Trichloroethene-contaminated aquifer material and groundwater from Ft. Lewis, WA were incubated for 200 days at ambient temperature (i.e., 10 degrees C) or heated to 100 degrees C for 10 days and cooled slowly over a period of 150 days to mimic a thermal treatment. Increases of up to 14 mM dissolved organic carbon were observed in the aqueous phase after heating. Redox conditions did generally not change during heating in the laboratory experiment, and only minor changes occurred as an effect of heat treatment in the field. The conditions were slightly manganese/iron-reducing in two sediments and possibly sulfate-reducing in the third sediment based on production of up to 0.20 mM dissolved iron and 0.15 mM dissolved manganese and consumption of 0.08 mM sulfate. The calculated energy gain of less than -20 kJ/mol H2 for iron and sulfate reduction as well as methane production indicated that these processes were thermodynamically favorable. Sulfate reduction and methane production occurred in the unheated microcosms upon lactate amendment. Little or no reduction of the redox level was identified in heated lactate-amended microcosms, possibly because of limited microbial activity. Because the redox conditions, pH, and alkalinity remained within normal aquifer levels upon heating, bioaugmentation may be feasible for stimulating anaerobic dechlorination in heated samples or in future field applications.     

241Am migration in a sandy aquifer studied by long-term column experiments

The migration behavior of 241Am(III) in a sandy aquifer was studied under near-natural conditions by long-term column experiments of more than 1 year duration. Columns with 50 cm length and 5 cm in diameter were packed with aeolian quartz sand and equilibrated with two different groundwaters having an original dissolved organic carbon concentration (DOC) of 1.1 and 7.2 mg x dm(-3), respectively,from the Gorleben site (Lower Saxony, Germany). In each experiment, 1 cm3 of Am-spiked groundwater ([Am] = 0.2 to 2 micromol x dm(-3)) was injected into the column. The flow rate of the groundwater was adjusted to 0.28 m x d(-1). A small colloid-borne Am fraction was found to elute together with tritiated water. After 414 and 559 days, respectively, the experiments were terminated. Whereas the nonsorbing tracer of tritiated water would have covered a distance of about 350 m in that time period, the maximum of the Am activity was detected between 32 and 40 mm column length. Applying selective dissolution analysis to the sand surface, Am was found to be preferentially bound to iron hydroxide/oxide sites. From this Am distribution, a retardation factor R of about 10(4) was determined and compared to static batch experiments. The Am breakthrough was calculated forthe conditions of the column experiment     

Vulnerability of recently recharged groundwater in principal [corrected] aquifers of the United States to nitrate contamination

Recently recharged water (defined here as <60 years old) is generally the most vulnerable part of a groundwater resource to nonpoint-source nitrate contamination. Understanding at the appropriate scale the interactions of natural and anthropogenic controlling factors that influence nitrate occurrence in recently recharged groundwater is critical to support best management and policy decisions that are often made at the aquifer to subaquifer scale. New logistic regression models were developed using data from the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program and National Water Information System for 17 principal aquifers of the U.S. to identify important source, transport, and attenuation factors that control nonpoint source nitrate concentrations greater than relative background levels in recently recharged groundwater and were used to predict the probability of detecting elevated nitrate in areas beyond the sampling network. Results indicate that dissolved oxygen, crops and irrigated cropland, fertilizer application, seasonally high water table, and soil properties that affect infiltration and denitrification are among the most important factors in predicting elevated nitrate concentrations. Important differences in controlling factors and spatial predictions were identified in the principal aquifer and national-scale models and support the conclusion that similar spatial scales are needed between informed groundwater management and model development.     

Long-term natural attenuation of carbon and nitrogen within a groundwater plume after removal of the treated wastewater source

Disposal of treated wastewater for more than 60 years onto infiltration beds on Cape Cod, Massachusetts produced a groundwater contaminant plume greater than 6 km long in a surficial sand and gravel aquifer. In December 1995 the wastewater disposal ceased. A long-term, continuous study was conducted to characterize the post-cessation attenuation of the plume from the source to 0.6 km downgradient. Concentrations and total pools of mobile constituents, such as boron and nitrate, steadily decreased within 1-4 years along the transect. Dissolved organic carbon loads also decreased, but to a lesser extent, particularly downgradient of the infiltration beds. After 4 years, concentrations and pools of carbon and nitrogen in groundwater were relatively constant with time and distance, but substantially elevated above background. The contaminant plume core remained anoxic for the entire 10-year study period; temporal patterns of integrated oxygen deficit decreased slowly at all sites. In 2004, substantial amounts of total dissolved carbon (7 mol C m(-2)) and fixed (dissolved plus sorbed) inorganic nitrogen (0.5 mol N m(-2)) were still present in a 28-m vertical interval at the disposal site. Sorbed constituents have contributed substantially to the dissolved carbon and nitrogen pools and are responsible for the long-term persistence of the contaminant plume. Natural aquifer restoration at the discharge location will take at least several decades, even though groundwater flow rates and the potential for contaminant flushing are relatively high.     

Groundwater or floodwater? Assessing the pathways of metal exports from a coastal acid sulfate soil catchment

Daily observations of dissolved aluminum, iron, and manganese in an estuary downstream of a coastal acid sulfate soil (CASS) catchment provided insights into how floods and submarine groundwater discharge drive wetland metal exports. Extremely high Al, Fe, and Mn concentrations (up to 40, 374, and 8 mg L(-1), respectively) were found in shallow acidic groundwaters from the Tuckean Swamp, Australia. Significant correlations between radon (a natural groundwater tracer) and metals in surface waters revealed that metal loads were driven primarily by groundwater discharge. Dissolved Fe, Mn, and Al loads during a 16-day flood triggered by a 213 mm rain event were respectively 80, 35, and 14% of the total surface water exports during the four months of observations. Counter clockwise hysteresis was observed for Fe and Mn in surface waters during the flood due to delayed groundwater inputs. Groundwater-derived Fe fluxes into artificial drains were 1 order of magnitude higher than total surface water exports, which is consistent with the known accumulation of monosulfidic black ooze within the wetland drains. Upscaling the Tuckean catchment export estimates yielded dissolved Fe fluxes from global acid sulfate soil catchments on the same order of magnitude of global river inputs into estuaries.     

Geochemical reactions resulting from in situ oxidation of PCE-DNAPL by KMnO4 in a sandy aquifer

Although the potential for KMnO4 to destroy chlorinated ethenes in situ was first recognized more than a decade ago, the geochemical processes that accompany the oxidation have not previously been examined. In this study, aqueous KMnO4 solutions (10-30 g/L) were injected into an unconfined sand aquifer contaminated by the dense non-aqueous-phase liquid (DNAPL) tetrachloroethylene (PCE). The effects of the injections were monitored using depth-specific, multilevel groundwater samplers, and continuous cores. Two distinct geochemical zones evolved within several days after injection. In one zone where DNAPL is present, reactions between KMnO4 and dissolved PCE resulted in the release of abundant chloride and hydrogen ions to the water. Calcite and dolomite dissolved, buffering the pH in the range of 5.8-6.5, releasing Ca, Mg, and CO2 to the pore water. In this zone, the aqueous Ca/Cl concentration ratio is close to 5:12, consistent with the following reaction for the oxidation of PCE in a carbonate-rich aquifer: 3C2Cl4 + 5CaCO3(s) + 4KMnO4 + 2H+ --> 11CO2 + 4MnO2(s) + H2O + 12Cl- + 5Ca2+ + 4K+. In addition to Mg from dolomite dissolution, increases in the concentration of Mg as well as Na may result from exchange with K at cation-exchange sites. In the second zone, where lesser amounts of PCE were present, KMnO4 persisted in the aquifer for more than 14 months, and the porewater pH increased graduallyto between 9 and 10 as a resultof reaction between KMnO4 and H2O. A small increase in SO4 concentrations in the zones invaded by KMnO4 suggests that KMnO4 injections caused oxidation of sulfide minerals. There are important benefits of carbonate mineral buffering during DNAPL remediation by in situ oxidation. In a carbonate-buffered system, Mn(VII) is reduced to Mn(IV) and is immobilized in the groundwater by precipitating as insoluble manganese oxide. Energy-dispersive X-ray spectroscopy analyses of the manganese oxide coatings on aquifer mineral grains have detected the impurities Al, Ca, Cl, Cu, Pb, P, K, Si, S, Ti, U, and Zn indicating that, similar to natural systems, precipitation of manganese oxide is accompanied by coprecipitation of other elements. In addition, the consumption of excess KMnO4 by reaction with reduced minerals such as magnetite will be minimized because the rates of these reactions increase with decreasing pH. Aquifer cores collected after the KMnO4 injections exhibit dark brown to black bands of manganese oxide reaction products in sand layers where DNAPL was originally present. Mineralogical investigations indicate that the manganese oxide coatings are uniformly distributed over the mineral grains. Observations of the coatings using transmission electron microscopy indicate that they are on the order of 1 microm thick, and consequently, the decrease in porosity through the formation of the coatings is negligible.     

Naturally occurring contamination in the Mancos Shale

Some uranium mill tailings disposal cells were constructed on dark-gray shale of the Upper Cretaceous Mancos Shale. Shale of this formation contains contaminants similar to those in mill tailings. To establish the contributions derived from the Mancos, we sampled 51 locations in Colorado, New Mexico, and Utah. Many of the groundwater samples were saline with nitrate, selenium, and uranium concentrations commonly exceeding 250,?000, 1000, and 200 μg/L, respectively. Higher concentrations were limited to groundwater associated with shale beds, but were not correlated with geographic area, stratigraphic position, or source of water. The elevated concentrations suggest that naturally occurring contamination should be considered when evaluating groundwater cleanup levels. At several locations, seep water was yellow or red, caused in part by dissolved organic carbon concentrations up to 280 mg/L. Most seeps had (234)U to (238)U activity ratios greater than 2, indicating preferential leaching of (234)U. Seeps were slightly enriched in (18)O relative to the meteoric water line, indicating limited evaporation. Conceptually, major ion chemical reactions are dominated by calcite dissolution following proton release from pyrite oxidation and subsequent exchange by calcium for sodium residing on clay mineral exchange sites. Contaminants are likely released from organic matter and mineral surfaces during weathering.     

Dissolution of an emplaced source of DNAPL in a natural aquifer setting

Field-scale dissolution of a multicomponent DNAPL (dense nonaqueous-phase liquid) source intentionally emplaced below the water table is evaluated in a well-characterized natural aquifer setting. The block-shaped source contained 23 kg of a trichloromethane, trichloroethene, and perchloroethene mixture homogeneously distributed at 5% saturation of pore space. Dissolution was monitored for 3 yr via down-gradient samplers (1-m fence) and occasional intra-source sampling. Although intra-source equilibrium dissolution was shown and endorsed by supporting modeling and literature lab data, less than equilibrium concentrations were predominantly monitored in the 1-m fence. This was ascribed to significant by-passing of the source by groundwater flow due to its low permeability relative to the aquifer and associated dilution of concentrations emitted from the source. Heterogeneous source dissolution occurred despite the relative homogeneity of the source and aquifer and was ascribed to dissolution fingering, which has not been previously field-demonstrated. Bulk bypass of groundwater flow around the source zone caused slow dissolution rates, with 77% of the source remaining after 3 yr and a projected longevity of approximately 25 yr. Observed dissolution fingering would have significantly increased longevity as it increasingly caused intra-source bypass of remaining DNAPL. Our dissolution interpretations were endorsed by additional data collected after 6 yr during source remediation via permanganate oxidation.     

Effects of increasing acidity on metal(loid) bioprecipitation in groundwater: column studies

Large-scale column experiments were carried out over a period of 545 days to assess the effect of increasing acidity on bacterial denitrification, sulfate reduction, and metal(loid) bioprecipitation in groundwater affected by acid mine drainage. At a groundwater pH of 5.5, denitrification and Cu2+ removal, probably via malachite (Cu2(OH)2CO3) precipitation, were observed in the ethanol-amended column. Sulfate reduction, sulfide production, and Zn2+ removal were also observed, with Zn2+ removal observed in the zone of sulfate reduction, indicating likely precipitation as sphalerite (ZnS). Se6+ removal was also observed in the sulfate reducing zone, probably as direct bioreduction to elemental selenium via ethanol/acetate oxidation or sulfide oxidation precipitating elemental sulfur. A step decrease in groundwater pH from 5.5 to 4.25 resulted in increased denitrification and sulfate reduction half-lives, migration of both these redox zones along the ethanol-amended column, and the formation of an elevated Cu2+ plume. Additionally, an elevated Zn2+ plume formed in the previous sulfate reducing zone of the ethanol-amended column, suggesting dissolution of precipitated sphalerite as a result of the reduction in groundwater pH. As Cu2+ passed through the zone of sphalerite dissolution, SEM imaging and EDS detection suggested that Cu2+ removal had occurred via chalcocite (Cu2S) or covellite (CuS) precipitation.