H14D-01 INVITED
Redox Processes and Water Quality of Selected Principal Aquifer Systems
Redox conditions in 15 Principal Aquifer systems across the United States, and their impact on several water- quality issues, were assessed from a large data base collected by the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey. The logic of these assessments was based on the observed ecological succession of electron acceptors such as dissolved oxygen, nitrate, and sulfate and threshold concentrations of these substrates needed to support active microbial metabolism. Similarly, the utilization of solid-phase electron acceptors such as Mn(IV) and Fe(III) are indicated by the production of dissolved manganese and iron. Applying this framework to a number of Principal Aquifers in the United States suggests that the occurrence and distribution of redox processes were controlled largely by climate, geology, hydrology, and anthropogenic factors that, in combination, influenced ground-water residence time and the availability of electron acceptors and donors. For the natural and anthropogenic contaminants assessed in this study, it appears that considering redox conditions as defined by the framework explained many of the observed water- quality trends at the regional scale of this analysis. An important finding of this study is that samples indicating mixed redox processes provide information on redox heterogeneity that affects the observed fate and transport of natural and anthropogenic contaminants and therefore are useful for assessing common water-quality issues.
H14D-02 INVITED
Paleohydrologic controls on methanogenesis in organic-rich saline aquifers
Freshwater recharge into the margins of sedimentary basins, during periods of continental glaciation, stimulated microbial methane generation in organic-rich shales and coal beds, by significantly diluting the ambient formation water salinity. Subglacial recharge may have also transported microorganisms and nutrients into the subsurface environment. Methane is generated by a diverse consortium of both acetoclastic and CO2-reducing methanogenic Archaea, and adsorbed onto the organic matter. These shallow methane accumulations account for ~20% of the total U.S. natural gas production. Anaerobic microbial metabolism of shales and coals is in part controlled by the volume of pore waters and fluid composition, amount of extractable organic matter and intermediary substrates, reservoir temperature, and mass transport processes that provide essential rock-derived nutrients and organic acids. Methanogens are most active in low salinity environments (<2.5 mol/L Cl) with no SO4, and at the interfaces between confining units and adjacent aquifers where diffusion dominates. Microbial degradation of organic matter generates high alkalinity concentrations, which may induce calcite precipitation in shale fractures and coal cleats, which can in turn modify the subsurface hydrology. Microbial methanogenesis also imparts a strong control on the cycling of carbon, H2, and other elements in the subsurface environment. This presentation will focus on the timing of recharge and establishment of microbial communities within the Upper Devonian black shales, Pennsylvanian coal beds, and overlying glacial drift in the Illinois Basin, and the importance of continued groundwater flow on active methane generation and accumulation. There is an approximately 65-70 per mil depletion in 13C of CH4, relative to the precursor CO2 in the Upper Devonian shales, Pennsylvanian coals, and glacial drift. In addition, there is a linear correlation between the dD values of co- produced formation waters and CH4. Isotope mass-balance modeling results confirm that these isotopic shifts can be produced by coupled acetate fermentation and CO2-reduction. The lowest d13C values for CO2 and CH4 are found in the shallow glacial drift (-14 to 8 per mil, -80 to -68 per mil, respectively), where the permeable aquifers are a relatively open system, rapidly flushed by modern recharge. In contrast, the deep Upper Devonian shales have relatively positive d13C values for CO2 and CH4 (6 to 20 per mil, -56 to -50 per mil, respectively), indicating that methane has been generated over relatively long time scales (at least since the Late Pleistocene) in a closed system. The Pennsylvanian coal beds have intermediary d13C values for CO2 and CH4 (-8 to 11 per mil, -66 to -56 per mil, respectively), and contain Holocene groundwaters. Understanding the hydrobiogeochemical processes active within fractured shales and coal beds is important for energy resources, as well as CO2 sequestration.
H14D-03 INVITED
Effects of Pumping and Well Disinfection on Arsenic Release to Well Water
In areas of northeastern Wisconsin, arsenic-bearing sulfides and iron oxides are distributed throughout a sandstone aquifer used for domestic water supplies. Aqueous arsenic concentrations exceed 10 μg/L in approximately 20% of wells in this region. These wells are often subjected to in situ chlorine disinfection to control nuisance or pathogenic bacteria. Field-based experiments investigating the effects of pumping and well disinfection showed that under non-pumping conditions, the geochemical environment in a domestic well is strongly reducing. Aqueous arsenic ranged from 10 to 18 μg/L, and the number of all tested groups of microorganisms (As(III)-reducing, Fe(III)-reducing, sulfate-reducing, As(V)-oxidizing, and aerobic microorganisms) increased 0.3 to 2.4 orders of magnitude in the well water under non-pumping conditions. The diverse populations of anaerobic and aerobic microorganisms reflect the complexity of the borehole environment. The number of Fe(III)-reducing bacteria correlates with As(III) concentrations, suggesting that microbially- facilitated reduction of iron (hydr)oxides contributes to the relatively rapid rise in aqueous arsenic observed under non-pumping conditions. Pumping the well introduces up to 1 mg/L of oxygen into the well water. The change in redox imposed by pumping decreased the number of anaerobic As(III)-reducing, Fe(III)-reducing, and sulfate-reducing microorganisms by 1 to 1.7 orders of magnitude. Aqueous arsenic also decreased during pumping (<7 μg/L), indicating that low-arsenic groundwater recharges the well. Chlorine disinfection produced strongly oxidizing conditions in the well for one hour. Treatment reduced the numbers of all microorganisms tested, but the populations recovered within three weeks. This suggests that either fresh formation water re-inoculated the well or that biofilm and scale in the well harbored some microbes from the disinfectant. Post-disinfection arsenic concentrations were similar to those measured prior to treatment. Although arsenic release under non-pumping conditions appears to be microbially mediated, well disinfection did not provide a sustained improvement in well- water quality.
H14D-04
Processes and Parameters Controlling the Extent of Methanogenic Conditions in the Unsaturated Zone of a Crude Oil Spill Site
Gas concentrations measured in the vadose zone at a crude oil spill site near Bemidji, MN, show that a large area near the oil body is currently dominated by methanogenic conditions. Away from the oil body methane concentrations decrease as it is degraded by methanotrophic bacteria under aerobic conditions. Numerical simulations have been conducted to quantify the contributions of the relevant transport and reaction processes to the production and attenuation of methane in the vadose zone. Methane is generated in the vadose zone by anaerobic degradation of oil and is also added by fluxes from the capillary fringe and the saturated zone. Gas diffusion and advection contribute to the transport of methane in the lateral direction and towards the ground surface. Attenuation of methane concentrations occurs through aerobic oxidation in the presence of methanotrophic bacteria. Critical parameters were varied within bounds provided by field data and previous studies. Simulation results confirm that the layered sediment structure present at the site plays a significant role in explaining the observed distribution of gases in the vadose zone. The presence of a low permeability lens in the area upgradient from the source results in higher moisture contents, limiting diffusion of oxygen into the zone of methane production, and contributes to the spread of methane. Diffusion was identified as the most significant transport mechanism for gases in the vadose zone. However, field-observed zones of depleted and enriched N2 and Ar concentrations could only be explained by the development of advective fluxes induced by reactive processes (methanogenesis and methanotrophy). The zones of gas production are characterized by slightly increased total gas pressures and low concentrations of N2 and Ar, while zones of gas consumption show slightly depressed total gas pressures and high concentrations of N2 and Ar. The simulations suggest that the advective flux that develops between these zones contributes up to 15% of the total methane flux.
H14D-05
Control of groundwater recharge-discharge on coupled N-processing across the sediment- water interface of floodplain sediments
From headwater agricultural streams to floodplain sloughs, denitrification is a common and environmentally important redox mediated reaction that occurs as dissolved NO3- is transported across the sediment- water interface within these systems. Factors influencing denitrification rates include carbon quality, NO3- availability, and the presence of O2. Here we present findings illustrating the influence of net groundwater recharge-discharge on nitrogen fate within two floodplain sloughs. Using a combination of slough-scale flood measurements (within 2 floodplain sloughs) and labeled 15NO3- additions, we show that coupled nitrogen removal is 4 times higher in the recharging slough. Our results suggest that O2 delivery to the sediment-water interface of the recharging slough resulted in measurable nitrification, relative to the discharging slough where nitrification was not detected. The redox profiles (e.g. Fe) and hydrologic gradients are consistent with the deeper penetration of O2 into the sediments within the recharging slough. Although the recharge/discharge N-flux was small relative to the overall N-balance within each slough, the subtle changes in recharge/discharge altered O2 availability and redox conditions near the sediment-water interface enhancing coupled N-removal within the recharging slough. These results suggest that O2 delivery across the sediment-water interface (e.g. hyporheic environments) may enhance net N-removal, especially if NO3- is limiting.
H14D-06
Ponds and Rice Fields: The Hydrology and Chemistry of Aquifer Recharge in Bangladesh
The shallow aquifer in Bangladesh, which provides drinking water for millions and irrigation water for innumerable rice fields, is severely contaminated with naturally occurring arsenic. Water balance calculations show that surface ponds and irrigated rice fields are the primary sources of recharge to this contaminated aquifer. Recharge from an individual rice field is both temporally and spatially heterogeneous, whereas flow from a pond is more constant and uniform through the pond sediments. Rice field recharge is focused through bunds (the berms surrounding the field), and depends on irrigation intervals. Field flow patterns are controlled by cracks and the development of an unsaturated zone. The water chemistry of these two recharge sources is distinctly different. Compared to the rice fields, ponds contribute recharge with a higher organic carbon load and increased concentrations of solutes associated with anoxic microbial respiration. The differences in the recharge behavior and solute loads of these two sources may explain the spatial patterns of groundwater chemistry that control arsenic concentrations.
H14D-07
Redox controls on solute transfer between shallow sediments and bottom-waters: Chemical and isotopic evidence from two Antarctic Dry Valley Lakes
The water chemistry studies of the McMurdo Dry Valley Lakes of Antarctica indicate a complex history of recent and past climate fluctuations. During warmer periods, the reduction in ice-cover results in augmented productivity and deposition of organic matter, coupled with a greater influx of oxic waters due to ice-melt. During cooler periods, the ice cover increases, and the oxidation of organic matter on the lake bottoms results in euxinic conditions. In this investigation, we present chemical and isotopic evidence that interstitial waters in sediments and bottom-water chemistry of Lake Fryxell and Lake Joyce are influenced by these changes. At Lake Fryxell, the deep-waters are characterized by 3He/4He and 36Cl/Cl ratios that indicate mixing between radiogenically derived 4He and 36Cl from basement rocks and He and Cl from meteoric sources. In contrast, Lake Joyce presents 3He/4He ratios which are and order of magnitude lower, while the 36Cl/Cl ratios are a order of magnitude higher. Both values indicate some limitations in the exchange between deep waters and oxic surface waters. The waters also become more depleted in deuterium and O-18 with increasing depth. Iodine bottom-water concentrations in both Lake Fryxell and Lake Joyce are roughly an order or magnitude less than the sediment pore waters directly below them. Bottom sediments in both lakes have most of the solid- phase iodine bound to it in the upper 5cm. Upon deeper burial, the sediment-bound iodine appears to be released into deeper pore waters and then rescavenged by the uppermost sediments. In both lakes, oxidation of organic matter, coupled with sulfate reduction, is consistent with the observed iodine concentrations. In the Lake Fryxell sediments, however, the iodine is less effectively retained in the upper sediments than in Lake Joyce, and seeps back into the water column. Interestingly, both the deepest waters of Lakes Joyce and Lake Fryxell have high 129I/I ratios suggesting that anthropogenic 129I may have been scavenged by organic matter at the lake surface, then settled to the lake bottom and was released into the deep waters. Apart from this anomaly, 129I/I ratios decrease with depth. Sediments host 129I/I ratios which are significantly lower than those of the lake water, indicate the either the scavenging of iodine has persisted in these lakes for extended periods of time or that there is a deep subsurface brine source of iodine. The presence and isotopic composition of iodine in the lake waters and shallow sediments is consistent with diagenesis and oxidation of organic matter in sediments. It also appears to be indirectly coupled to non-reactive lake water constituents such as helium and chloride. The melt-water input and ice cover changes which influence 36Cl/Cl and 3He/4He ratios also ultimately determine the oxidation state of the lake waters and the immediate subsurface.