Subsurface Redox Conditions Research Articles

Enhanced cycling of nitrogen and metals during rapid infiltration: Implications for managed recharge

We present results from a series of plot-scale field experiments to quantify physical infiltration dynamics and the influence of adding a carbon-rich, permeable reactive barrier (PRB) for the cycling of nitrogen and associated trace metals during rapid infiltration for managed aquifer recharge (MAR). Recent studies suggest that adding a bio-available carbon source to soils can enhance denitrification rates and associated N load reduction during moderate-to-rapid infiltration (≤1 m/day). We examined the potential for N removal during faster infiltration (>1 m/day), through coarse and carbon-poor soils, and how adding a carbon-rich PRB (wood chips) affects subsurface redox conditions and trace metal mobilization. During rapid infiltration, plots amended with a carbon-rich PRB generally demonstrated modest increases in subsurface loads of dissolved organic carbon, nitrite, manganese and iron, decreases in loads of nitrate and ammonium, and variable changes in arsenic. These trends differed considerably from those seen during infiltration through native soil without a carbon-rich PRB. Use of a carbon-rich soil amendment increased the fraction of dissolved N species that was removed at equivalent inflowing N loads. There is evidence that N removal took place primarily via denitrification. Shifts in microbial ecology following infiltration in all of the plots included increases in the relative abundances of microbes in the families Comamonadaceae, Pseudomonadaceae, Methylophilaceae, Rhodocyclaceae and Sphingomonadaceae, all of which contain genera capable of carrying out denitrification. These results, in combination with studies that have tested other soil types, flow rates, and system scales, show how water quality can be improved during infiltration for managed recharge, even during rapid infiltration, with a carbon-rich soil amendment.

Regionally variable timing and duration of celadonite formation in the Troodos lavas (Cyprus) from Rb-Sr age distributions

Celadonite (K[Mg2+,Fe2+][Fe3+,Al3+](Si4O10)(OH2)) is a common void-filling mineral in ocean floor and ophiolite lavas that forms during low-temperature, off-axis, hydrothermal alteration. Its occurrence provides important information about the sink of K from seawater into the upper oceanic crust. New age determinations for celadonitic samples from a ~20 km section of the lavas in the Troodos ophiolite, along with published ages, show that most celadonite formed within the first ~20 Myrs after crust accretion. However, significant regional differences in the duration of celadonite formation are observed. In comparison to a region of flat paleo-seafloor, in a paleo-seafloor topographic low celadonite started forming later and continued forming for ~20 Myrs longer. The paleo-seafloor low is associated with abundant hydrothermal sediments which are interpreted to have played a key role in controlling the chemical conditions required for celadonite precipitation. It is hypothesised that celadonite formed for longer largely because these hydrothermal sediments acted as a source for labile Fe for a longer time than in the region where the Fe required for celadonite formation was mainly leached from silicate phases. In the region of flat paleo-seafloor, sediment did not begin accumulating for ~20 Myrs after crustal accretion allowing prolonged oxidative alteration of the upper crust. This led to Fe released by silicate phase breakdown being incorporated in stable Fe3+ containing phases, such as Fe-oxyhydroxides, rather than being transported as aqueous Fe2+ to sites where celadonite could form. If this model is correct, the uptake of K from seawater into the oceanic crust may be highly spatially heterogeneous and intimately linked to the subsurface redox conditions and the distribution of hydrothermal sediments.

Atmospheric Electricity Influencing Biogeochemical Processes in Soils and Sediments.

The Earth’s subsurface represents a complex electrochemical environment that contains many electro-active chemical compounds that are relevant for a wide array of biologically driven ecosystem processes. Concentrations of many of these electro-active compounds within Earth’s subsurface environments fluctuate during the day and over seasons. This has been observed for surface waters, sediments and continental soils. This variability can affect particularly small, relatively immobile organisms living in these environments. While various drivers have been identified, a comprehensive understanding of the causes and consequences of spatio-temporal variability in subsurface electrochemistry is still lacking. Here we propose that variations in atmospheric electricity (AE) can influence the electrochemical environments of soils, water bodies and their sediments, with implications that are likely relevant for a wide range of organisms and ecosystem processes. We tested this hypothesis in field and laboratory case studies. Based on measurements of subsurface redox conditions in soils and sediment, we found evidence for both local and global variation in AE with corresponding patterns in subsurface redox conditions. In the laboratory, bacterial respiratory responses, electron transport activity and H2S production were observed to be causally linked to changes in atmospheric cation concentrations. We argue that such patterns are part of an overlooked phenomenon. This recognition widens our conceptual understanding of chemical and biological processes in the Earth’s subsurface and their interactions with the atmosphere and the physical environment.

Fate of leaf litter deposits and impacts on oxygen availability in bank filtration column studies

Degradation of particulate organic carbon (POC) such as leaf litter might deplete dissolved oxygen within the upper layers of bank filtration, an efficient and robust barrier for pathogens and for various organic micro-pollutants (OMP) in water supply systems worldwide. The degradation of OMP during bank filtration depends on the redox conditions. The present study aimed at identifying the impacts and fates of different local leaves on the oxygen consumption and the possible biological degradation of indicator OMP. Oxygen concentrations initially decreased within the columns from around 8 mg/L in the influent to low concentrations indicating extensive consumption within a short travel distance. Still a substantial oxygen consumption was observed after 250 days. OMP concentrations were not significantly affected by the microbial processes. A layer of calcium carbonate crystallites was observed on the POC layer. Some leaf fragments appeared to be persistant towards degradation and the carbon content relative to nitrogen and sulfur contents decreased within 250 days. The results demonstrate that trees at bank filtration sites might have a strong long-term impact on the subsurface redox conditions.

Combination of aquifer thermal energy storage and enhanced bioremediation: resilience of reductive dechlorination to redox changes.

To meet the demand for sustainable energy, aquifer thermal energy storage (ATES) is widely used in the subsurface in urban areas. However, contamination of groundwater, especially with chlorinated volatile organic compounds (CVOCs), is often being encountered. This is commonly seen as an impediment to ATES implementation, although more recently, combining ATES and enhanced bioremediation of CVOCs has been proposed. Issues to be addressed are the high water flow velocities and potential periodic redox fluctuation that accompany ATES. A column study was performed, at a high water flow velocity of 2 m/h, simulating possible changes in subsurface redox conditions due to ATES operation by serial additions of lactate and nitrate. The impacts of redox changes on reductive dechlorination as well as the microbial response of Dehalococcoides (DHC) were evaluated. The results showed that, upon lactate addition, reductive dechlorination proceeded well and complete dechlorination from cis-DCE to ethene was achieved. Upon subsequent nitrate addition, reductive dechlorination immediately ceased. Disruption of microorganisms’ retention was also immediate and possibly detached DHC which preferred attaching to the soil matrix under biostimulation conditions. Initially, recovery of dechlorination was possible but required bioaugmentation and nutrient amendment in addition to lactate dosing. Repeated interruption of dechlorination and DHC activity by nitrate dosing appeared to be less easily reversible requiring more efforts for regenerating dechlorination. Overall, our results indicate that the microbial resilience of DHC in biosimulated ATES conditions is sensitive to redox fluctuations. Hence, combining ATES with bioremediation requires dedicated operation and monitoring on the aquifer geochemical conditions.Electronic supplementary materialThe online version of this article (doi:10.1007/s00253-015-7241-6) contains supplementary material, which is available to authorized users.

Thioarsenic species associated with increased arsenic release during biostimulated subsurface sulfate reduction.

Introduction of acetate into groundwater at the Rifle Integrated Field Research Challenge (Rifle, CO) has been used for biostimulation aimed at immobilizing uranium. While a promising approach for lowering groundwater-associated uranium, a concomitant increase in soluble arsenic was also observed at the site. An array of field data was analyzed to understand spatial and temporal trends in arsenic release and possible correlations to speciation, subsurface redox conditions, and biogeochemistry. Arsenic release (up to 9 μM) was strongest under sulfate reducing conditions in areas receiving the highest loadings of acetate. A mixture of thioarsenate species, primarily trithioarsenate and dithioarsenate, were found to dominate arsenic speciation (up to 80%) in wells with the highest arsenic releases; thioarsenates were absent or minor components in wells with low arsenic release. Laboratory batch incubations revealed a strong preference for the formation of multiple thioarsenic species in the presence of the reduced precursors arsenite and sulfide. Although total soluble arsenic increased during field biostimulation, the termination of sulfate reduction was accompanied by recovery of soluble arsenic to concentrations at or below prestimulation levels. Thioarsenic species can be responsible for the transient mobility of sediment-associated arsenic during sulfidogenesis and should be considered when remediation strategies are implemented in sulfate-bearing, contaminated aquifers.

Elevated arsenic and manganese in groundwaters of Murshidabad, West Bengal, India

High levels of geogenic arsenic (As) and manganese (Mn) in drinking water has led to widespread health problems for the population of West Bengal, India. Here we delineate the extent of occurrences of As and Mn in Murshidabad, where the contaminated aquifers occur at shallow depths between 35 and 40m and where access to safe drinking water is a critical issue for the local population.A total of 78 well-water samples were taken in 4 blocks on either side of the river Bhagirathi: Nabagram and Kandi (west, Pleistocene sediments), Hariharpara and Beldanga (east, Holocene sediments). High As, total iron (FeT) and low Mn concentrations were found in waters from the Holocene gray sediment aquifers east of the river Bhagirathi, while the opposite was found in the Pleistocene reddish-brown aquifer west of the river Bhagirathi in Murshidabad. Speciation of As in water samples from Holocene sediments revealed the dominant species to be As(III), with ratios of As(III):AsT ranging from 0.55 to 0.98 (average 0.74). There were indications from saturation index estimations that Mn solubility is limited by the precipitation of MnCO3. Tubewells from high As areas in proximity to anthropogenic waste influx sources showing high molar Cl/Br ratios, low SO42− and low NO3− demonstrate relatively lower As concentrations, thereby reducing As pollution in those wells.Analyses of core samples (2 in each of the blocks) drilled to a depth of 45m indicate that there is no significant variation in bulk As (5–20mg/kg) between the Holocene and Pleistocene sediments, indicating that favorable subsurface redox conditions conducive to mobilization are responsible for the release of As. The same applies to Mn, but concentrations vary more widely (20–2000mg/kg). Sequential extraction of Holocene sediments showed As to be associated with ‘specifically sorbed-phosphate-extractable’ phases (10–15%) and with ‘amorphous and well crystalline Fe-oxyhydroxide’ phases (around 37%) at As-contaminated well depths, suggesting that the main As release mechanisms could be either competitive ion exchange with PO43−, or the dissolution of Fe oxyhydroxides. In the Pleistocene sediments Mn is predominantly found in the easily exchangeable fraction.

Retardation of arsenic transport by oxidized Holocene aquifer sediments of West Bengal, India

• As(III) adsorption by oxidized Holocene aquifer sediments at pH 6–9 was studied. • No pH dependency was observed for As(III) adsorption by oxidized Holocene sediments. • K d values for As(III) are comparable with those for As(V) in West Bengal, India. • Low adsorption capacity of Holocene sediments demonstrates significant As transport. The transport of arsenic (As) in groundwater is known to be considerably controlled by the adsorption behavior of aquifer sediments and changes in subsurface redox conditions. Here we report wet chemical analyses of As(III) adsorption onto oxidized, low As containing Holocene aquifer sediments from West Bengal, India. Batch experiments in an open laboratory atmosphere show that the slow removal of As(III) from aqueous solution is a resulting effect of adsorption and oxidation which is not significantly influenced by the changes in pH (6.0–9.1). The isotherm data were fitted with Freundlich and Langmuir equations and maximum adsorption density ( Γ max ≈ 0.40 mmol kg −1 ) obtained is lower than that for As(V) at pH 7.5 (0.95 mmol kg −1 ). The estimated distribution coefficient maxima ( K d max ) values are within the ranges of K d values previously reported in case of Holocene aquifer sediments of West Bengal, India and Bangladesh. The low adsorption capacity of oxidized Holocene aquifer sediments in this study compared to oxidized Pleistocene sediments of Bangladesh is attributed to coarser grain sizes and low abundance of Fe-oxyhydroxides and micas which act as a potential sink of As. This study has important implications for As(III) attenuation and transport in the shallow aquifers (especially around the zone of ‘maximum’ water table fluctuations, i.e., 10–20 m depth below ground surface) when oxygen-depleted (anoxic) Holocene aquifers get in contact with oxygen (e.g., by irrigation/groundwater drawdown) and also for predicting possible future risk associated with its transport from shallow aquifers to present day “low-As” containing deeper aquifers of West Bengal, India.

Microbial life associated with low‐temperature alteration of ultramafic rocks in the Leka ophiolite complex

Water-rock interactions in ultramafic lithosphere generate reduced chemical species such as hydrogen that can fuel subsurface microbial communities. Sampling of this environment is expensive and technically demanding. However, highly accessible, uplifted oceanic lithospheres emplaced onto continental margins (ophiolites) are potential model systems for studies of the subsurface biosphere in ultramafic rocks. Here, we describe a microbiological investigation of partially serpentinized dunite from the Leka ophiolite (Norway). We analysed samples of mineral coatings on subsurface fracture surfaces from different depths (10-160 cm) and groundwater from a 50-m-deep borehole that penetrates several major fracture zones in the rock. The samples are suggested to represent subsurface habitats ranging from highly anaerobic to aerobic conditions. Water from a surface pond was analysed for comparison. To explore the microbial diversity and to make assessments about potential metabolisms, the samples were analysed by microscopy, construction of small subunit ribosomal RNA gene clone libraries, culturing and quantitative-PCR. Different microbial communities were observed in the groundwater, the fracture-coating material and the surface water, indicating that distinct microbial ecosystems exist in the rock. Close relatives of hydrogen-oxidizing Hydrogenophaga dominated (30% of the bacterial clones) in the oxic groundwater, indicating that microbial communities in ultramafic rocks at Leka could partially be driven by H2 produced by low-temperature water-rock reactions. Heterotrophic organisms, including close relatives of hydrocarbon degraders possibly feeding on products from Fischer-Tropsch-type reactions, dominated in the fracture-coating material. Putative hydrogen-, ammonia-, manganese- and iron-oxidizers were also detected in fracture coatings and the groundwater. The microbial communities reflect the existence of different subsurface redox conditions generated by differences in fracture size and distribution, and mixing of fluids. The particularly dense microbial communities in the shallow fracture coatings seem to be fuelled by both photosynthesis and oxidation of reduced chemical species produced by water-rock reactions.

Microbial Functional Gene Diversity with a Shift of Subsurface Redox Conditions during In Situ Uranium Reduction

To better understand the microbial functional diversity changes with subsurface redox conditions during in situ uranium bioremediation, key functional genes were studied with GeoChip, a comprehensive functional gene microarray, in field experiments at a uranium mill tailings remedial action (UMTRA) site (Rifle, CO). The results indicated that functional microbial communities altered with a shift in the dominant metabolic process, as documented by hierarchical cluster and ordination analyses of all detected functional genes. The abundance of dsrAB genes (dissimilatory sulfite reductase genes) and methane generation-related mcr genes (methyl coenzyme M reductase coding genes) increased when redox conditions shifted from Fe-reducing to sulfate-reducing conditions. The cytochrome genes detected were primarily from Geobacter sp. and decreased with lower subsurface redox conditions. Statistical analysis of environmental parameters and functional genes indicated that acetate, U(VI), and redox potential (E(h)) were the most significant geochemical variables linked to microbial functional gene structures, and changes in microbial functional diversity were strongly related to the dominant terminal electron-accepting process following acetate addition. The study indicates that the microbial functional genes clearly reflect the in situ redox conditions and the dominant microbial processes, which in turn influence uranium bioreduction. Microbial functional genes thus could be very useful for tracking microbial community structure and dynamics during bioremediation.

Arsenic contamination of groundwater in the Kathmandu Valley, Nepal, as a consequence of rapid erosion

Elevated levels of arsenic (As) in groundwater in the flood plain of the Ganges River have been well-documented over the past decades. The objective of this study was to measure As and the transition elements normally associated with As in the Kathmandu Valley in Nepal, a heavily populated tectonic valley in the upper reaches of the Ganges River system. Water samples were collected from six shallow tubewells (depth < 50 m), eight deep tubewells and 13 dug wells and stone spouts. Electrical conductivity, temperature and pH were measured on-site and concentrations of As, Fe, Cu, Ni, Co, Mn, Zn and Cr were measured with a spectrophotometer. Five tubewells and four dug wells had As levels exceeding the Nepal Interim Standard (As = 0.05 mg/L). There was no statistically significant clustering of As levels either with depth or horizontal location. Arsenic was uncorrelated with either Fe (R2 = 0.096), Mn (R2 = 0.0004) or any combination of transition elements (R2 < 0.083), which is inconsistent with both the reduction-dissolution and the sulfide oxidation models for As release. The geometric mean As level of groundwater (As = 0.015 mg/L) was indistinguishable from the geometric mean As level of surface water (As = 0.013 mg/L) obtained from 48 river samples from the Kathmandu Valley in a previous study. We are suggesting that elevated groundwater As results not from subsurface redox conditions, but from losing streams with elevated As, which is a consequence of rapid erosion caused by a combination of monsoon climate, tectonic uplift and deforestation.

Low-molecular weight hydrocarbons in vent fluids from the Main Endeavour Field, northern Juan de Fuca Ridge

Despite its location on sediment-free basalt, vent fluids from the Main Endeavour Field (MEF) contain chemical species that indicate fluids have interacted with sediments during circulation. We report on the distribution and isotopic abundances of organic compounds ( C 1– C 3 alkanes and alkenes, benzene and toluene) in fluids collected from the Main Endeavour Field (MEF) in July, 2000, to understand the processes that regulate their abundances and characterize fluid sources. Aqueous organic compounds are derived from the thermal alteration of sedimentary organic matter and subsequently undergo further oxidation reactions during fluid flow. Fluid:sediment mass ratios calculated using ΣNH 4 concentrations indicate that the sediments are distal to the MEF, resulting in a common reservoir of fluids for all of the vents. Following the generation from sediment alteration, aqueous organic compounds undergo secondary alteration reactions via a stepwise oxidation reaction mechanism. Alkane distributions and isotopic compositions indicate that organic compounds in MEF fluids have undergone a greater extent of alteration as compared to Middle Valley fluids, either due to differences in subsurface redox conditions or the residence time of fluids at subsurface conditions. The distributions of the aromatic compounds benzene and toluene are qualitatively consistent with the subsurface conditions indicated by equilibration of aqueous alkanes and alkanes. However, benzene and toluene do not achieve chemical equilibrium in the subsurface. Methane and CO 2 also do not equilibrate chemically or isotopically at reaction zone temperatures, a likely result of an insufficient reaction time after addition of CO 2 from magmatic sources during upflow. The organic geochemistry supports the assumption that the sediments with which MEF fluids interact has the same composition as sediments present in Middle Valley itself, and highlight differences in subsurface reaction zone conditions and fluid flow pathways at these two sites.

Three‐dimensional self‐potential inversion for subsurface DNAPL contaminant detection at the Savannah River Site, South Carolina

Self‐potential (SP) data are collected over an area of approximately 250 m2 at the Department of Energy Savannah River Site where there is known subsurface dense nonaqueous phase liquid (DNAPL) contamination. Nonpolarizing electrodes are used to measure the SP signal on a two‐dimensional (2‐D) surface grid with 2‐m spacing in both horizontal directions, and four borehole arrays with 3.7‐m electrode spacing and 25.6‐m total depth. The primary contaminants, tetrachloroethylene (PCE) and trichloroethylene (TCE), are known to undergo redox reactions in the environment. Variations in the subsurface redox conditions are proposed as an electrochemical source for the SP signals measured in this investigation. A 3‐D self‐potential inversion algorithm is used to find an electrical current source model, taking into account the resistivity structure derived from a 3‐D spectral induced polarization survey at the same field location. The sources and sinks of electrical current can be related to the zones of relative high or low redox potential and are therefore interpreted in the context of contaminated areas. These results are reasonably correlated with DNAPL concentration data obtained from several ground‐truth well measurements, indicating that the SP sources can be an indicator of contaminated areas where electrochemical source mechanisms are active. In several cases, however, the SP sources and contaminant concentrations are not correlated, reflecting the spatial variability of biogeochemical parameters in the Earth that control the SP response in addition to concentration. More extensive geochemical ground‐truth information is therefore needed to validate the self‐potential source inversion methodology and develop it as a predictive tool in the context of contaminated sites.

Bench- and pilot-scale applications of electrochemical peroxidation: A new remedial concept

Electrochemical peroxidation (ECP) is a proprietary process that utilizes sacrificial iron electrodes and stochiometrically balanced applications of hydrogen peroxide to efficiently destroy aqueous phase contaminants. In laboratory trials it has been successful in reducing, often to non-detectable levels, BTEX, fuel additives, chlorinated solvents, and polychlorinated biphenyls in ground waters. The process has also been found effective in reducing the chemical and biological oxygen demand of industrial waste water. Agency-approved pilot tests will be conducted at two gasoline spill sites during 2000 where traditional pump and treat methods have proven ineffectual because of ground water chemistry or subsurface hydrologic conditions. The ECP process utilizes a tripartite treatment strategy consisting of 1) ex situ chemical oxidation; 2) in situ oxidation by reinjection of treated water with residual oxidants at the head of the plume; and 3) reestablishment of aerobic biodegradation by alteration of subsurface redox conditions. In contrast to other in situ oxidation treatment methods, dissolved iron is derived electrochemically, negating the need for ferrous salt addition. Dilute hydrogen peroxide (3 percent) is incrementally added to maximize oxidation efficiency and eliminate safety and environmental concerns accompanying the use of highly concentrated solutions. Results of laboratory trials and the geological and geochemical considerations of upcoming pilot-scale applications are presented. Other potential applications currently under investigation include combination with other remedial processes (e.g. permeable barriers and hydrogen release compounds) to insure complete and rapid contaminant mineralization.

Spatial and temporal gradients in aquifer oxidation‐reduction conditions

The study was undertaken to identify principal oxidizing and reducing chemical species in groundwater with the goal of determining the utility of platinum electrode (Eh) measurements to characterize subsurface redox conditions. Serial measurements of Eh and groundwater analyses were conducted in oxic and suboxic environments over more than a 2‐year period for major ionic, oxidized, and reduced species. Vertical gradients in measured Eh values in the oxic groundwater environments can exceed −40 mV/m depth. In the poorly poised, oxic groundwaters, the Eh measurements correlated reasonably well with calculated values based on analytical determinations of O2 and H2O2 which was detected persistently in the 10−8 to 10−9 M range. The equilibrium calculated Eh values from this and other redox couples did not correlate well with measured Eh values over the 2‐year study period. In the suboxic range, the average calculated values based on the Fe3+/Fe2+ coupled correlated well with averaged measured values. The results support the need to determine redox pairs in groundwater as a supplement to either calculated “equilibrium” Eh values or Pt electrode Eh measurements.