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At least 253 records · Page 14Linked to original sources

USGS2015

Relating subsurface temperature changes to microbial activity at a crude oil-contaminated site

Crude oil at a spill site near Bemidji, Minnesota has been undergoing aerobic and anaerobic biodegradation for over 30 years, creating a 150–200 m plume of primary and secondary contaminants. Microbial degradation generates heat that should be measurable under the right conditions. To measure this heat, thermistors were installed in wells in the saturated zone and in water-filled monitoring tubes in the unsaturated zone. In the saturated zone, a thermal groundwater plume originates near the residual oil body with temperatures ranging from 2.9 °C above background near the oil to 1.2 °C down gradient. Temperatures in the unsaturated zone above the oil body were up to 2.7 °C more than background temperatures. Previous work at this site has shown that methane produced from biodegradation of the oil migrates upward and is oxidized in a methanotrophic zone midway between the water table and the surface. Enthalpy calculations and observations demonstrate that the temperature increases primarily result from aerobic methane oxidation in the unsaturated zone above the oil. Methane oxidation rates at the site independently estimated from surface CO 2 efflux data are comparable to rates estimated from the observed temperature increases. The results indicate that temperature may be useful as a low-cost measure of activity but care is required to account for the correct heat-generating reactions, other heat sources and the effects of focused recharge.

Minnesota↗

USGS2017

Effects of solid-liquid separation and storage on monensin attenuation in dairy waste management systems

Environmental release of veterinary pharmaceuticals has been of regulatory concern for more than a decade. Monensin is a feed additive antibiotic that is prevalent throughout the dairy industry and is excreted in dairy waste. This study investigates the potential of dairy waste management practices to alter the amount of monensin available for release into the environment. Analysis of wastewater and groundwater from two dairy farms in California consistently concluded that monensin is most present in lagoon water and groundwater downgradient of lagoons. Since the lagoons represent a direct source of monensin to groundwater, the effect of waste management, by mechanical screen separation and lagoon aeration, on aqueous monensin concentration was investigated through construction of lagoon microcosms. The results indicate that monensin attenuation is not improved by increased solid-liquid separation prior to storage in lagoons, as monensin is rapidly desorbed after dilution with water. Monensin is also shown to be easily degraded in lagoon microcosms receiving aeration, but is relatively stable and available for leaching under typical anaerobic lagoon conditions.

Journal of Environmental Management↗

USGS2016

Internal loading of phosphorus in western Lake Erie

This study applied eight techniques to obtain estimates of the diffusive flux of phosphorus (P) from bottom sediments throughout the western basin of Lake Erie. The flux was quantified from both aerobic and anaerobic incubations of whole cores; by monitoring the water encapsulated in bottom chambers; from pore water concentration profiles measured with a phosphate microelectrode, a diffusive equilibrium in thin films (DET) hydrogel, and expressed pore waters; and from mass balance and biogeochemical diagenetic models. Fluxes under aerobic conditions at summertime temperatures averaged 1.35 mg P/m 2 /day and displayed spatial variability on scales as small as a centimeter. Using two different temperature correction factors, the flux was adjusted to mean annual temperature yielding average annual fluxes of 0.43–0.91 mg P/m 2 /day and a western basin-wide total of 378–808 Mg P/year as the diffusive flux from sediments. This is 3–7% of the 11,000 Mg P/year International Joint Commission (IJC) target load for phosphorus delivery to Lake Erie from external sources. Using these average aerobic fluxes, the sediment contributes 3.0–6.3 μg P/L as a background internal contribution that represents 20–42% of the IJC Target Concentration of 15 μg P/L for the western basin. The implication is that this internal diffusive recycling of P is unlikely to trigger cyanobacterial blooms by itself but is sufficiently large to cause blooms when combined with external loads. This background flux may be also responsible for delayed response of the lake to any decrease in the external loading.

Lake Erie↗

USGS2026

PFAS remediation in a bioelectrochemical system inoculated with the west branch consortium (WBC-2)

Groundwater contamination by per - and polyfluoroalkyl substances (PFAS) poses a persistent environmental and public health concern. This study evaluates a two-chambered bioelectrochemical system (BES) inoculated with the West Branch Consortium (WBC-2) for PFAS remediation. Under an applied cathodic potential of −450 mV (versus Ag/AgCl), the BES with active WBC-2 achieved >99.0% perfluorooctanesulfonic acid (PFOS) removal within 21 days in deionized water with culture medium and > 98.9% removal of PFOS, perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA), and perfluorohexanesulfonic acid (PFHxS) in contaminated groundwater after 102 days. Intermediate formation (e.g., PFOA, 6:2 fluorotelomer sulfonate (6:2 FTS), perfluoropropionic acid (PFPrA), perfluorobutanoic acid (PFBA)) and background-corrected fluoride release were consistent with PFOS transformation under anaerobic reducing conditions potentially involving defluorination. Following repeated PFOS spikes (100 μg/L on Days 0, 50, and 399), PFOA, PFPrA, and PFBA accumulated over 664 days. Despite being the dominant accumulated compound, PFOA accounted for <1.8% of the total spiked PFOS mass. Minimal PFOS transformation occurred in controls without active WBC-2, highlighting the importance of microbial metabolism. Biofilm analysis revealed dense colonization of rod-shaped bacteria on carbon fiber brushes. Enrichment of Bacillus , Agrobacterium , and other low-abundance taxa suggests selective adaptation to BES and PFAS conditions. These findings highlight BES driven by electrochemically stimulated microbial activity as a promising strategy for PFAS remediation.

Journal of Water Process Engineering↗

USGS2025

Tracking diagenetic alteration of magnetic susceptibility in thrust ridge and slope basin sediments of the Cascadia margin (ODP Sites 1249 and 1252; IODP Site U1325)

We investigated sediment core records from the Cascadia Margin (Ocean Drilling Program Sites 1249 and 1252 at Hydrate Ridge; Integrated Ocean Drilling Program Site U1325 offshore Vancouver Island) using a Zr/Rb heavy mineral proxy from X-ray fluorescence (XRF) core scanning to identify intervals of primary detrital magnetic susceptibility (κ) and predict intervals where diagenesis caused magnetite dissolution by hydrogen sulfide. We also measured total sulfur (TS) content, grain size distributions, total organic carbon (TOC) content, and the magnetic mineral assemblage to further constrain the role of diagenesis on κ. Understanding how κ can be used to better characterize the varied effects of detrital and diagenetic signals in marine settings is important for understanding biogeochemical cycling and records of paleoenvironmental change. The upper 100 m of slope basin Site 1252 contains multiple intervals (> 90 m total) of decreased κ correlated with elevated TS content, consistent with dissolution of magnetite and precipitation of pyrite, iron monosulfides, and/or elemental sulfur. Similarly at the other slope basin site, Site U1325, κ is lower and TS is elevated in the interval between 24 and 51 mbsf, due to sulfide formation. At both slope basin sites, these low κ intervals correspond with high TOC, suggesting the possibility that organoclastic sulfate reduction (OSR) is likely a major driver of diagenetic alteration of κ at these sites. High TS:TOC ratios at Site U1325 suggests anaerobic oxidation of methane (AOM) during sulfate-methane transition zone (SMTZ) migration may have contributed to alteration of κ. In contrast, within the upper 90 m of Site 1249, a methane seep site at the summit of Hydrate Ridge, κ is almost entirely altered by diagenetic processes, with much of the low κ explained by a high degree of iron sulfide formation, while some intervals are affected by precipitation of magnetic iron sulfides that maintain or even increase κ. The presence of abundant methane seepage and gas hydrate as well as chemosynthetic seafloor fauna at this site, suggests that sulfide is released to the water column and AOM, rather than OSR, drives diagenetic alteration of κ at this site. Overall, the slope basin sites show episodic variation of κ that is influenced by TOC content, likely driven by changes in marine primary productivity and sedimentation rate, while the seep site shows consistently altered κ with lower TS content and no correlation with TOC. Methane seep environments likely experience loss of hydrogen sulfide to the water column and oxidation of hydrogen sulfide by seafloor seep fauna, which limits the amount of solid phase sulfur (pyrite, iron monosulfides, elemental sulfur) that can be precipitated within the sediments. In contrast, the migration of a buried SMTZ at slope basin sites results in enhanced sulfur precipitation within the sediments. This integrated magnetic and geochemical approach reveals the diagenetic production pathway and residence time of sulfide with the sediment column ultimately controls the style and degree of diagenetic loss of κ in marine sediments. This approach works best in environments with unaltered reference intervals/sites, consistent magnetic mineralogy, and clay-to-silt grain sizes.

Cascadia margin, Pacific Ocean↗

USGS2008

Assessing sulfate reduction and methane cycling in a high salinity pore water system in the northern Gulf of Mexico

Pore waters extracted from 18 piston cores obtained on and near a salt-cored bathymetric high in Keathley Canyon lease block 151 in the northern Gulf of Mexico contain elevated concentrations of chloride (up to 838 mM) and have pore water chemical concentration profiles that exhibit extensive departures (concavity) from steady-state (linear) diffusive equilibrium with depth. Minimum ??13C dissolved inorganic carbon (DIC) values of -55.9??? to -64.8??? at the sulfate-methane transition (SMT) strongly suggest active anaerobic oxidation of methane (AOM) throughout the study region. However, the nonlinear pore water chemistry-depth profiles make it impossible to determine the vertical extent of active AOM or the potential role of alternate sulfate reduction pathways. Here we utilize the conservative (non-reactive) nature of dissolved chloride to differentiate the effects of biogeochemical activity (e.g., AOM and/or organoclastic sulfate reduction) relative to physical mixing in high salinity Keathley Canyon sediments. In most cases, the DIC and sulfate concentrations in pore waters are consistent with a conservative mixing model that uses chloride concentrations at the seafloor and the SMT as endmembers. Conservative mixing of pore water constituents implies that an undetermined physical process is primarily responsible for the nonlinearity of the pore water-depth profiles. In limited cases where the sulfate and DIC concentrations deviated from conservative mixing between the seafloor and SMT, the ??13C-DIC mixing diagrams suggest that the excess DIC is produced from a 13C-depleted source that could only be accounted for by microbial methane, the dominant form of methane identified during this study. We conclude that AOM is the most prevalent sink for sulfate and that it occurs primarily at the SMT at this Keathley Canyon site.

Marine and Petroleum Geology↗

USGS2008

Thermal stability of ladderane lipids as determined by hydrous pyrolysis

Anaerobic ammonium oxidation (anammox) has been recognized as a major process resulting in loss of fixed inorganic nitrogen in the marine environment. Ladderane lipids, membrane lipids unique to anammox bacteria, have been used as markers for the detection of anammox in marine settings. However, the fate of ladderane lipids after sediment burial and maturation is unknown. In this study, anammox bacterial cell material was artificially matured by hydrous pyrolysis at constant temperatures ranging from 120 to 365 ??C for 72 h to study the stability of ladderane lipids during progressive dia- and catagenesis. HPLC-MS/MS analysis revealed that structural alterations of ladderane lipids already occurred at 120 ??C. At temperatures >140 ??C, ladderane lipids were absent and only more thermally stable products could be detected, i.e., ladderane derivatives in which some of the cyclobutane rings were opened. These diagenetic products of ladderane lipids were still detectable up to temperatures of 260 ??C using GC-MS. Thus, ladderane lipids are unlikely to occur in ancient sediments and sedimentary rocks, but specific diagenetic products of ladderane lipids will likely be present in sediments and sedimentary rocks of relatively low maturity (i.e., C31 hopane 22S/(22S + 22R) ratio <0.2 or ????/(???? + ???? + ????) ratio of >0.5). ?? 2008 Elsevier Ltd.

Organic Geochemistry↗

USGS2015

Biogeochemical environments of streambed-sediment pore waters with and without arsenic enrichment in a sedimentary rock terrain, New Jersey Piedmont, USA

Release of arsenic (As) from sedimentary rocks has resulted in contamination of groundwater in aquifers of the New Jersey Piedmont Physiographic Province, USA; the contamination also may affect the quality of the region's streamwater to which groundwater discharges. Biogeochemical mechanisms involved in the release process were investigated in the streambeds of Six Mile Run and Pike Run, tributaries to the Millstone River in the Piedmont. At Six Mile Run, streambed pore water and shallow groundwater were low or depleted in oxygen, and contained As at concentrations greater than 20 μg/L. At Pike Run, oxidizing conditions were present in the streambed, and the As concentration in pore water was 2.1 μg/L. The 16S rRNA gene and the As(V) respiratory reductase gene, arrA , were amplified from DNA extracted from streambed pore water at both sites and analyzed, revealing that distinct bacterial communities that corresponded to the redox conditions were present at each site. Anaerobic enrichment cultures were inoculated with pore water from gaining reaches of the streams with acetate and As(V). As(V) was reduced by microbes to As(III) in enrichments with Six Mile Run pore water and groundwater, whereas no reduction occurred in enrichments with Pike Run pore water. Cloning and sequencing of the arrA gene indicated 8 unique operational taxonomic units (OTUs) at Six Mile Run and 11 unique OTUs at Pike Run, which may be representative of the arsenite oxidase gene arxA . Low-oxygen conditions at Six Mile Run have favored microbial As reduction and release, whereas release was inhibited by oxidizing conditions at Pike Run.

New Jersey↗

USGS2020

Degradation of RDX (Hexahydro-1,3,5-trinitro-1,3,5-triazine) in contrasting coastal marine habitats: Subtidal non-vegetated (sand), subtidal vegetated (silt/eel grass), and intertidal marsh

Hundreds of explosive-contaminated marine sites exist globally, many of which contain the common munitions constituent hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Quantitative information about RDX transformation in coastal ecosystems is essential for management of many of these sites. Isotopically labelled RDX containing 15 N in all 3 nitro groups was used to track the fate of RDX in three coastal ecosystem types. Flow-through mesocosms representing subtidal vegetated (silt/eel grass), subtidal non-vegetated (sand) and intertidal marsh ecosystems were continuously loaded with isotopically labelled RDX for 16–17 days. Sediment, pore-water and overlying surface water were analyzed to determine the distribution of RDX, nitroso-triazine transformation products (NXs) and nitrogen containing complete mineralization products, including ammonium, nitrate+nitrite, nitrous oxide and nitrogen gas. The marsh, silt, and sand ecotypes transformed 94%, 90% and 76% of supplied RDX, respectively. Total dissolved NXs accounted for 2%–4% of the transformed 15 N-RDX. The majority of RDX transformation in the water column was by mineralization to inorganic N (dissolved and evaded; 64%–78% of transformed 15 N-RDX). RDX was mineralized primarily to N 2 O (62–74% of transformed 15 N-RDX) and secondarily to N 2 (1–2% of transformed 15 N-RDX) which exchanged with the atmosphere. Transformation of RDX was favored in carbon-rich lower redox potential sediments of the silt and marsh mesocosms where anaerobic processes of iron and sulfate reduction were most prevalent. RDX was most persistent in the carbon-poor sand mesocosm. Partitioning of 15 N derived from RDX onto sediment and suspended particulates was negligible in the overall mass balance of RDX transformation (2%–3% of transformed 15 N-RDX). The fraction of 15 N derived from RDX that was sorbed or assimilated in sediment was largest in the marsh mesocosm (most organic carbon), and smallest in the sand mesocosm (largest grain size and least organic carbon). Sediment redox conditions and available organic carbon stores affect the fate of RDX in different coastal marine habitats.

Connecticut, New York↗

USGS2021

Repeated large-scale mechanical treatment of invasive Typha under increasing water levels promotes floating mat formation and wetland methane emissions

Invasive species management typically aims to promote diversity and wildlife habitat, but little is known about how management techniques affect wetland carbon (C) dynamics. Since wetland C uptake is largely influenced by water levels and highly productive plants, the interplay of hydrologic extremes and invasive species is fundamental to understanding and managing these ecosystems. During a period of rapid water level rise in the Laurentian Great Lakes, we tested how mechanical treatment of invasive plant Typha × glauca shifts plant-mediated wetland C metrics. From 2015 to 2017, we implemented large-scale treatment plots (0.36-ha) of harvest (i.e., cut above water surface, removed biomass twice a season), crush (i.e., ran over biomass once mid-season with a tracked vehicle), and Typha -dominated controls. Treated Typha regrew with approximately half as much biomass as unmanipulated controls each year, and Typha production in control stands increased from 500 to 1500 g-dry mass m −2 yr −1 with rising water levels (~10 to 75 cm) across five years. Harvested stands had total in-situ methane (CH 4 ) flux rates twice as high as in controls, and this increase was likely via transport through cut stems because crushing did not change total CH 4 flux. In 2018, one year after final treatment implementation, crushed stands had greater surface water diffusive CH 4 flux rates than controls (measured using dissolved gas in water), likely due to anaerobic decomposition of flattened biomass. Legacy effects of treatments were evident in 2019; floating Typha mats were present only in harvested and crushed stands, with higher frequency in deeper water and a positive correlation with surface water diffusive CH 4 flux. Our study demonstrates that two mechanical treatments have differential effects on Typha structure and consequent wetland CH 4 emissions, suggesting that C-based responses and multi-year monitoring in variable water conditions are necessary to accurately assess how management impacts ecological function.

Michigan↗

USGS2012

Microbial transformations of arsenic: Mobilization from glauconitic sediments to water

In the Inner Coastal Plain of New Jersey, arsenic (As) is released from glauconitic sediment to carbon- and nutrient-rich shallow groundwater. This As-rich groundwater discharges to a major area stream. We hypothesize that microbes play an active role in the mobilization of As from glauconitic subsurface sediments into groundwater in the Inner Coastal Plain of New Jersey. We have examined the potential impact of microbial activity on the mobilization of arsenic from subsurface sediments into the groundwater at a site on Crosswicks Creek in southern New Jersey. The As contents of sediments 33–90 cm below the streambed were found to range from 15 to 26.4 mg/kg, with siderite forming at depth. Groundwater beneath the streambed contains As at concentrations up to 89 μg/L. Microcosms developed from site sediments released 23 μg/L of As, and active microbial reduction of As(V) was observed in microcosms developed from site groundwater. DNA extracted from site sediments was amplified with primers for the 16S rRNA gene and the arsenate respiratory reductase gene, arrA , and indicated the presence of a diverse anaerobic microbial community, as well as the presence of potential arsenic-reducing bacteria. In addition, high iron (Fe) concentrations in groundwater and the presence of iron-reducing microbial genera suggests that Fe reduction in minerals may provide an additional mechanism for release of associated As, while arsenic-reducing microorganisms may serve to enhance the mobility of As in groundwater at this site.

New Jersey↗

USGS2026

Aging of biosolids in fields prolongs the release of biosolid-borne PFAS and promotes the release of select PFCA in water

Land application of biosolids can introduce PFAS into croplands. Biosolids transported to surface waters through runoff may subsequently release PFAS into aquatic environments. The objective was to evaluate the effects of the aging of biosolids and the presence of sediment on the release of biosolid-borne PFAS to water at the water-sediment interface under anaerobic conditions. Through two independent rounds of microcosm experiments, we demonstrate that by day 136, most of the PFAS compounds (e.g., perfluorocarboxylic acids, or PFCAs) released <10 ng per g biosolids dry weight, except for 5:3 FTCA. Kinetically, the aging process substantially delayed the release of some long-chain PFAS (i.e., PFNA, PFDA, and PFOS) to water, due to the collapse of the porous structure of biosolids during aging. After day 136, the amount of long-chain PFAS released from aged biosolids reached levels similar to those released from the fresh biosolids due to the recovery of the pore structure during the release experiment. Mass balance analysis showed considerably higher, final masses of PFPeA, PFHxA, and PFOA than their initial masses, suggesting that precursors were biotransformed into these compounds during the release experiment, and biosolid aging led to elevated biotransformation and release of these three PFCAs. Sediment had little effect on the kinetics or extent of PFAS release from biosolids, except that the extent of FTCA release was reduced in the presence of sediment potentially due to adsorption. Overall, biosolid aging can delay the release of long-chain PFAS and promote the biotransformation to and release of PFCAs.

Water Research↗

USGS2000

Fractionation of selenium isotopes during bacterial respiratory reduction of selenium oxyanions

Reduction of selenium oxyanions by microorganisms is an important process in the biogeochemical cycling of selenium. Numerous bacteria can reduce Se oxyanions, which are used as electron acceptors during the oxidation of organic matter in anoxic environments. In this study, we used a double spike ( 82 Se and 74 Se) thermal ionization mass spectrometry technique to quantify the isotopic fractionation achieved by three different species of anaerobic bacteria capable of accomplishing growth by respiratory reduction of selenate [SeO 4 2− or Se(VI)] or selenite [SeO 3 2− or Se(IV)] to Se(IV) or elemental selenium [Se(0)] coupled with the oxidation of lactate. Isotopic discrimination in these closed system experiments was evaluated by Rayleigh fractionation equations and numerical models. Growing cultures of Bacillus selenitireducens , a haloalkaliphile capable of growth using Se(IV) as an electron acceptor, induced a 80 Se/ 76 Se fractionation of −8.0 ± 0.4‰ (instantaneous ϵ value) during reduction of Se(IV) to Se(0). With Bacillus arsenicoselenatis , a haloalkaliphile capable of growth using Se(VI) as an electron acceptor, fractionations of −5.0 ± 0.5‰ and −6.0 ± 1.0‰ were observed for reduction of Se(VI) to Se(IV) and reduction of Se(IV) to Se(0), respectively. In growing cultures of Sulfurospirillum barnesii , a freshwater species capable of growth using Se(VI), fractionation was small initially, but near the end of the log growth phase, it increased to −4.0 ± 1.0‰ and −8.4 ± 0.4‰ for reduction of Se(VI) to Se(IV) and reduction of Se(IV) to Se(0), respectively. Washed cell suspensions of S. barnesii induced fractionations of −1.1 ± 0.4‰ during Se(VI) reduction, and −9.1 ± 0.5% for Se(IV) reduction, with some evidence for smaller values (e.g., −1.7‰) in the earliest-formed Se(0) results. These results demonstrate that dissimilatory reduction of selenate or selenite induces significant isotopic fractionation, and suggest that significant Se isotope ratio variation will be found in nature.

Geochimica et Cosmochimica Acta↗

USGS2001

Progression of natural attenuation processes at a crude oil spill site: II. Controls on spatial distribution of microbial populations

A multidisciplinary study of a crude-oil contaminated aquifer shows that the distribution of microbial physiologic types is strongly controlled by the aquifer properties and crude oil location. The microbial populations of four physiologic types were analyzed together with permeability, pore-water chemistry, nonaqueous oil content, and extractable sediment iron. Microbial data from three vertical profiles through the anaerobic portion of the contaminated aquifer clearly show areas that have progressed from iron-reduction to methanogenesis. These locations contain lower numbers of iron reducers, and increased numbers of fermenters with detectable methanogens. Methanogenic conditions exist both in the area contaminated by nonaqueous oil and also below the oil where high hydrocarbon concentrations correspond to local increases in aquifer permeability. The results indicate that high contaminant flux either from local dissolution or by advective transport plays a key role in determining which areas first become methanogenic. Other factors besides flux that are important include the sediment Fe(II) content and proximity to the water table. In locations near a seasonally oscillating water table, methanogenic conditions exist only below the lowest typical water table elevation. During 20 years since the oil spill occurred, a laterally continuous methanogenic zone has developed along a narrow horizon extending from the source area to 50–60 m downgradient. A companion paper [J. Contam. Hydrol. 53, 369–386] documents how the growth of the methanogenic zone results in expansion of the aquifer volume contaminated with the highest concentrations of benzene, toluene, ethylbenzene, and xylenes.

Journal of Contaminant Hydrology↗

USGS1999

Geochemical heterogeneity of a gasoline-contaminated aquifer

The scale of biogeochemical reactions was studied in a physically and chemically heterogeneous surficial Coastal Plain aquifer contaminated by a gasoline spill. The physical heterogeneity of the aquifer is manifested in two hydrologic units, a shallow local aquifer of perched water and a regional sandy aquifer. Over the studied vertical interval of 21.3 ft (6.5 m), concentrations of reactive species varied by orders of magnitude, and the impact of biodegradation was expressed to widely varying degrees. A thin (3 ft thick) section of the perched-water zone was the most contaminated; total aromatic hydrocarbons were as high as 19.4 mg/l. Hydrocarbons were degraded by microbially mediated reactions that varied over short vertical distances and time. Anaerobic processes dominated within the low-permeability clay unit, whereas in the more permeable sandy layers nitrate reduction and aerobic degradation occurred. Hydrocarbons were more persistent over time in the low-permeability layer due to the limited availability of electron acceptors for degradation. The microbial degradation of hydrocarbons was linked to sulfate and iron reduction in the clay unit and led to alterations in the aquifer solids; electron microscopy revealed the presence of FeS minerals encrusting primary aquifer grains. High concentrations of Fe 2+ in groundwater, up to 34.5 mg/l, persist in kinetic disequilibrium in the presence of elevated H 2 S levels of 1.0 mg/l. Assessment of aquifer heterogeneities and groundwater contamination was possible due to sample discrimination at a scale of approximately 2 ft (∼0.6 m), a much finer resolution than is attempted in many remedial investigations of polluted aquifers. The information obtained in this type of study is essential to the development of models capable of estimating the fate of hydrocarbons at a site scale.

Journal of Contaminant Hydrology↗

USGS1998

Flow of river water into a karstic limestone aquifer: 2. Dating the young fraction in groundwater mixtures in the Upper Floridan aquifer near Valdosta, Georgia

Tritium/helium-3 ( 3 H/ 3 He) and chlorofluorocarbon (CFCs, CFC–11, CFC–12, CFC–113) data are used to date the young fraction in groundwater mixtures from a karstic limestone aquifer near Valdosta, Georgia, where regional paleowater in the Upper Floridan aquifer receives recharge from two young sources—the flow of Withlacoochee River water through sinkholes in the river bed, and leakage of infiltration water through post-Eocene semi-confining beds above the Upper Floridan aquifer. In dating the young fraction of mixtures using CFCs, it is necessary to reconstruct the CFC concentration that was in the young fraction prior to mixing. The 3 H/ 3 He age is independent of the extent of dilution with older ( 3 H-free and 3 He trit -free) water. The groundwater mixtures are designated as Type-1 for mixtures of regional paleowater and regional infiltration water and Type-2 for mixtures containing more than approximately 4% of river water. The fractions of regional paleowater, regional infiltration water, and Withlacoochee River water in the groundwater mixtures were determined from Cl − and δ 18 O data for water from the Upper Floridan aquifer at Valdosta, Georgia. The chlorofluorocarbons CFC–11 and CFC–113 are removed by microbial degradation and/or sorption processes in most anaerobic (Type-2) groundwater at Valdosta, but are present in some aerobic Type-1 water. CFC–12 persists in both SO 4 -reducing and methanogenic water. The very low detection limits for CFCs (approximately 0.3 pg kg −1 ) permitted CFC–11 and CFC–12 dating of the fraction of regional infiltration water in Type-1 mixtures, and CFC-12 dating of the river-water fraction in Type-2 mixtures. Overall, approximately 50% of the 85 water samples obtained from the Upper Floridan aquifer have CFC–12-based ages of the young fraction that are consistent with the 3 H concentration of the groundwater. Because of uncertainties associated with very low 3 H and 3 He content in dilute mixtures, 3 H/ 3 He dating is limited to the river-water fraction in Type-2 mixtures containing more than about 10% river water. Of the 41 water samples measured for 3 H/ 3 He dating, dilution of 3 H and low 3 He concentration limited 3 H/ 3 He dating to 16 mixtures in which 3 H/ 3 He ages are defined with errors ranging from ±2 to ±7.5 a (1 σ). After correction for dilution with (assumed) CFC-free regional infiltration water and regional paleowater in the Upper Floridan aquifer, adjusted CFC–12 ages agree with 3 H/ 3 He ages within 5 a or less in 7 of the 9 co-dated Type-2 mixtures. Tritium data and dating based on both CFC–11 and CFC–12 in Type-1 mixtures indicate that travel times of infiltration water through the overlying Post-Eocene semi-confining beds exceed 35 a. The CFC and 3 H/ 3 He dating indicate that the river fraction in most groundwater entered the groundwater reservoir in the past 20 to 30 a. Few domestic and municipal supply wells sampled intercept water younger than 5 a. Calculated velocities of river water in the Upper Floridan aquifer downgradient of the sinkhole area range from 0.4 to 8.2 m/d. Radiocarbon data indicate that ages of the regional paleowater are on the 10 000-a time scale. An average lag time of approximately 10 to 25 a is determined for discharge of groundwater from the surficial and intermediate aquifers above the Upper Floridan aquifer to the Withlacoochee River.

Georgia↗

USGS2016

Hydrologic controls on nitrogen cycling processes and functional gene abundance in sediments of a groundwater flow-through lake

The fate and transport of inorganic nitrogen (N) is a critically important issue for human and aquatic ecosystem health because discharging N-contaminated groundwater can foul drinking water and cause algal blooms. Factors controlling N-processing were examined in sediments at three sites with contrasting hydrologic regimes at a lake on Cape Cod, MA. These factors included water chemistry, seepage rates and direction of groundwater flow, and the abundance and potential rates of activity of N-cycling microbial communities. Genes coding for denitrification, anaerobic ammonium oxidation (anammox), and nitrification were identified at all sites regardless of flow direction or groundwater dissolved oxygen concentrations. Flow direction was, however, a controlling factor in the potential for N-attenuation via denitrification in the sediments. Potential rates of denitrification varied from 6 to 4500 pmol N/g/h from the inflow to the outflow side of the lake, owing to fundamental differences in the supply of labile organic matter. The results of laboratory incubations suggested that when anoxia and limiting labile organic matter prevailed, the potential existed for concomitant anammox and denitrification. Where oxic lake water was downwelling, potential rates of nitrification at shallow depths were substantial (1640 pmol N/g/h). Rates of anammox, denitrification, and nitrification may be linked to rates of organic N-mineralization, serving to increase N-mobility and transport downgradient.

Massachusetts↗

USGS2020

Tracing the uptake of Hg(II) in an iron-reducing bacterium using mercury stable isotopes

Anaerobic microorganisms play a key role in the biological mercury (Hg) cycle due to their ability to produce bioaccumulative neurotoxic methylmercury (MeHg). However, despite recent advances, how bacteria accumulate inorganic Hg [Hg(II)] prior to methylation is largely unknown. In this study, we applied Hg stable isotopes to measure changes in cellular compartments of Geobacter sulfurreducens and a nonmethylating mutant strain to investigate intracellular transport of Hg(II). Both strains accumulated intracellular Hg(II) that was lower in δ 202 Hg relative to dissolved extracellular Hg(II), demonstrating mass-dependent fractionation during uptake. Hg reduction by the mutant strain (50% Hg concentration loss in 24 h) resulted in higher δ 202 Hg values of cellular Hg than in wild-type cells. Further observations showed increasing δ 202 Hg values in dissolved extracellular MeHg and Hg(II) but decreasing δ 202 Hg values of intracellular Hg(II) in wild-type G. sulfurreducens suggesting that external Hg pools may be the proximate source of Hg for methylation in this bacterium. This investigation demonstrates that cellular uptake is comprised of multiple processes and transformations that influence Hg(II) prior to methylation, which can impart distinct isotopic signatures to Hg(II) and MeHg pools in the environment.

Environmental Science and Technology Letters↗