H13G-1663
Effect of mixing on oxygen reduction, denitrification, and isotopic fractionation in a heterogeneous aquifer in an agricultural setting.
Large fluxes of nitrate in agricultural settings threaten water quality worldwide. In groundwater, the transport and fate of anthropogenic nitrate can be influenced by denitrification, which occurs under anaerobic conditions. It is difficult to characterize denitrification because (1) waters from multiple flow paths commingle in the aquifer due to dispersion and (2) a sample typically includes water from multiple lithologies. For a site near Merced, California, geostatistical simulations, flow modeling, and backward random walk particle tracking were applied to simulate the influence of mixing on apparent reaction rates, redox boundaries, and fractionation of stable isotopes of nitrogen. Results show that dispersion has a strong effect on the distribution of redox indicators. When reactions are rapid, mixing can create the appearance of gradual redox transitions that mask the actual reaction rates and the degree of isotopic fractionation.
H13G-1664
Denitrification-Coupled Iron(II) Oxidation: A Key Process Regulating the Fate and Transport of Nitrate, Phosphate, and Arsenic in a Wastewater-Contaminated Aquifer
Denitrification in the subsurface is often viewed as a heterotrophic process. However, some denitrifiers can also utilize inorganic electron donors. In particular, Fe(II), which is common in many aquifers, could be an important reductant for contaminant nitrate. Anoxic iron oxidation would have additional consequences, including decreased mobility for species like arsenic and phosphate, which bind strongly to hydrous Fe(III) oxide. A study was conducted in a wastewater contaminant plume on Cape Cod to assess the potential for denitrification- coupled Fe(II) oxidation. Previous changes in wastewater disposal upgradient of the study area had resulted in nitrate being transported into a portion of the anoxic zone of the plume and decreased concentrations of Fe(II), phosphate, and arsenic. A series of anoxic tracers (groundwater + nitrate + bromide) were injected into the unaffected, Fe(II)-containing zone under natural gradient conditions. Denitrification was stimulated within 1 m of transport (4 days) for both low and high (100 & 1000 μM) nitrate additions, initially producing stiochiometric quantities of nitrous oxide (>300 μM N) and trace amounts of nitrite. Subsequent injections at the same site reduced nitrate even more rapidly and produced less nitrous oxide, especially over longer transport distances. Fe(II) and nitrate concentrations decreased together and this was accompanied by an increase in colloidal Fe(III) and decreases in pH, total arsenic, and phosphate concentrations. All plume constituents returned to background levels several weeks after the tracer tests were completed. Groundwater microorganisms collected on filters during the tracer test rapidly and immediately reduced nitrite and oxidized Fe(II) in 3-hr laboratory incubations. Several pure cultures of Fe(II)-oxidizing denitrifying bacteria were isolated from core material and subsequently characterized. All of the isolates were mixotrophic, simultaneously oxidizing organic carbon and Fe(II). These results demonstrate that denitrification-coupled Fe(II) oxidation can readily occur within anoxic groundwater; that it could be key to controlling iron speciation in certain situations and that the outcome of the process can impact the mobility of other chemical species that are not directly involved in the oxidation-reduction reaction.
H13G-1665
Geologic and Hydrologic Controls on Denitrification in Denitrifying Layer-Bearing Groundwater Systems at Watershed Scales
Nitrate contamination of ground water from agricultural practices may degrade groundwater quality and contribute to the eutrophication of surface water. Field evidence and geochemical analysis indicate removal of nitrate by denitrification in denitrifying layers (redox zones) situated in groundwater systems. The potential for denitrification in the denitrifying layers may considerably impacted nitrate levels in the aquifers and, in turn, the streams of the pertinent watersheds. Denitrification in the aquifers may be influenced by different geologic and hydrologic factors such as extent and thickness of the denitrifying layer below the watersheds, hydraulic conductivity of the geologic formations, and the regional dip of the underlying strata. In this study, groundwater flow and nitrate fate and transport are modeled, by using a powerful computer modeling package, in the regional aquifers beneath the agricultural watersheds at a specific site. The groundwater flow and nitrate fate & transport numerical models were calibrated and verified successfully by implementing an integrated modeling approach. The models simulated a significant reduction in long-term nitrate loading into the streams from the agricultural practices due to denitrification in aquifer sediments. Sensitivity analysis through numerical simulations has been performed to investigate the relative significance of various geologic and hydrologic factors possibly influencing denitrification in aquifers. The interesting modeling findings have been achieved from this investigation that may have implications on the effective controls of nitrate contamination of groundwater and surface water systems in watershed scales.
H13G-1666
A metabolism-based modeling approach to redox transformations in a nitrate- and uranium- contaminated aquifer at Oak Ridge, Tennessee, USA
Microbial metabolisms control groundwater chemistry and influence the fate of both natural and anthropogenic contaminants in aquifers. Recent advances in groundwater microbiology revealed diverse microorganisms competing for limited energy resources in aquifers. To predict accurately the kinetics of redox transformations, we took into account both microbial metabolic diversity and limited energy resources and developed a new biogeochemical reaction model for microbially catalyzed redox reactions in a nitrate- and uranium-contaminated aquifer at the FRC site, Oak Ridge, east Tennessee. The new model describes the metabolisms of diverse microorganisms in the aquifer as a network of metabolic reactions. It considers various interactions among the pathways, such as inhibition, syntrophism, and competition. The model predicts the rates of each pathway by taking into account the balance of electrons, energy, and nutrients during microbial metabolisms. To incorporate the close interactions between microorganisms and aquifer environments, the new model also considers geochemical reactions pertinent to microbial metabolisms, including abiotic redox reactions, mineral precipitation and dissolution, surface adsorption, and ion exchange. The catalytic capacity of individual branch of the network (i.e., kinetic parameters and biomass concentration of functional groups) is then determined by fitting the modeling results to a laboratory microcosm experiment on sediments from the FRC site. The best-fit model is then applied to predict the kinetics of microbial metabolisms during a field push-pull test on the aquifer. The new model predicts well the progress of redox transformations observed during the field experiment, including denitrfication, ammonification, iron reduction, sulfate reduction, and methanogenesis. The results revealed the key roles of metabolic diversity and energy availability in controlling the kinetics of redox reactions in aquifers. These results also demonstrated the potential of the new model to extrapolate the results of laboratory experiments directly to natural environments.
H13G-1667
Hydraulic control for manipulating subsurface conditions for in situ experiments of uranium(VI) bioremediation
A field test on in-situ subsurface bioremediation of uranium (VI) is underway at the Y-12 National Security Complex in the Oak Ridge Reservation, Oak Ridge, TN. A four-well system, including two downgradient extraction and two upgradient injection wells were installed to create an inner cell, which functioned as the treatment zone, nested within an outer cell, which protected the inner cell from the influence of regional flow. The proposed four- well system has several advantages in the subsurface flow field manipulation: (1) the recirculation ratio within the nested inner cell is less sensitive to the regional flow direction; (2) a transitional recirculation zone between the inner and outer cells can capture flow leakage from the inner cell, minimizing the release of untreated contaminants; (3) the size of the recirculation zone and residence times can be better controlled within the inner cell by changing the pumping rates. A three-phase remediation strategy was applied in this experiment. It included first removing nitrate prior to stimulation of U(VI) reduction, then adjusting the pH to levels favorable for activity of U(VI)-reducing bacteria, i.e., to about neutral values, and finally adding electron donor to the in-situ reactor to foster reduction and immobilization of U(VI). Tracer tests and bioremediation experiments demonstrated that the designed multiple-well system and the experimental strategy were successful in creating favorable subsurface chemical and biological conditions for uranium bioremediation.
H13G-1668
Sulfur Isotopes as Indicators of Bacterial Sulfate Reduction Processes Influencing Field Scale Uranium Bioremediation
An in-situ acetate amendment at a DOE Uranium Mill Tailings Remedial Action (UMTRA) site near Rifle, CO demonstrated successful reduction of aqueous U(VI), to less soluble U(IV) through stimulated microbial activity. U(VI) reduction rates were highest during iron reduction and decreased with the onset of sulfate reduction. However, sustained U(IV) attenuation was observed following subsequent termination of the acetate amendment. These findings illustrate the importance of the transition between iron and sulfate reducing conditions in stimulating bioreduction of uranium. The sulfur isotope compositions of sulfate and sulfide were measured through this transition in order to explore the utility of these data in tracking the extent of microbial sulfate reduction and to assess the stability of sulfide precipitates. Samples for isotopic analyses and aqueous measurements of sulfate, ferrous iron, U(VI) and acetate were collected in one background well and three monitoring wells down-gradient of the acetate injection. Results show an increase of up to 7‰ in the δ34S of sulfate at the onset of sulfate reduction, followed by a return to background δ34S values of -8‰ following cessation of the acetate amendment. The δ34S values of sulfide increased from roughly -20‰ at the onset of sulfate reduction to a maximum of -0.8‰ during peak sulfate removal, followed by a gradual return to values of roughly -28‰ upon cessation of the acetate amendment. These data present a unique perspective on the processes governing the bioreduction experiment in that the sulfate isotopes are a function of both transport and mixing processes, whereas the sulfide isotopes represent biogenic sulfide that is rapidly removed from the aqueous phase. Thus a comparable enrichment in sulfate isotopic data noted in the closest and furthest wells from the injection gallery suggest bioreduction in both of these locations, while a larger increase in sulfide isotopic values in the closest well indicates greater rates of sulfate reduction closer to the injection gallery. In addition, a steady decline in δ34S of sulfide concurrent with increased sulfide concentrations following cessation of acetate amendment suggests that this increase is not a result of reoxidation of precipitated FeS species. FeS precipitates formed during the height of sulfate reduction therefore appear to be stable on a timeframe of months following acetate amendment and may support the stability and long-term sequestration of precipitated U(IV).
H13G-1669
Redox and Recharge Controls on Sub-Surface Radium-Isotope Mobility.
Groundwater geochemical systems are known to be highly complex, driven by a wide variety of physical, chemical and biological mechanisms. A north-south transect of wells in the Sheehan well-field (Memphis, TN) that border an aquitard window were used to sample the confined Memphis Aquifer. Short-lived (223Ra and 224Ra) and long-lived isotopes (226Ra and 228Ra) of radium were examined in the colloidal and dissolved groundwater phases to constrain the geochemical mechanisms affecting the Memphis Aquifer. Measured isotope activities increase from the periphery to the center of the measured transect by 0.5 to 1.0 orders of magnitude for all Ra-isotopes. Wells east of the transect show an additional increase of 0.3 orders of magnitude for short-lived Ra-isotopes. Additionally, 223Ra/226Ra and 224Ra/226Ra activity- ratios increased as a function of distance from the local aquitard window. The results suggest 1) low Ra-isotope activities close to the aquitard window by association with the solid-phase, and 2) the increase of short-lived over long-lived Ra-isotope activities further away from the aquitard window due to alpha-recoil.
H13G-1670
The role of redox conditions in controlling arsenic release from a tailings aquifer
At an abandoned arsenic mine both geology and hydrology play important roles in controlling redox conditions, which in turn influence arsenic release from minerals to groundwater. The shallow aquifer at the site is comprised of mine tailings, which contain a variety of arsenic-bearing minerals, including arsenopyrite, scorodite, and As-rich iron oxides. Based on sequential extraction, most of the As in the mine tailings (> 80%) is associated with iron oxides. Groundwater is microaerophillic but recharge introduces DO almost instantaneously to the shallow aquifer, which contributes to oxidation of arsenopyrite in the aquifer. Because conditions in the aquifer are not strongly reducing, release of As from iron oxides due to reductive dissolution does not appear to be a significant As release mechanism. However, desorption may play a role in releasing high concentrations (up to 5 mg/L) of arsenic to groundwater.
H13G-1671
Effects of Chlorine Promoted Oxidation on Arsenic Release from Sulfide Minerals
High arsenic concentrations (>100 ppb) have been measured in wells completed in the Ordovician St. Peter sandstone aquifer of eastern Wisconsin. The primary source of arsenic is As-bearing sulfide minerals within the aquifer. Periodic disinfection of wells by chlorination may facilitate arsenic release to groundwater by increasing the rate of sulfide mineral oxidation. During typical well disinfection procedures, aquifer solids exposed along uncased portions of wells remain in direct contact with chlorine disinfection solutions for up to twenty-four hours. Due to the redox sensitivity of arsenic mobility in groundwater, it is important to evaluate the effect of repeatedly adding oxidizers to an arsenic impacted aquifer system. This study focuses on abiotic processes that mobilize arsenic from the solid phase during controlled exposure to chlorinated solutions. Two St. Peter samples with As concentrations of 21 and 674 ppm were selected for the experiments. Before reaction, the aquifer mineralogy is characterized using scanning electron microscopy (SEM) and electron microprobe analysis (EMPA). The samples are then reacted with solutions of 60 mg/L free chlorine, 1200 mg/L free chlorine, or nanopure water (control) at pH 7.0 and pH 8.5. These parameters represent typical solution chemistries present within the wells after disinfection. Solutions are sampled periodically during the experiments and analyzed for As, Fe, other trace metals such as Co, Mo, Cr, and Ni, and sulfate. Analysis of the post-reaction solids using SEM, EMPA, laser ablation ICP-MS and Raman techniques are used to document the changes in mineralogy due to chlorination and to document which solid phases contain As.
H13G-1672
Depth Transects of Sediment Age, Reflectance, and Aquifer Arsenic in the Bac Bo Plain, Vietnam: Implications for Groundwater Arsenic Heterogeneity in the Red River Delta
Shallow aquifer groundwater arsenic heterogeneity is well documented in many of the fluvial regions of Asia. To this day, the cause for the heterogeneity remains poorly understood in part because of the heterogeneity of sediment properties inherent to a young floodplain depositional environment. In April 2006, a needle-sampler device was used to obtain depth transects of both sediment and porewater samples as the first step towards understanding the heterogeneous subsurface environment. Depth transects were taken between sites with opposing trends in tube-well arsenic, e.g. low As cluster to high As cluster, in a stable, fault controlled river bend in Van Phuc, Vietnam, to determine how sediment properties such as grainsize and reflectance relate to dissolved arsenic. Luminescence dating of aquifer sands and He3/H3 dating of the groundwater was conducted to investigate how aquifer redox conditions and groundwater arsenic evolve in Van Phuc's river-bend over time. Results indicate that localized zones of sediments enriched in leachable arsenic exist at depth within the aquifer and provide a mostly local source of arsenic. It also appears that groundwater flow encountering sediments of different ages can account for some of the observed spatial patterns of groundwater heterogeneity. Compiled with observations that reducing groundwaters with both low dissolved and extractable arsenic exist in the upper deltaic region, it appears that heterogeneities in shallow floodplain arsenic can be driven by magnitude differences in sediment-labile arsenic. Finally, given that these localized packages of arsenic enrichment correspond to a particular in-filling type of facies, it appears that fluvial cycling along the delta is the underlying cause of arsenic heterogeneity in the shallow floodplain.
H13G-1673
Seasonal Variation in Arsenic Speciation in a Shallow Aquifer
Seasonal variation in arsenic speciation and concentration in sediments in a shallow aquifer were studied with respect to changes in water table elevation and rainfall. Sediment cores were collected at different times from 2004 to 2007 at a marsh site adjacent to San Francisco Bay (in East Palo Alto, CA), which experiences a strong wet winter/dry summer seasonality. The site is a former pesticide manufacturing plant that has undergone remediation and surface capping. Mobilization of post-remediation residual arsenic in sediments is inhibited by natural subsurface attenuation. This study examines the dynamics of seasonal changes on groundwater level and subsurface redox conditions, and its potential impact on arsenic mobilization. Cores (3-cm diameter) were collected from depths of 2.7-3.5 m, intersecting the range of seasonal elevation change in the water table. Groundwater level from a nearby well was monitored with continuous data logging. Sediment samples were analyzed for total and extractable element concentrations and characterized by arsenic and iron synchrotron X-ray absorption spectroscopy (XANES and EXAFS). Three distinct redox zones are recognized spectroscopically within the shallow aquifer: a reduced zone in unsaturated sediments (~0.5-1.5 m depth) where arsenic is present as As-sulfide phases (orpiment or realgar); a transition zone between reduced and oxidized zones at the depth of water table (~1.5 m ±0.5 m) with mixed arsenic oxidation states; and an oxidized zone permeated with oxic groundwaters (~1.5-3 m depth) where only As(V) is present. Sediment samples from the reduced zone had arsenic concentrations from 50 to 150 mg kg-1. Arsenic concentrations decreased to a minimum in the transition zone to 20 mg kg-1, and reached a maximum in the oxidized zone (around 200 mg kg-1). Arsenic XANES spectra showed a progressive change from mostly arsenic sulfides in the upper reduced sediments (component sum ~100%\) to a mixture of orpiment and As(V) deeper in the reduced zone (80%\ orpiment, 20%\ As(V)). In the transition zone, the As(III)/(V) ratio varied with depth, in general from 90%\ As(III)/10%\ As(V) at 1.30 m to 30%\ As(III)/60%\ As(V) at 2.00 m. In samples from between 2.50-2.70 m, As(V) was the main species, although the spectroscopic data indicated a small amount of orpiment and As(III). Iron XANES spectra indicated the formation of pyrite in some samples with arsenic sulfides. Comparisons of annual data showed that the limits of the transition zone were shifted vertically due to regional precipitation and elevation changes in the water table. During the dry season when water table elevation was at a minimum, sorbed As(III) was the dominant species in the partially saturated transition zone. In the winter, water table rise causes oxidative dissolution of sulfides in the flooded reduced zone, with persistent sulfide minerals at shallower depths that are not always saturated annually.
H13G-1674
Biogeochemical Cycling at Soil Interfaces in the Vadose Zone and its Impact on Hydraulic Conductivity
Much research has focused on understanding and predicting chemical fate and transport in subsurface systems to protect drinking water reserves and ecosystem health. However, chemical changes that occur in the unsaturated zone due to processes such as mineral-water interactions, desorption, or biogeochemical cycling have often been neglected. In particular, the effects of soil structure (i.e. layers, lenses, macropores, or fractures) on these processes remain poorly understood. This study focuses on characterizing the linkages between geochemical processes, hydrologic flow, and microbial activity in the vadose zone using packed soil columns. We constructed three laboratory soil columns: a homogenized medium-grained sand, homogenized organic-rich silty clay, and a sand-over-clay layered column. Both upward and downward infiltration of water was evaluated during experiments to simulate rising water table and rainfall events respectively. In situ collocated probes measured soil water content, matric potential, and Eh. Water samples extracted by lysimeter were analyzed for major cations and anions, ammonium, organic acids, alkalinity, Fe2+, and total sulfide. Enhanced biogeochemical cycling was observed in the layered column. For example, concentrations of the electron acceptor sulfate were two-fold greater in the layered column than in either of the homogeneous columns likely due to increased oxidation/reduction reactions. Rainfall events enhanced denitrification in the layered column through the addition of NO3- via enhanced ammonium oxidation. Biogeochemical cycling was directly linked to hydrologic flow and varied as a function of water infiltration direction (upward/downward). Enhanced biogeochemical activity produced mineral crusts and biofilms that decreased overall hydraulic conductivity. Preliminary results suggest that changes in the vadose zone occur too rapidly for the system to achieve redox equilibrium and suggest that a new conceptual framework to analyze and predict biogeochemical cycling in the vadose zone is needed.
H13G-1675
Microbially Induced Temperature Changes in a Petroleum Hydrocarbon Plume
The degradation reactions of organic contaminants are often exothermic. Given this, the degradation of organic contaminants in an aquifer should produce measurable temperature increases if the heat is generated faster than it is dissipated. The groundwater contaminant plume at a crude oil spill site near Bemidji, Minnesota, USA, has been undergoing aerobic and anaerobic biodegradation for 28 years. At this site, the theoretical degradation of 100 mg/L phenol, a representative compound, under aerobic conditions could generate a 2°C increase in groundwater temperature with no heat loss and an aquifer heat capacity of 2,494 J/L-°C. The temperature in the aquifer was measured with an accurate thermistor (<e;±0.01°C) that was lowered to multiple depths in 13 monitoring wells along a groundwater flowpath. The measurements were taken from 0.15 to 12.62 m below the water table. Temperatures ranged from 6.35°C in the background aquifer to 9.19°C just under the crude oil source. These data revealed a thermal plume co-located with a previously observed area of BTEX biodegradation under iron-reducing and methanogenic conditions. The results indicate that evidence of exothermal microbial reactions within contaminant plumes can be detected using sensitive and detailed temperature measurements in wells.
H13G-1676
Electron Donor Potential of Eastern North Dakota Shale Formations
We have a network of 16 in situ mesocosms (ISMs) used to study aquifer denitrification at 9 sites in North Dakota and Minnesota. The site in the Elk Valley aquifer in northeastern North Dakota has the highest denitrification rates and the greatest concentration of electron donors (organic carbon ~0.4%, pyrite as S ~0.4%, and ferrous iron ~0.3%) in the sediments. In contrast, denitrification rates at our other ISM sites are lower (or even below detection), as are the electron donor concentrations in the sediments. Knowing that the sediments at all of our ISM sites were placed as outwash during the last Wisconsinan glaciation (~12ka), we wonder what caused the variation in electron donor supplies in our aquifers. It has been suggested that variations is the electron donor concentrations in the various Late Cretaceous shale strata exposed during glaciation may help explain the variation in electron donor concentrations in overlying aquifers formed nearby. The purpose of this study is to evaluate the amount of electron donors available in these bedrock units. Bedrock samples (n = 39) from 20 sites in eastern North Dakota were obtained by drilling during the summer of 2006 in conjunction with the North Dakota State Water Commission. Samples were frozen before analysis for bulk mineralogical content by X-ray diffraction, organic carbon, pyrite as inorganic S, and ferrous iron contents. It was hypothesized that the Pierre Shale would have the highest donor concentration, but it appears that other formations, such as the Carlile and Greenhorn contain higher concentrations of electron donors. Organic carbon concentrations in the Pierre (< 0.01% to 1.0%) are relatively low while concentrations in the Carlile (3.5% – 6.5%) and Greenhorn (~8.5%) are significantly higher. Pyrite as inorganic S concentrations in the Pierre (< 0.01% to 0.2%), Carlile (0.3% – 0.5%), and Greenhorn (~0.5%) are similarly distributed. In the future, it may be possible to create a qualitative index (low, med, high) of aquifer denitrification capacity based on the probable source of the parent material. This index could then be used to focus, in a cost-effective fashion, more extensive and expensive geochemical analysis on specific aquifers or sites.
H13G-1677
Evaluation of Reactive Mixtures for Treatment of Mine Drainage From a Waste Rock Storage Area in Northern Saskatchewan, Canada
A column experiment has been conducted to evaluate the performance of three reactive mixtures which may be used in a permeable reactive barrier (PRB) for the treatment of low quality mine drainage water from a waste rock storage area in northern Saskatchewan, Canada. The key element of concern in the drainage water is dissolved Ni, which occurs at approximately 13 mg/L. The water is low pH ~4.3, oxidized, contains high concentrations of dissolved sulfate (4400-4750 mg/L), Al (45 mg/L), Zn (3 mg/L), Co (3 mg/L) and relatively low concentrations of other dissolved heavy metals and iron. Three columns, each containing one of the mixtures, were constructed: column A (peat/lime/limestone/gravel), column B (peat/zero valent iron (ZVI) filings (20%/vol)/limestone/gravel), and column C (peat/ZVI filings (10%/vol)/limestone/gravel). The experimental results have shown that the mixtures promote bacterially-mediated sulfate reduction and metal removal by precipitation of metal sulfides, metal precipitation, and adsorption under relatively high pH conditions (pH of 7 to 8). Reducing conditions (Eh of 0 to -200 mV) have developed in all of the columns, from the highly oxidized influent water (Eh of +500 to +600 mV). Hydrogen sulfide is detected in the effluent water, and dissolved sulfate concentrations decrease by several hundred mg/L. Based on sulfate removal, sulfate reduction occurs more strongly in columns B and C than column A. All of the columns are removing Ni to below the limit of detection (typically < 0.01 mg/L); however, the removal rate in column A is slower than in columns B and C and has decreased over time. Most other metals are removed to low concentrations in all of the columns. The results suggest that while the longevity of mixtures including ZVI will be much longer than mixtures containing only peat, considering economic aspects, the PRB consisting of only peat could also be an alternative option, if breakthrough time can be predicted and replacement of peat can be conducted in a timely manner. This study shows that the use of reactive mixtures that facilitate microbial activities and redox reactions in subsurface could be a valuable means to remove various metal contaminants originated from mine drainage sites.
H13G-1678
A method for determining ultra-low concentration of acid-volatile sulfides in sediments and its implication for redox status of aquifer
Relatively high concentrations (7~140 £gmoleg-1) of acid volatile sulfides (AVS), commonly found in marine or organic-rich sediments, were generally determined by methods with detection limit of 1 £gmoleg-1. However, AVS in pristine fresh-water sediments is much lower than 1 £gmoleg-1 and is undetectable by existing conventional methods. We report a new method, the detection limit of which is as low as 1 nmoleg-1, for measuring ultra-low AVS in sediments. The improved experimental procedures include: (1) adding an anti-oxidant buffer to prevent samples from oxidation, (2) shortening the time of trapping hydrogen sulfide gas, and (3) using ¡§methylene blue method¡¨ instead of ¡§iodometric titration¡¨ to detect sulfide concentration. By using the method, we found that AVS in sediments from freshwater aquifers at the Huwei site on the Choshui fan-delta, western Taiwan, falls in the range of 1~61 nmoleg-1. We suggested that the vertical variations of sediment AVS were controlled by redox reactions occurred in each aquifer, groundwater table fluctuations induced by Quaternary sea level change, and lithology of each aquifer. The data of ultra-low concentration of AVS can provide a new insight into hydrochemical evolution of coastal fresh-water aquifers.