H33H-01 13:40h
Declining Temporal Variability of Major Ions Concentrations but not Arsenic Concentration as Function of Groundwater age in the Ganges-Brahmaputra-Meghna Delta of Bangladesh
Elevated arsenic concentrations in groundwater of the Ganges-Brahmaputra-Meghna (GBM) delta region pose a devastating health threat. One issue of considerable uncertainty is the magnitude of temporal variations in groundwater As concentrations. Despite the importance of the issue, there are very few high quality, long-term, time series data for groundwater As from the region. This presentation addresses the issue with a series of samples from Araihazar, Bangladesh, that were collected biweekly or monthly between 2001 and 2004 and analyzed by high resolution inductively-coupled plasma mass spectrometry. The method is precise (better than 5 percent long-term reproducibility for As and generates data for 32 other elements. Our study site comprises six nests of monitoring wells installed in an area where the hydrology and geochemistry have been carefully studied. Dissolved As concentrations range from $<$ 1 ug/L to 600 ug/L in the shallow Holocene aquifer (upper 30 m) and mostly $<$ 1 ug/L (As $<$ 1-6.6 ug/L) in the deep Pleistocene aquifer ($>$ 30 m depth). Dating of shallow aquifer water resulted in tritium-helium ages ranging from $<$ 1 yr to $>$ 40 yr. Deep Pleistocene aquifer water was 14C-dated and found to be up to thousands of years old. In the shallow aquifers, variations in groundwater As concentrations in all but 2 out of 28 wells did not display any discernable pattern linked to seasonal water table fluctuations. In addition, As concentration fluctuations were limited (0-15 percent) in all 28 wells. The two wells displayed seasonal pattern of As variations (10-15 percent) were particularly shallow (depths 6 m and 9 m). Redox-sensitive indicators such as Fe and Mn in both wells also displayed seasonal pattern. In contrast to As, most of wells with tritium-helium ages $<$21 years showed much larger variations in dissolved Cl- concentrations (10-43 percent) and total dissolved cations (5-31 percent) concentrations. Temporal variability was considerably reduced for groundwater $>$21 years old: 6-7 percent and 10-14 percent, major cations and anions, respectively. This suggests that despite the apparent influence of recharge on major ion concentrations, dissolved As concentrations in most shallow aquifers are relatively well buffered. No notable variability ($<$2 percent) in As concentrations was observed in groundwaters from the Pleistocene aquifer.
H33H-02 13:55h
The Effect Of Floodplain Evolution On The Distribution Of Arsenic In The Shallow Aquifer Of Araihazar, Bangladesh
The proliferation of geogenic arsenic has been documented in several fluvial systems, including areas of Inner Mongolia and the deltas off the Amazon River and in the Bengal Basin. Currently, however, our understanding of the arsenic cycling within many of these systems has yet to be explained geologically. In this study, we focus on determining how floodplain evolution affects the arsenic heterogeneity of the groundwater in Araihazar, Bangladesh. During two successive field studies, more than 100 augers were obtained in an effort 1) to correlate 10$^{3}$m-scaled distributions of groundwater arsenic to patterns of modern and relict stream channels, 2) test whether fine-scale (10$^{1-2}$m) patterns of groundwater arsenic are controlled by local fluvial subenvironments such as crevasse splay deposits, point bars, and levees, 3) to determine if anomalies in fine-scale distribution of groundwater are geologically controlled by lateral discontinuities in stratigraphy, and 4) examine the effects of anthropogenic floodplain modification on arsenic within the shallow aquifer. Observations from these surveys integrated within a GIS digital elevation model suggest that villages with low levels of tubewell arsenic are localized along sandy relict channel levee and bar features. Geochronology of the floodplain sediments determined using optically-stimulated luminescence and Pb-210 activity are in agreement with one another and indicate that a major avulsion event occurred ~500ya. This abandonment phase left behind a floodplain with several low-lying areas that have been since then silting in. Spatial analysis of the combined data indicates that villages proximal to these silting-in areas host higher levels of tubewell arsenic. Our findings also suggest that there are anthropomorphic disturbances on the geomorphology of the area, as villages in the lower-lying areas were observed building-up their elevation using fine-grained, nearby sediments. Ultimately, this study suggests that the heterogeneity of arsenic within Araihazar, as well as other dynamic fluvial systems, originates from a variety in facies (i.e., channel fill, levee, and infilling floodplain) deposited in response to the evolution of the floodplain.
H33H-03 14:10h
Depth Transects Of Sediment Properties And Aquifer Arsenic In Araihazar, Bangladesh: Implications For Arsenic Mobilization
Shallow aquifer groundwater arsenic heterogeneity is well documented in the floodplain of Bangladesh. The cause for this heterogeneity is poorly understood in part because of the heterogeneity of sediment properties inherent to a young floodplain depositional environment. In January 2004, a Needle-sampler device was used to obtain depth transects of both sediment and pore water samples as the first step towards understanding the heterogeneous subsurface environment. A total of 15 depth transects sampled at 0.3 m intervals to a depth of 15 m below the surface were obtained between sites with opposing trends in tube-well arsenic, e.g. low As cluster (average As = 2 $\mu$g/L) to high As cluster (average As = 383 $\mu$g/L). Labile sediment properties, including phosphate-extractable As, FeII/Fe ratio as extracted by 1 M HCl, and diffuse spectral reflectance were obtained by field analysis. Filtered porewater samples were also analyzed by HR ICP-MS for dissolved arsenic, while sediments were further evaluated for bulk organic C (e.g. loss upon ignition) and grain size. Results indicate significant correlations exist (P $<$ 0.01) between grainsize, reduced iron, sediment reflectance, and organic matter. In addition, these parameters likewise covary (P $<$ 0.05) with lognormal trends in leachable and dissolved fractions of aquifer arsenic. When taken in context with the hydrology of the area, the resultant pattern of sediments and aquifer chemistry indicates that localized zones of sediments enriched in leachable arsenic exist at depth within the aquifer and provide a mostly local source of arsenic. Thus, villages with elevated levels of arsenic at relatively shallow depths within the floodplain are located 1) over sites with reducing groundwaters and high levels of leachable arsenic or 2) down the flowpath from sediment with elevated extractable As. 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.
H33H-04 14:25h
Fate of Arsenic Within the Lower Humboldt River Basin, Nevada, Under High Flow Conditions
The Humboldt Wildlife Management Area (HWMA) has been identified as a wetland ecosystem in decline because of the dominance of salt-tolerant submergent vegetation, loss of indigenous fauna over time, and episodic bird kills. Previous investigations within the HWMA indicated that certain trace elements, including arsenic, were greater than geochemical background concentrations, and in some cases, exceeded protective ecological criteria established by the U.S. Environmental Protection Agency. From 1998 through 2001, arsenic was characterized within the lower Humboldt River system during high-flow conditions. The lower Humboldt River and associated irrigation drains are the primary sources of surface water for HWMA. Arsenic in surface-water samples occurred primarily as dissolved arsenate with concentrations ranging from 14 to 238 ppb. Arsenic concentrations increased downriver with the highest concentrations occurring in Humboldt Lake waters. Although arsenite concentrations were less than U.S. Environmental Protection Agency ecological criteria, the criterion for the watering of livestock (200 ppb) was exceeded in wetland surface waters near the mouth of the river. Arsenic concentrations in wetland sediments ranged from 19 to 57 ppm. Pore-water collected from these sediments contained arsenic concentrations ranging from 90 to 524 ppb of which 5 to 74 percent was arsenite. The Canadian Interim Sediment Quality Guideline (5.9 ppm) and consensus based toxicity-threshold arsenic concentration (9.79 ppm) were exceeded in all wetland-sediment samples. Arsenic concentrations in surface-water samples collected from the outflow of the Humboldt Sink indicate that arsenic may be flushed from the HWMA wetland ecosystem under high-flow conditions.
H33H-05 14:40h
Arsenic speciation in the shallow aquifer, Churchill County, Nevada
Wells that tap the shallow aquifer in Churchill County, Nevada over a period of seven months have shown significant fluctuations in the As(III):As(V) proportion. Eleven wells were chosen that showed substantial fractions of each species, and were sampled once-monthly near the farming town of Fallon beginning in February, 2004 to the present. The region has an arid climate receiving approximately eight inches of rain annually. The Carson River flows through the region, however, most of its waters has been diverted to the Newlands Project, which provides irrigation water to surrounding farms. Groundwater recharge, therefore, is dominated by infiltration of this irrigation water, which is supplied primarily in the spring and summer months. Water tables in the area are fairly shallow (between 2.5 and 7 meters) and fluctuate seasonally due to irrigation. Total arsenic concentrations of between 10 and 440 ppb exist in the wells, although As concentrations have remained fairly stable during the sampling period. Two wells have shown complete reversal of the As(III):As(V) ratio, some have shown remarkable stability, and others have shown significant variation from the ratio expected from pH-pE diagrams for arsenate and arsenite. Additional groundwater constituents are being sampled to determine their contribution to the speciation process, and deuterium and oxygen-18 analysis is on-going to determine the provenance of groundwaters at the sampling locations. Adsorption/desorption pertaining to iron oxides and hydroxides as well as oxidation-reduction are being investigated to determine their contribution to variations in speciation.
H33H-06 14:55h
Thio-arsenic species in alkaline, hypersaline, meromictic Mono Lake, California
Mono Lake had been meromictic for 8 years when sampled (Feb-Nov, 2002). Arsenic speciation was determined by IC-ASRS-ICP-MS of samples preserved in the field by flash-freezing in liquid nitrogen. Arsenic speciation was dominated by arsenate when oxygen was detectable, but shifted to dominance by reduced species once oxygen was no longer detectable. Thio-arsenic species were the dominant form of As found in sulfidic waters. Maxima of thio-arsenic species with stoichiometries consistent with mono-, di- and tri-thio-arsenic occurred in succession as sulfide concentration increased. A compound with a stoichiometry consistent with tri-thio-arsenic was the dominant As species ($>$50% of total As) in high sulfide ($>$2 mM) bottom water. Comparison of the data with a simple equilibrium model suggested that the distributions do not represent simple chemical equilibria driven by sulfide concentration. Lower concentrations of total As in bottom water relative to surface water suggest precipitation of As/S mineral phases in response to sulfide accumulation during prolonged anoxia.
http://www.monolake.uga.edu
H33H-07 15:10h
Fate of Arsenic at a Landfill Plume Discharge Point Into Surface Water, Central Massachusetts
Fate of dissolved arsenic in ground water at the point of discharge into surface water raises a concern about the public health and the environment due to a potential of arsenic accumulation in the bottom sediments. Dissolved arsenic may become sorbed on amorphous precipitates of ferric hydroxides formed by a changing redox potential during the discharge and then locally deposited. Many landfill plumes and natural reducing ground waters in Central Massachusetts have dissolved arsenic well above the EPA 10 $\mu$g/L MCL level that could cause a local arsenic accumulation. The area of this study stretches along several man-made ponds in North Central Massachusetts located down gradient from (1) a landfill plume in which dissolved arsenic ranges between 50 and 700 $\mu$g/L occasionally up to 5,000 $\mu$g/L; (2) reduced natural ground water where dissolved arsenic can be as high as 200 $\mu$g/L; and (3) oxygenated natural ground water with dissolved arsenic levels below 10 $\mu$g/L. Bottom sediments were collected at 34 separate sites and additional samples were obtained from two to three foot cores at a subset of 9 sites. All samples were analyzed for extractable As, Fe, Ni, and other metals. Samples of bottom sediments near a discharge of the landfill plume form a layer highly enriched in Fe and As. Iron approaches the composition of 100% ferric hydroxides (390,000 and 420,000 mg/kg of Fe respectively). Corresponding arsenic values are 6,800 and 3,900 mg/kg which are nearly 100x the typical background values of arsenic in soils in this area. Arsenic concentrations in other sediments of the landfill plume area have a range from 220 to 2,000 mg/kg. Depth gradients from core sections indicate a decreasing As and Fe trend, yet significantly elevated arsenic levels are still detected at 2 ft depth (30 to 1,200 mg/kg of As). Sediments from the area where a reducing natural water mixes with the surface water have arsenic values that are 3 to 5 times the background soils (As range in the sediments: 66 to 140 mg/kg); no arsenic was detected at 2 ft depth (typical detection limit between 10 and 20 mg/kg). Sediments from the discharge area of oxygenated ground water have arsenic values indistinguishable from the background soils.
H33H-08 15:25h
Arsenic Mobility Under Sulfate Reducing Conditions
At a former landfill site in southern Maine approximately 300 ppb arsenic has been observed in groundwater over the last two decades. Laboratory and field measurements support the hypothesis that this arsenic originates within the underlying glaciofluvial sediments containing natural arsenic at concentrations of approximately 6 ppm. Arsenic is mobilized under the landfill by reducing conditions induced by decomposition of organic-rich landfill leachate. The feasibility of arsenic removal by in situ oxidation was investigated with laboratory and pilot field experiments. The high redox buffering capacity of the aquifer solids makes this remediation strategy very difficult to accomplish. A more promising remediation strategy may involve the sequestration of arsenic through the formation of solid phase sulfides under sulfate-reducing conditions. To test this hypothesis, laboratory microcosm experiments were conducted with sediment from beneath the landfill. Acetate was added to the sediments to stimulate sulfate reducing conditions. Microcosms were monitored for changes to the solid and aqueous phase chemistry along with changes to the microbial community. The addition of acetate enabled the native microbial community to establish sulfate reducing conditions. The production of sulfide coincided with a decrease in the observed iron and arsenic concentrations. Over ten days, roughly 70 to 80% of the dissolved arsenic and $>$99% of the dissolved iron was removed from solution. Arsenic was subsequently partially remobilized, possibly due to continued sulfate reduction and an increase in pH. Results indicated that laboratory manipulations of the microbial community and subsurface redox state were able to lower the dissolved arsenic concentrations.