H11E-0819
Phase Partitioning of Common Alcohols With BTEX Compounds in Water: Comparison Between Modeling and Experimental Results
This study compares the modeling and experimental results on the equilibrium phase partitioning behavior of three common alcohols (ethanol, isopropanol, and methanol) in a two-phase system consisting of water and a BTEX compound. A previously developed computer program is used to generate ternary phase diagrams for each alcohol-water-NAPL mixture combination, where the required activity coefficients are estimated using the UNIFAC model. A set of laboratory experiments is conducted to determine the maximum single-phase water content for every alcohol-water-NAPL mixture combination considered in this study, where the initial volume composition is 85 percent alcohol and 15 percent NAPL. Comparison of experimental results against UNIFAC- derived modeling results shows good agreement for mixtures containing ethanol and methanol, but relatively poor agreement for mixtures containing isopropanol.
H11E-0820
Sequestration Mechanisms of Arsenic Mobilized during In Situ Bioremediation of Chlorinated Solvents
The mobilization and down-gradient attenuation of naturally occurring arsenic (As) was studied at a site where anaerobic bioremediation was employed to treat a tetrachloroethene plume at a former military base. Elevated levels of As in groundwater were induced by anthropogenic introduction of organic substrates, which generated an anaerobic reducing zone that extends ~30 m down gradient of the injection zone. However, ~60 m down gradient and outside of the reducing zone, As and Fe groundwater concentrations have remained below their detection limits, indicating natural attenuation. In this study, As sequestration behavior and mechanisms were investigated in laboratory batch and column uptake experiments and characterized by As, Fe, and Mn X-ray absorption spectroscopy (XAS). Batch sorption experiments showed similar As(III) and As(V) sorption on sediments at high total As concentration (10-3 M), but slightly more As(V) than As(III) sorption at low total As concentration (10-6 and 10-5). At the same As concentration, As uptake in sterilized samples was not significantly different from untreated samples, indicating a lack of microbial influence on As sorption. More Fe(II) was released to solution in samples reacted with As compared to a control sediment with no As, and slightly more Fe(II) was released in the presence of As(III) than As(V). XAS of unreacted core sediments collected from 40-42 m depth showed that native As (15- 24 ppm) was present as As(V). Sediments reacted in laboratory experiments with As(V) solutions retained their As(V) speciation after sorption, as indicated by the XANES spectra. Sediments reacted with As(III) showed spectroscopic evidence for oxidation to As(V). The amount of As(III) oxidation was proportional to the total dissolved concentration, with the majority of sorbed As oxidized to As(V) at low As(III) concentrations and most sorbed As remaining as As(III) at high concentrations. EXAFS results are consistent with adsorption of As as the primary mechanism of retention by sediment minerals. Comparison of unreacted and reacted Fe XANES showed no changes in spectral features. The Mn XANES spectra, however, indicated a spectral shift in absorption maxima to lower energy between unreacted sediments and As(III)-reacted sediments. Qualitatively, the absorption shift can be interpreted as a slightly higher proportion of reduced to oxidized Mn in the reacted sediment compared with the unreacted sediment. These results indicate that the sediment has a limited sorption and abiotic oxidation capacity for As, and that reduction of sediment Mn plays a role in the oxidation of sorbed As(III).
H11E-0821
Natural Arsenic Mobilization by Counterion Effect, Ogallala Aquifer, Southern High Plains, Texas, USA
A sharp contrast in arsenic levels between the northern part (median 4.3 ppb) and southern part (median 12 ppb – more than half of the water wells exceed the MCL of 10 ppb) of the Ogallala Aquifer in the Southern High Plains (SHP) coincides with a change in several aquifer characteristics (from north to south: decrease in saturated thickness, decrease in water table depth, and increase in total dissolved solids). Earlier analyses ruled out cotton crops and associated historical application of arsenic defoliants as the source of groundwater arsenic contamination. The most likely source of arsenic is adsorption onto Fe-Mn (oxyhydr)oxides, similar to arsenic sources in most semiarid, oxidizing systems and mobilization through a change in environmental conditions. Changes along flow lines in the southern Ogallala aquifer of the SHP include, in addition to increasing levels of arsenic and other oxyanions, increasing TDS (from <500 to >2000 ppm) and evolution from a calcium- bicarbonate water type to a sodium chloride/sulfate type. There is, however, no change in pH, which remains around neutral. Increase in pH is often described as an important factor in arsenic mobilization. It is hypothesized that waters of the southern Ogallala aquifer of the SHP mix with arsenic-poor waters from the underlying Triassic Dockum Formation releasing sorbed arsenic in the process. Change from Ca to Na type water has been documented in laboratory experiments as a mechanism for mobilizing sorbed arsenic. Several other studies, including those on phosphate, an ion with chemical properties very similar to arsenate, in allied fields (soil science, water treatment) strongly support this counterion effect as a mechanism for mobilizing arsenic.
H11E-0822
Dissolved Calcium and Magnesium Carbonates Promote Arsenate Release From Ferrihydrite in Flow Systems
Field data from water systems around the world have shown that arsenic can reach toxic concentrations in dynamic groundwater systems. This is generally in contrast to analogous static systems at circumneutral pH, where arsenic is strongly retained by sorption to iron (hydr)oxides. Our research examines the effect of calcium and magnesium carbonates on As(V) mobility. In both dynamic flow and static experiments, arsenate was pre- sorbed to poorly crystalline iron hydroxides (1-10% sorption capacity), with varying aqueous compositions including calcium, magnesium, carbonate, sulfate, lactate, and other common groundwater species (pH 7.5-8). Thus we investigated how the dissolution of common carbonate minerals, specifically CaCO3 and MgCO3, affect arsenic behavior in the context of groundwater solutions. Under static (batch) conditions, no measurable arsenic (<10 μg/L) is released into solutions containing alkaline earth metals (AEMs) and carbonates. When elevated concentrations of AEMs and carbonate are introduced by dynamic flow, however, arsenic is mobilized at up to 500 μg/L, releasing significant proportions the total arsenic present. This is only the case when both of these species are present; with other common ion pairs, little to no arsenic is released. These results indicate that arsenate adsorption is kinetically controlled under flow conditions, resulting in very different mobility relative to otherwise equivalent static systems. Furthermore, the combination of alkaline earth metals and carbonates promotes As(V) mobility in column-based systems. We propose that these phenomena indicate a combination of physical and chemical effects by which diffusion limitation becomes dominant in limiting arsenic sorption in flow systems. Many carbonate-buffered aquifers, as well as those undergoing rapid mineralization of organic matter, could be affected by these processes of AEM-carbonate-limited sorption and increased arsenic mobility.
H11E-0823
Controls on Aqueous Arsenic Mobilization in the Treasure Valley Shallow Aquifer, Southwestern Idaho
A study has been undertaken to elucidate the geochemical mechanisms by which arsenic is being stored, released and transported in the shallow sedimentary aquifer beneath the Western Snake River Plain, Idaho where groundwater arsenic concentrations exceed 100 ug/L. While arid, this region is extensively irrigated for agriculture and we are evaluating the effects of the infiltration of irrigation waters from surface and subsurface sources on the release of arsenic to the aquifer system. Our initial results indicate that while arsenic concentrations exhibit high variation over short lateral distances, throughout the affected area the highest aqueous concentrations occur in conjunction with high dissolved oxygen and low iron and manganese concentrations and near the water table. While sequential extraction and batch experiments performed on unirrigated shallow sediments from the basin show solid phase arsenic at or near average crustal abundances (n.d. to 29 mg/kg, average being 1.77 mg/kg), resultant concentrations of water soluble arsenic to >20 ug/L and of specifically adsorbed arsenic to >100 ug/L, with the highest being >500 ug/L, were observed. The highest aqueous concentrations were associated with surficial aeolian material and iron oxide coatings and concretions. Arsenic associated with the oxide operational fraction ranged from 1 to 29 mg/kg, average being 4.15 mg/kg. Analysis of flood irrigation waters indicate very little to no arsenic is currently being released from irrigated surface sediments. This data suggests that water soluble and specifically adsorbed arsenic has been flushed from the surficial sediments and that oxides/oxyhydroxides in the vadose zone and shallow aquifer may be acting as both sources and sinks for aqueous phase arsenic.
H11E-0824
Potential bedrock source of groundwater arsenic anomaly in northeastern Chihuahua City, Chihuahua, Mexico
Contaminant sources of arsenic are often very difficult to identify. It is rare that specific rock units can be identified to which groundwater anomalies can be attributed. In this study, primary arsenic minerals, such as a Y-As bearing phase and a Sr-Al-As phase, have been identified in the Cenozoic volcanic tuff from El Mimbre area, at the northeast part of Tabalaopa Basin, the City of Chihuahua. Tabalaopa Basin is one of the sources for groundwater of the City of Chihuahua. The volcanic strata and the unconsolidated Quaternary deposit serve as the groundwater reservoir. The El Mimbre area demonstrates elevated groundwater arsenic concentrations, with 5 wells having values greater than 20 ppb. Small hills of Cenozoic volcanic tuff lie immediately up gradient to the northeast adjacent to the groundwater anomaly. Electron microprobe elemental x-ray maps have been applied to detect arsenic distribution in the samples. The volcanic rocks are reddish welded ash flow tuff and rhyolite with mainly sanidine, quartz, and biotite. The Y-As phase (a possible hydrated chernovite YAsO4) exists as inclusions in the biotite. The Y-As phase could be the primary arsenic mineral formed in the crystallization stage of the magma. After the eruption, the vapor phase from the diagenetic welding of volcanic tuff formed the euhedral Sr-Al- As phase in the lithophysal cavities. The wavelength peak scan and quantitative analysis present that the euhedral arsenic minerals are mainly arsenogoyazite (SrAl3[(OH)5(AsO4)2])H2O. There are euhedral alkali feldspars and ilmenite co-existing with arsenogoyazite, so the arsenogoyazite could be the vapor phase derived. The arsenic mineral petrogenesis sequences can be 1) the formation of chernoviet, 2) vapor stage alteration and the precipitation of arsenogoyazite in the cavities. It is hypothesized that the relative soluble arsenogoyazite is leached by meteoric water and provides the arsenic in the nearby groundwater. Based on current study, the arsenic anomaly in the Tabalaopa Basin aquifer can be mainly from the arsenic minerals in the adjacent volcanic rocks.
H11E-0825
Sensor Measurements and Sediment Incubations Indicate Diurnal Redox Cycling Associate With Arsenic Mobilization at a Bangladeshi Rice Paddy
The presence of arsenic in the groundwater has led to the largest environmental poisoning in history; tens of millions of people in the Ganges Delta continue to drink groundwater that is dangerously contaminated with arsenic (As). Rice fields receive large loads of arsenic with irrigation water and provide recharge to the underlying aquifer. It is currently not known whether rice fields are a sink or source of arsenic in the hydrologic system. In the dry season, as As(III)-containing minerals are oxidized, As(V) is released and will adhere to Fe hydr(oxide) minerals. When sediments are inundated with water, reducing conditions will then drive reduction of Fe hydr(oxides) and release of As. We have been intensively studying a field site in Munshiganj, Bangladesh with extremely high levels of arsenic in groundwater (up to 1.2 mg/L). To better understand geochemical and microbial processes leading to As mobilization in surface sediment, we deployed sensors to take temporally dense measurements across our experimental rice paddy. Data collected in both 2006 and 2007 showed trends in geochemical parameters indicating that diurnal, possibly plant-induced, processes may be important. Over a two month period, nitrate concentrations decrease consistently each day as ammonium levels increase, presumably through temperature driven reductive processes. Nitrate concentrations in the subsurface then increase while ammonium levels decrease, possibly due to root oxygen leakage or rapid infiltration of oxygen rich surface water. Using sediment from the rice paddy and artificial irrigation water, laboratory microcosms were constructed to simulate the diurnal cycles observed at the field site. In carbon-ammended treatments, Fe and As cycling can occur on the order of days. Oscillations in redox conditions on diurnal as well as seasonal time scales may be important in the mobilization of arsenic into aquifers. By elucidating As mobilization mechanisms at an experimental rice paddy, this work could ultimately lead to solutions that minimize As exposure in critical populations.
H11E-0826
Mobilization of Arsenic in Groundwater of Holocene Delta, Indian Segment
Recent incidences of high arsenic exposure from drinking water and human suffering in SE Asia notably in Bengal Delta Plain (BDP) appear to be quite different from the global scenario. The source of arsenic in groundwater of BDP is considered to be geogenic and mostly restricted to Holocene sediments. Several models (oxidation of As-rich pyrite, reductive dissolution of Fe(III)-Fe(II) systems, anion exchange of sorbed As mostly phosphate) have been put forward. However, the models have their own weakness and the paper deals with such discrepancies to focus the arsenic sourcing (primary, secondary and tertiary), release mechanism and mobilization in Holocene deltas. The paper also deals with groundwater quality and (hydro) chemistry vis-a-vis alluvial Holocene sediment. The stratigraphic profiling indicates that the shallow aquifers (< 40m) are significant contributors (~91 percent, n = 497) of high arsenic in groundwater. The major hydrochemical features are low Eh (range: - 149-37 mv; mean: - 53mv) and nitrate (range 0.001- 1.72; mean 0.13 mg/L) followed by high alkalinity (range 100 – 630 mg/L; mean 280 mg/L), iron (1.23- 38.4, mean 7.48 mg/L), phosphate (range 0.002 – 3. 99 mg/L; mean 0.56 mg/L), hardness (range 46-490 mg/L; mean 235) and sulphate (0.28 – 48.62 mg/L; mean 6.43 mg/L). Groundwater (hydro) chemistry indicates that the affected aquifers are reducing in nature. The four different local situations (sanitation, surface water bodies, land use and sanitation coupled with eco-system) are identified and most important is sanitation where groundwater arsenic concentration is highest (up to 1180 mg/L) among the different local situation. Arsenic hotspots are identified as a multi-level phenomena and unveiled greater complexities at a deeper level. Morphology and sediment profiling of BDP deltas are influencing arsenic sourcing where sandy useable aquifers are identified with overbank deposits rich in arsenic host environment. Shallow aquifers ( 5-70 m) are often enriched with DOC ( range- 1.2- 7.0 mg/L) where as deep aquifers ( 100- 304 m ) are relatively containing low amount of DOC ( 0.4- 1.9 mg/L). This reflects the influence of local processes at shallow depths where oxidative as well as microbial degeneration of carbon sink is playing crucial role in arsenic mobilization. This also suggests that the break down of organic matter is the principal process in the shallow reducing aquifers with high/low arsenic along with high / low – coupled redox sensitive species and DOC. The presence of DOC in shallow aquifers also indicates that organic matter is relatively young and more reactive. High PCO2 values, relatively high redox sensitive elements, low Eh and absence of DO are the hydrogeochemical fingerprints of the BDP shallow aquifers. Delta building processes are also important to explain arsenic sourcing and mobilization (deeper arsenics- bearing middle delta aquifers with low concentration of chloride as well as deeper saline tidal deposits aquifers of lower delta with / without low arsenic). Both the deeper aquifers are also containing high bicarbonate. This indicates that mineral carbonates (both simple and complex) are also playing important role in arsenic mobilization at least in deeper aquifers. The study also deals with the role of secondary minerals (mica/clay) in arsenic mobilization. The XPS studies on mica further strengthen that the surface chemistry and role of Fe (II) are also important issue to understand the difference of high/low arsenic in between shallow and deep aquifers in BDP.