H13L-01
Arsenic and Humic Substances in Alluvial Aquifers of Bangladesh and Taiwan: A Comparative Study
Humic substances in groundwater samples from the arsenicosis area in Bangladesh, northern Taiwan and the Blackfoot disease (BFD) area in southwestern Taiwan were characterized by Fluorescence Spectroscopy (FS), and Fourier Transform Infrared Spectroscopy (FTIR) analyses. As, Mn, Fe, Sr, Se levels in these groundwaters were measured by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). Major ions and selected water parameters including pH, electrical conductivity (EC), oxidation reduction potential (ORP), and dissolved oxygen (DO) were also determined. Groundwater As concentration ranges from 1.4 to 140 μg/L in the alluvial aquifers located in the Chapai-Nawabganj district of Bangladesh. As levels in groundwater ranges from 0.5 to 560 μg/L in the Ilan Plain of northern Taiwan. Geothermal waters in the Beitou hot springs contain high concentrations of inorganic As (up to 3,975 μg/L); geothermal activity is likely responsible for the significant discharge of arsenic to the downstream Kwandu Plain. As levels in the BFD area of southwestern Taiwan ranges from 25 μg/L to 967 μg/L. Interestingly, groundwater arsenic in the BFD area of southwestern Taiwan correlates positively with strong fluorescence (maximum relative fluorescence intensity upto 495) and the content of humic substances. In contrast, As-rich groundwaters from Chapai-Nawabganj district of Bangladesh and northern part of Taiwan generally have relatively low content of humic substances with weak fluorescence (maximum relative fluorescence intensity upto 65 and 121, respectively). Moreover, results of FTIR analysis show that humic substances extracted from water samples of the Taiwan BFD area contain phenolic and amines groups of humic substances, which tend to form organo-metal complexes with As and other trace elements. High levels of As and humic substances probably play a critical role in causing the Black foot disease in Chia-Nan plain of southwestern Taiwan.
H13L-02
Subsurface Occurrence, Distribution, and Speciation of Arsenic in the Southern High Plains
H13L-03 INVITED
Mechanisms of Arsenic Mobilization and Attenuation in Subsurface Sediments
This talk will review molecular mechanisms of As mobilization and attenuation in subsurface sediments using examples from recent field studies that represent a range in oxidation-redox (redox) potential. As a ubiquitous trace element in sediments, As speciation and fate is linked to the abundance and biogeochemical behavior of the generally more abundant redox-active elements Fe, S, and Mn. All four elements are subject to oxidation, reduction, and pH-dependent processes such as sorption, desorption, precipitation, and dissolution, and which may include both biotic and abiotic reaction steps. We have used spectroscopic interrogation and geochemical modeling to characterize As speciation in subsurface sediments in several contrasting environments, including high and low S and Fe settings. Aquifers most at risk for contamination by As include those that are rich in organic matter and nutrients, stimulating high rates of microbial reduction and creating anoxic conditions, but limited in labile or available S and/or Fe that remove As by precipitation or adsorption. In subsurface sediments with low labile S and Fe, laboratory experiments and spectroscopic studies suggest that sediment Mn minerals are important in the oxidation of sorbed As(III) to As(V), but that they have a limited oxidation capacity. Arsenic attenuation and mobilization in the subsurface are affected by seasonal variations when hydraulic conditions are influenced by surface infiltration, which may induce transitions from oxidized to reduced conditions (or vice versa) in porewater.
H13L-04
Groundwater Flowpath Analysis and Arsenic and Selenium Trends Beneath a Drained Marsh
An evolving groundwater flowpath was studied beneath a historic marshland. Located in San Diego Creek Watershed of Orange County California, the historic "Swamp of the Frogs Marsh" was drained in the late 1800s. Today, groundwater beneath the historic marsh is suboxic to oxic. Groundwater was sampled at nine points along the groundwater flowpath. Along the first half of the 1 km flowpath, groundwater moves through the outer fringes of the historic swamp. There, groundwater does not change in concentration and remains slightly saline, whereas arsenic increases from 13 ug/L to 90 ug/L dissolved As. Selenium increases from 50 to 228 ug/L dissolved Se in this upper region. Over 98 percent of the arsenic and selenium along the first half of the flowpath is in the form of arsenate and selenate, the most oxidized forms of these ions. Iron and manganese are not detectable, and almost all of the nitrogen is in the form of nitrate. Along the lower half of the flowpath, groundwater moves into and through the interior of the historic marsh. There, salinity doubles and we observe a correlative increase of chloride, bromide, sulfate, and arsenic; the latter reaching 196 ug/L dissolved As. Selenium decreases substantially to about 60 ug/L Se along the lower flowpath. Over 98 percent of the arsenic is arsenate, but only 85 percent of the selenium is selenate in the lower flowpath region, the rest is present at selenite, an intermediate redox form of Se. Organic forms of arsenic and selenium are not detected along the full flowpath. There is a small amount of manganese (<70 ug/L) and trace amounts of ammonium in the lower flowpath area, but no detectable iron. Oxygen and deuterium isotope values do not change along the full flowpath, eliminating pure evaporation as a possible explanation for salinity and arsenic enrichment in the lower flowpath area. Based on our data, we propose a model of selenium oxidation and arsenic desorption (but not reductive dissolution) from sites on iron and aluminum oxides and clays within the upper part of the flowpath, and leaching of evaporative efflorescent salts (containing arsenate) in the lower flowpath region. Geomorphic and geochemical analysis suggest that the lower flowpath region, in the interior of the historic marsh, was once a phreatic salt playa.
H13L-05
Arsenic Release Mechanism in an Intensively Irrigated Agricultural Region of the Alluvial Aquifer, Eastern Arkansas, USA
Twenty one of 118 wells from the alluvial aquifer in the Bayou Bartholomew watershed in eastern Arkansas exceed the maximum contaminant level (MCL) of 10 ìg/L for As. This watershed is one of the most intensive agricultural ground-water irrigation regions in the U.S.A. It was hypothesized that the intensive ground-water irrigation caused significant water level fluctuation that could affect the geochemical evolution and mobilization of As in the alluvial aquifer. In order to test the hypothesis and conceptualize the As mobilization mechanism, laboratory column experiments and field work were conducted. Ground-water level and quality was monitored at 3 monitoring sites (a shallow and a deep well at each site) and 21 irrigation wells. Disaggregated sediments from the borehole of 3 monitoring wells were packed in 6 in (D) * 2 ft (L) acrylic columns proportional to the field sediment profile in terms of lithology and thickness. Field collected ground water was passed through a pre- treatment column also packed with sediments collected in the field in order to a reducing environment similar to field conditions. The regenerated water was used as input for three separate columns: 1) a column exposed to air representing oxic water-level fluctuation, 2) a column isolated from air representing anoxic water-level fluctuation, and 3) a column isolated from air with continuous flow. ORP, pH, conductivity, DO, and temperature were measured in situ in the columns, and water was collected periodically for chemical analyses. Ground water was collected from the monitoring and irrigation wells during the recharge season (April 2007) and growing season (July 2007), and analyzed for major and trace ions. The ground-water quality was generally similar with laboratory column experiments (e.g. As: <5 – 88 ìg/L and 10.3 – 354 ìg/L, Fe: 0.016 – 38 mg/L and 0.029 – 50.5 mg/L for field and laboratory, respectively). Statistical and graphical analyses, and geochemical modeling with PHREEQC indicated that surface complexation of arsenic onto the iron oxyhydroxides and ion exchange were the main geochemical processes causing As mobilization and transport in the two columns representing water-level fluctuation, whereas mineral dissolution and ion exchange were the main geochemical processes operating in the continuous flow column. A small amount of competitive sorption and reductive dissolution of iron oxyhydroxides were observed in all columns. These distinct conditions in the laboratory (oxic and anoxic fluctuation, and continuous flow) are aggregated in the natural environment. Based on the laboratory column experiments and ground-water chemistry from the field samples, the following mechanisms were determined for arsenic mobilization: 1) In an oxidizing environment (during the growing season when the water level is lowered and/or fluctuated by irrigation) arsenic was sorbed onto the iron oxyhydroxides in the sediment, 2) when a reducing environment was developed, arsenic was released into the ground water by the reductive dissolution and the common ion effect, resulting in the increase of the arsenic concentrations during the spring season, and 3) infiltration of phosphate, carbon, and nitrogen increased the common ion effect (competitive sorption and interference with sorption) and the development of reducing condition during the recharge season.
H13L-06
Evaluating sources of arsenic to groundwater in the Mekong Delta based on coupled hydrologic and biogeochemical analyses
Tens of millions of people living on the large river deltas of Southeast Asia routinely consume groundwater with unsafe arsenic levels. While there is general agreement that arsenic is naturally derived, the particular sources of arsenic, as well as the mechanisms promoting its release from the solid-phase, remain unresolved, thereby limiting our ability to predict arsenic concentrations in space (between wells) and time (future concentrations). This uncertainty is attributed, in part, to a poor understanding of groundwater flow paths due to extensive irrigation pumping in the Ganges-Brahmaputra River system, where most research has focused. In order to elucidate the most important arsenic sources and the processes controlling arsenic contamination in Southeast Asian groundwater, we have established a field area within the minimally disturbed Mekong River Delta in Cambodia. While the Mekong Delta system in Cambodia has similar depositional history, regional hydrology, and biogeochemical conditions to other arsenic-contaminated deltaic aquifers of Asia, land use alteration, inclusive of irrigation, is minimal. Thus, the hydrology of our system remains governed by natural rather than anthropogenic processes, allowing us to formulate a steady-state, coupled hydrologic and biogeochemical model of arsenic release and transport. Using robust principles of mass balance, we show that, while liberation mechanisms within the deeper aquifer sediments may contribute arsenic to the groundwater, the majority of the dissolved arsenic is derived from arsenic release at the near-surface and is transported through the aquifer, a finding that has important implications for management of the arsenic crisis.
H13L-07
Hydrogeochemical assessment of arsenic in groundwater and its policy implication: a case study in Terai Basin, Nepal
Arsenic contamination at levels above the WHO guideline (10 ìg/l) in groundwater is a worldwide problem due to its detrimental effects on health and now known to be a problem also in the Terai Basin of Nepal, posing a serious threat to more than 10 million people. The distribution of arsenic in the basin, however, is patchy. The study emphasizes on the three different types of research into an interdisciplinary package that can be immediately useful to government agencies in Nepal trying to deal with groundwater contamination. They are: hydrogeological assessment of water sources and flow, geochemical analysis of groundwater, and assessment of practical public policy. Basic geochemical analysis gives the abundance and distribution of arsenic along with other physico-chemical parameters of groundwater, whereas, the hydrogeological assessment as an integral part of this study that assist in determining process of mobilization or attenuation of arsenic. Arsenic levels and other key parameters mainly pH, electrical conductivity, chemical oxygen demand, iron, and biological parameter as E-coli were observed at the various locations with different transmissivity values. The study suggests that the flushing rate of an aquifer plays an important role in arsenic content. High flushing rates of an aquifer lead to low levels of arsenic, however the mechanism of this process is still under study. Transmissivity the property of an aquifer that measures the rate at which ground water moves horizontally through a unit is the main factor for controlling flushing. Concentration maps overlaying the base transmissivity map reveals relation of groundwater movement and arsenic concentration. Understanding the relationship between groundwater movement and arsenic content helps planners protect uncontaminated aquifers from future contamination. Also assessment of public policy related to groundwater has identified important changes needed in the existing policy.
H13L-08
Hydrogeochemistry and adsorption behaviour of As-enriched sediment from Bengal Delta Plain, India
Sixty-six groundwater samples were collected from an area of 16 sq. km in Chakdaha block, West Bengal. High arsenic (1 to 475 μg/l) in anoxic groundwater is associated with relatively high Fe and pCO2. Groundwater redox potential is controlled by the Fe2+/Fe(OH)3 couple. X-ray fluorescence (XRF) analyses of 29 borehole sediments showed higher contents of As (8-14 mg/kg), Fe2O3 (5-9.6%) and MnO (0.07-0.15%) in silty clay whereas fine to medium sand have lower content of As (<5 mg/kg), Fe2O3 (1.15-3.9%) and MnO (0.02- 0.06%). Acid extractable As does not correlate with ascorbate extractable Fe oxyhydroxide. However, high Fe oxyhydroxide (10-12 g/kg) was obtained by ascorbate extraction in low As area. Organic carbon is low ~ 1% and groundwater temperature (26-31oC) may facilitate microbial process that leads to increase local reducing condition (redox traps). Speciation of Fe, Mn, HCO3- and As in groundwater samples was done by using MINTEQA2. Arsenate adsorption study performed on sandy riverbank sediment indicates very low affinity for As at pH ~7.5.