Applied Geochemistry (v.19, #2)

Arsenic in groundwater of sedimentary aquifers by Prosun Bhattacharya; Alan H Welch; Kazi Matin Ahmed; Gunnar Jacks; Ravi Naidu (163-167).

Behavior of arsenic and geochemical modeling of arsenic enrichment in aqueous environments by Ondra Sracek; Prosun Bhattacharya; Gunnar Jacks; Jon-Petter Gustafsson; Mattias von Brömssen (169-180).
Arsenic is present in aqueous environments in +III and +V oxidation states. In oxidizing environments, the principle attenuation mechanism of As migration is its adsorption on Fe(III) oxide and hydroxides. The adsorption affinity is higher for As(V) under lower pH conditions and for As(III) under higher pH conditions. Ferric oxide and hydroxides can dissolve under low Eh and pH conditions releasing adsorbed As. Oxidation-reduction processes often involve high organic matter content in sediments and also contamination by organics such as BTEX. Arsenic may desorb under high pH conditions. Changes of pH can be related to some redox reactions, cation exchange reactions driving dissolution of carbonates, and dissolution of silicates. In very reducing environments, where SO4 reduction takes place, secondary sulfide minerals like As-bearing pyrite and orpiment, As2S3, can incorporate As. Geochemical modeling can be divided into two principal categories: (a) forward modeling and (b) inverse modeling. Forward modeling is used to predict water chemistry after completion of predetermined reactions. Inverse modeling is used to suggest which processes take place along a flowpath. Complex coupled transport and geochemistry programs, which allow for simulation of As adsorption, are becoming available. A common modeling approach is based on forward modeling with surface complexation modeling (SCM) of As adsorption, which can incorporate the effect of different adsorbent/As ratios, adsorption sites density, area available for adsorption, pH changes and competition of As for adsorption sites with other dissolved species such as phosphate. The adsorption modeling can be performed in both batch and transport modes in codes such as PHREEQC. Inverse modeling is generally used to verify hypotheses on the origin of As. Basic prerequisites of inverse modeling are the knowledge of flow pattern (sampling points used in model have to be hydraulically connected) and information about mineralogy including As mineral phases. Case studies of geochemical modeling including modeling of As adsorption are presented.

Arsenic enrichment in groundwater of the alluvial aquifers in Bangladesh: an overview by K.Matin Ahmed; Prosun Bhattacharya; M.Aziz Hasan; S.Humayun Akhter; S.M.Mahbub Alam; M.A.Hossain Bhuyian; M.Badrul Imam; Aftab A Khan; Ondra Sracek (181-200).
Arsenic in the groundwater of Bangladesh is a serious natural calamity and a public health hazard. Most groundwater from the shallow alluvial aquifers (<150 m), particularly in the Holocene plain lands, are vulnerable to As-enrichment. Delta plains and flood plains of the Ganges–Brahmaputra river system are moderately to severely enriched and more than 60% of the tube wells are affected. Shallow aquifers in the Meghna river basin and coastal plains are extremely enriched with more than 80% of the tube wells affected. Aquifers in the Pleistocene uplands and Tertiary hills are low in As. The vertical lithofacies sequence of the sediments from highly enriched areas of the country show two distinct lithofacies associations—a dominantly sandy channel-fill association and a fine-grained over bank association. The sediments can be grouped into 4 distinct lithofacies, viz. clay, silty clay, silty sand and sand. Thin section petrography of the As-enriched aquifer sands shows that the sands are of quartzolithic type and derived from the collision suture and fold thrust belt of the recycled orogen provenance. Groundwater is characterized by circum-neutral pH with a moderate to strong reducing nature. The waters are generally of Ca–Mg–HCO3 or Ca–Na–HCO3 type, with HCO3 as the principal anion. Low SO4 2− and NO3 , and high dissolved organic C (DOC) and NH4 + concentrations are typical chemical characteristics of groundwater. The presence of dissolved sulfides in these groundwaters indicates reduction of SO4. Total As concentration in the analyzed wells vary between 2.5 and 846 μg l−1 with a dominance of As(III) species (67–99%). Arsenic(III) concentrations were fairly consistent with the DOC and NH4 + contents. The HNO3 extractable concentrations of As As NO 3 in the sediments (0.5–17.7 mg kg−1), indicate a significant positive correlation with FeNO3 , MnNO3 , AlNO3 and PNO3 . The concentrations of SNO3 (816–1306 mg kg−1) peaked in the clay sediments with high organic matter (up to 4.5 wt.%). Amounts of oxalate extractable As (Asox) and Fe (Feox) ranged between 0.1–8.6 mg kg−1and 0.4–5.9 g kg−1, respectively. Arsenicox was positively correlated with Feox, Mnox and Alox in these sediments. Insignificant amounts of opaque minerals (including pyrite/arsenopyrite) and the presence of high As contents in finer sediments suggests that some As is incorporated in the authigenically precipitated sulfides in the reducing sediments. Moreover, the chemical extractions suggest the presence of siderite and vivianite as solid phases, which may control the aqueous chemistry of Fe and PO4 3−. Reductive dissolution of Fe oxyhydroxide present as coatings on sand grains as well as altered mica (biotite) is envisaged as the main mechanism for the release of As into groundwater in the sandy aquifer sediments.

Redox control of arsenic mobilization in Bangladesh groundwater by Y. Zheng; M. Stute; A. van Geen; I. Gavrieli; R. Dhar; H.J. Simpson; P. Schlosser; K.M. Ahmed (201-214).
Detailed hydrochemical measurements, δ 34SSO4 and 3H analyses were performed on 37 groundwater samples collected during February 1999, January and March 2000 from 6 locations in eastern and southeastern Bangladesh to examine redox processes that lead to As mobilization in groundwater. The study sites were chosen based on available nation-wide As surveys to span the entire spectrum of As concentrations in Bangladesh groundwater, and to represent 3 of 5 major geological units of the Ganges-Brahmaputra Delta: uplifted Pleistocene terrace, fluvial flood plain and delta plain. Arsenic was found to be mobilized under Fe-reducing conditions in shallow aquifers (<35 m depth), presumably of Holocene age. It remained mobile under SO4-reducing conditions, suggesting that authigenic sulfide precipitation does not constitute a significant sink for As in these groundwaters. The redox state of the water was characterized by a variety of parameters including dissolved O2, NO3 , Mn2+, Fe2+ concentrations, and SO4 2−/Cl ratios. High dissolved [As] (> 50 μg/l; or > 0.7 μM ) were always accompanied by high dissolved [HCO3 ] (> 4 mM), and were close to saturation with respect to calcite. Groundwater enriched in As (200–800 μg/l; or 2.7–10.7 μM) and phosphate (30–100 μM) but relatively low in dissolved Fe (5–40 μM) probably resulted from re-oxidation of reducing, As and Fe enriched water. This history was deduced from isotopic signatures of δ 34SSO4 and 3H2O (3H) to delineate the nature of redox changes for some of the reducing groundwaters. In contrast, As is not mobilized in presumed Pleistocene aquifers, both shallow (30–60 m) and deep (150–270 m), because conditions were not reducing enough due to lack of sufficient O2 demand.

Mineralogical and geomicrobiological investigations on groundwater arsenic enrichment in Bangladesh by Junji Akai; Kaoru Izumi; Haruo Fukuhara; Harue Masuda; Satoshi Nakano; Takahisa Yoshimura; Hiroaki Ohfuji; Hossain Md Anawar; Kurumi Akai (215-230).
Sources of As in the Ganges sediments and microbial mechanisms of its release in groundwater were examined in the present study, where the authors have systematically examined the pertinent mineral species present in the sediments using XRD, TEM-EDS and EPMA techniques. The results show several As-bearing minerals in the Ganges sediments, in western Bangladesh. Iron-sulfide minerals consist of near-amorphous and/or crystalline precursors of framboidal pyrite and pyrite both of which contain As. Several types of Fe oxyhydroxides (oxides), which contain variable amounts of As were also found in muddy sediments. The content of As increases from Fe-oxides to the precursors of framboidal pyrite and pyrite. Four different chemical forms of As from the core sediments were determined. The sequentially extracted chemical forms are as follows: (1) acid soluble form (As mainly fixed in carbonates), (2) reducible form (As fixed in Fe- and/or Mn-oxides), (3) organic form, (4) insoluble form (As fixed mainly in sulfide and rarely in silicate minerals). Arsenic is dominantly sorbed on to Fe- and/or Mn-oxides, organic forms and sulfide minerals in most samples, although their relative abundances differ in different samples. Geomicrobial culture experiments were carried out to test the hypothesis that microbial processes play a key role in the release of As in groundwater. Batch culture and circulating water system experiments were designed using the sediments from Bangladesh. In the batch experiments, As was released at low Eh values a few days after adding nutrients containing glucose, polypepton and yeast extract, urea and fertilizer under a dominantly N2 atmosphere. This contrasts with the control experiments without nutrients. Circulating water experiments with sand layer in a N2 atmosphere showed similar results. These results support the hypothesis that microbial processes mediate the release of As into groundwater under reducing conditions. Glucose and polypepton used in the experiments may correspond to C and N sources, respectively. Younger sediments contain abundant organic matter, which is easily used by bacteria. So, the enhanced bacterial activity may correspond to simulation of accelerated natural diagenetic process using organic matter, or some fertilizer/wastewater effects.

Groundwater arsenic in the Chaco-Pampean Plain, Argentina by J. Bundschuh; B. Farias; R. Martin; A. Storniolo; P. Bhattacharya; J. Cortes; G. Bonorino; R. Albouy (231-243).
In large parts of rural Argentina people depend on groundwater whose As content exceeds the Argentine drinking water standards (0.05 mg l−1). The most affected areas are located in the Chaco-Pampean Plain, where aquifers comprise Tertiary loess deposits (in the Pampean Plain) and Tertiary and Quaternary fluvial and aeolian sediments (in the Chaco Plain). Robles county is located in the alluvial cone of the Dulce River consisting of loess (aeolian), and gravel, silt, sand and clay (alluvial) deposits. In the shallow aquifers, more than 48% of the 63 studied wells show As at toxic levels (maximum 4.8 mg l−1), while in the deep groundwater the concentration is below 0.05 mg l−1. The pH of the shallow groundwaters range between 6.5 and 9 and generally have high electrical conductivity with mean values of 2072 and 1693 μS/cm−1 in the years 1998 and 1999, respectively. Arsenic concentrations are high in the alkaline Na–HCO3 type groundwaters, where As correlates positively with Na+ and HCO3 . Moreover, As correlates positively with Mo, U, and V, while a negative correlation was observed with Ca2+ and Mg2+. The potential sources of groundwater As are: (i) layers of volcanic ash with 90% of rhyolitic glass; (ii) volcanic glass dispersed in the sediments; and (iii) clastic sediments of metamorphic and igneous origin. Great lateral variability in the concentration of groundwater As is caused by several hydrogeological and hydrogeochemical factors.

Groundwater arsenic variations: the role of local geology and rainfall by R. Rodrı́guez; J.A. Ramos; A. Armienta (245-250).
High concentrations of As were detected in the aquifer system of Zimapán Valley, Mexico. Arsenic is related to a multisource system, both natural and anthropogenic. The main source is As bearing-rocks. Groundwater As variations were detected in the more polluted urban deep well. Variations were correlated with the precipitation regime and with the hydrogeologic and geologic framework. An induced As transport mechanism related mainly with the rainfall regime is proposed.

Groundwater arsenic in the Verde Valley in central Arizona, USA by R.D Foust; P Mohapatra; A.-M Compton-O'Brien; J Reifel (251-255).
Forty-one water samples were collected and analyzed from throughout the Verde Valley watershed to identify the source of As in well water used for domestic and agricultural purposes. Each water sample was analyzed for anions, cations and trace chemical constituents by atomic absorption spectroscopy, anion chromatography and traditional wet chemical procedures. Arsenic concentrations ranged from 10 to 210 μg/l, with the highest values observed for water pooled on tailings from an abandoned Cu mine. Geostatistical analysis of the data revealed the primary source of As to be groundwater in contact with the Supai and Verde formations, as opposed to runoff from the abandoned mine tailings. Montezuma Well, a collapsed travertine spring, contained the highest levels of naturally occurring As (> 100 μg/l). Arsenic in Montezuma Well water was shown to be 100% arsenate. X-ray absorbance near edge spectra (XANES) of Potomogeton illinoiensis, an endemic plant species of Montezuma Well, demonstrate that As is absorbed as arsenate, reduced to arsenite in the plant and retained as an organic glutathione complex. XANES spectra of Montezuma Well sediments show 4 forms of As present: arsenate (∼54%), As(III)-glutathione complex (∼32%) and an As-organic complex (∼14%) containing dimethylarsinic acid and arsenobetaine. This is the first report of As(III)-glutathione in sediments.

A preliminary geochemical map for arsenic in surficial materials of Canada and the United States by Andrew E Grosz; Jeffrey N Grossman; Robert Garrett; Peter Friske; David B Smith; Arthur G Darnley; Eric Vowinkel (257-260).
Over the past 30 a, regional and national solid-phase geochemical surveys have been conducted by the United States Geological Survey and the Geological Survey of Canada. In the present paper the authors have examined the distribution of As in stream-sediment and soil samples of the US and Canada in terms of geologic and anthropogenic components. The results of the compilations indicate that the distribution of As in stream sediments, lake sediments, and soils in Canada and the US shows that most of the variability is controlled by the bed rock characteristics.