The influence of groundwater chemistry on arsenic concentrations and speciation in a quartz sand and gravel aquifer

Abstract

We examined the chemical reactions influencing dissolved concentrations, speciation, and transport of naturally occurring arsenic ͑As͒ in a shallow, sand and gravel aquifer with distinct geochemical zones resulting from land disposal of dilute sewage effluent. The principal geochemical zones were: ͑1͒ the uncontaminated zone above the sewage plume ͓350 M dissolved oxygen ͑DO͒, pH 5.9͔; ͑2͒ the suboxic zone ͑5 M DO, pH 6.2, elevated concentrations of sewage-derived phosphate and nitrate͒; and ͑3͒ the anoxic zone ͓dissolved iron͑II͒ 100-300 M, pH 6.5-6.9, elevated concentrations of sewage-derived phosphate͔. Sediments are comprised of greater than 90% quartz but the surfaces of quartz and other mineral grains are coated with nanometer-size iron ͑Fe͒ and aluminum ͑Al͒ oxides and/or silicates, which control the adsorption properties of the sediments. Uncontaminated groundwater with added phosphate ͑620 M͒ was pumped into the uncontaminated zone while samples were collected 0.3 m above the injection point. Concentrations of As͑V͒ increased from below detection ͑0.005 M͒ to a maximum of 0.07 M during breakthrough of phosphate at the sampling port; As͑III͒ concentrations remained below detection. These results are consistent with the hypothesis that naturally occurring As͑V͒ adsorbed to constituents of the coatings on grain surfaces was desorbed by phosphate in the injected groundwater. Also consistent with this hypothesis, vertical profiles of groundwater chemistry measured prior to the tracer test showed that dissolved As͑V͒ concentrations increased along with dissolved phosphate from below detection in the uncontaminated zone to approximately 0.07 and 70 M, respectively, in the suboxic zone. Concentrations of As͑III͒ were below detection in both zones. The anoxic zone had approximately 0.07 M As͑V͒ but also had As͑III͒ concentrations of 0.07-0.14 M, suggesting that release of As bound to sediment grains occurred by desorption by phosphate, reductive dissolution of Fe oxides, and reduction of As͑V͒ to As͑III͒, which adsorbs only weakly to the Fe-oxide-depleted material in the coatings. Results of reductive extractions of the sediments suggest that As associated with the coatings was relatively uniformly distributed at approximately 1 nmol/g of sediment ͑equivalent to 0.075 ppm As͒ and comprised 20%-50% of the total As in the sediments, determined from oxidative extractions. Quartz sand aquifers provide high-quality drinking water but can become contaminated when naturally occurring arsenic bound to Fe and Al oxides or silicates on sediment surfaces is released by desorption and dissolution of Fe oxides in response to changing chemical conditions.

Groundwater chemistry and arsenic mobilization in the Holocene flood plains in south-central Bangladesh

“…Mobilization of As into the groundwater of shallow alluvial aquifers in the Bengal Basin through natural processes involving reductive dissolution of Fe(III)-oxyhydroxide is widely accepted as the principal mechanism (Bhattacharya et al 1997(Bhattacharya et al , 2002a(Bhattacharya et al , b, 2006Nickson et al 1998Nickson et al , 2000BGS and DPHE 2001;Smedley and Kinniburgh 2002;Dowling et al 2002;Anawar et al 2003;Ahmed et al 2004;Horneman et al 2004;McArthur et al 2004). Solid phases, such as Mn/Al-oxyhydroxides as well as phyllosilicates, are also known to play an important role in the As cycling in the aquifers (Breit et al 2001;Kent and Fox 2004;Saunders et al 2005;Chakraborty et al 2007von Brömssen et al 2008). Mobilization of As due to oxidation of pyrite (Chowdhury et al 1999) and widespread application of phosphate fertilizer (Acharyya et al 2000) is not consistent with the geochemical evidence revealed by several studies (Nickson et al 2000;BGS and DPHE 2001;2002aHarvey et al 2002;Zheng et al 2004Zheng et al , 2005Swartz et al 2004;Horneman et al 2004;Ahmed et al 2004;Hasan et al 2007).…”

Kinetics of sorption and abiotic oxidation of arsenic(III) by aquifer materials

“…In oxic environments, release of sediment-bound arsenic has been observed in response to elevated concentrations of phosphate (Kent and Fox, 2004) and oxidation of sulfide minerals at low pH values (McCreadie et al, 2000;Schreiber et al, 2000). In anoxic environments, sediment-bound arsenic can be mobilized by reductive dissolution or transformation of Fe(III) hydroxide coatings and reduction of arsenate (As(V)) to arsenite (As(III)) (e.g., Belzile and Tessier, 1990;Ahmann et al, 1997;Harrington et al, 1998;Cummings et al, 1999;McArthur et al, 2001;Hö hn et al, 2001;Harvey et al, 2002;Horneman et al, 2004;Kent and Fox, 2004;Swartz et al, 2004;van Geen et al, 2004).…”

“…The exact pH adsorption behavior may be attributed to the acid-base characteristics of aqueous As(V) and As(III), and acid-base characteristics and adsorption affinity of surface functional groups on the solids (Davis and Kent, 1990;Dzombak and Morel, 1990). Competitive or cooperative effects induced by the presence of other adsorbing anions or cations can significantly influence the adsorption and mobility of As(III) and As(V) (Wilkie and Hering, 1996;Darland and Inskeep, 1997;Gräfe et al, 2004;Kent and Fox, 2004).…”

Accumulation of arsenic in plants from arsenic contaminated irrigation water

“…Gediz Ilıca ve Balçova jeotermal alanlarında sıcak sular, alüvyon akifer ve yüzeysel sular için en önemli kirletici kaynaklar olarak belirlenmiştir. Kaynak ve kuyu suyu As içerikleri Gediz Ilıca alanında 0.104-0.172 mg/L, Balçova'da ise 0.164-1.420 mg/L arasında bulunmuştur [14].…”