DISTRIBUTION AND CHARACTERIZATION OF LIPID BIOMARKERS AND TRACE METALS IN AN ARSENIC CONTAMINATED PLEISTOCENE AQUIFER IN WEST BENGAL, INDIA

Arsenic (As) contamination in groundwater of East Indian deltaic plains has been intensively studied for more than a decade. Reductive dissolution of As bearing Fe(III)-hydroxides involving in situ microbial processes is one of the most accepted processes for As remobilization in the aquifers. Organic matter (OM) present in sediments as well as in groundwater is suggested to sustain the microbial flora which impact As cycling. In this study, during installation of a drinking water well, 26 sediment sections of 6 m each were retrieved up till 156 m from a brown sand Pleistocene aquifer. The sections were logged and analyzed for grain size and C content. Various geochemical analyses in these samples included sequential extraction of trace metals, and extraction of different lipid classes. Arsenic (As) concentration ranged from 2 to 21 mg/kg, and is abundant in clay-rich samples associated with crystalline oxides and silicate minerals. Arsenic showed significant correlation with Fe (p = 0.00; r = 0.75), and also with Mn and S in all sediment fractions suggesting pyrite bound As bearing minerals. Previous studies have indicated presence of mature petroleum derived hydrocarbons in the As contaminated grey sand aquifers (GSA), which are used for microbial metabolism during As cycling. In contrast, the Pleistocene brown sand aquifers (BSA) in this study with low As levels (labeled as ‘As-safe aquifers’) lack presence of these organic compounds. Instead the lipid profiles indicate predominantly fatty acids, and high terrigenous input with preferential preservation of n-alkanes over n-alkanols; sterols are mostly absent. In addition, the gene signals in environmental DNA extracted from BSA groundwater indicate bacterial assemblages participating in biogeochemical cycling of As are absent. These results imply that OM type and bacterial assemblages play an important role in driving As biogeochemical cycling in the BSA and GSA aquifers.