TIME SERIES ANALYSIS OF ARSENIC CONCENTRATIONS IN 24 MONITORING WELLS IN ARAIHAZAR, BANGLADESH

Naturally occurring arsenic in drinking water is a worldwide problem but at the local scale concentrations can vary over short length scales both vertically and horizontally. However, given the spatial variability few studies (e.g. McArthur, 2010 for 8 years) have proceeded long enough to understand temporal variability. The goal of this study was to examine the temporal variability of arsenic and groundwater chemistry of 4 vertical wells nests with a total of 24 wells ranging in depth from 5 to 90m in both Holocene and Pleistocene aquifers in Araihazar, Bangladesh that were monitored from ~2000 to 2017. Water chemistry, 17 anions and cations, including As, Fe, Mn, S, Cl, and Br, were monitored expanding a study that originally went from 2000-2003. An average of 63 samples were collected per well. Multiple statistical tests were used to assess trends and correlations. Out of a possible 408 time series across the species and wells, 182 were constant, 133 increased, and 93 decreased. Arsenic was constant in 6 wells, increased in 9 and decreased in 9. Rates of As change in individual wells ranged from -9.3 to 4.3 ug/l/yr. Anthropogenic impacted anions (e.g. Cl) geologic impacted elements (e.g. Sr) and redox sensitive elements (e.g. Fe) all changed over time in the majority of wells. When examining the 4 vertical profiles at the well nests, the As appeared more stable with slow changes in the mass (0 to 1.2% per year), center of mass (-0.63 to 0.54% per year), and spread (0 to 2.8% per year); zero, first, and second moments, respectively. Spearman correlations were used to compare changes in As to other elements at each well. Out of a possible 379 correlations 189 were uncorrelated, 153 were positively correlated, and 37 were negatively correlated with As. For all cations at least 50% of wells were positively correlated with As whereas for all anions except phosphorous the majority of wells were uncorrelated to As and the correlations tended to be negative. The change in chloride, most likely an anthropogenic constituent, indicates that humans are impacting water quality. However As is correlated with cations and redox sensitive elements and not anions indicating that As release, redox reactions, and mineral weathering are linked but cannot be separated from changes in groundwater flow and mixing that are occurring in the region.