Influence of hydrological and hydrogeochemical parameters on arsenic variation in shallow groundwater of southwestern Taiwan

Shallow groundwater in the Choushui river alluvial fan of southwestern Taiwan contains high As contents. Arsenic concentrations in most of wells varied spatial–temporally. The purpose of this study is to quantitatively evaluate the hydrological and hydrogeochemical factors that influence As concentration in the shallow groundwater of southwestern Taiwan. Dynamic factor analysis (DFA), a dimension reduction technique that considers the time component for studying multivariate time series, is adopted to obtain underlying common trends of response time series and influences of explanatory variables. The major As-associated hydrological and hydrogeochemical parameters were identified and applied to predict the variation of As concentrations in groundwater. The results showed that As fluctuation in groundwater were governed by the rainfall and river water recharges in both dry and wet seasons. Rapid infiltration of surface water caused a rise in groundwater elevation in the rainy season and created a reductive environment, inducing reductive dissolution of As-bearing Fe (hydr)oxides. Dissolved As concentration was subsequently transported by fast groundwater flow to downstream areas. Moreover, extensive pumping for agricultural and aquacultural demands masked the influences of groundwater elevation on variations of As concentrations and led to critical seawater intrusion. In non-saline wells, Fe reduction and subsequent secondary mineralization between dry and wet seasons was responsible for As mobility. Sulfate reduction in saline wells promoted sulfide precipitation and restrained As mobility in wet season. Collectively, hydrological and hydrogeochemical variables controlled the temporal variation and spatial distribution of As concentration, respectively, in the shallow groundwater of southwestern Taiwan.