TECTONIC AND SEDIMENTOLOGICAL CONTROLS ON GROUNDWATER ARSENIC MOBILIZATION IN THE GANGES FLOODPLAIN OF NEPAL

Tectonic setting plays a major role in the distribution of groundwater arsenic in the headwaters of the Ganges River in south Asia. Mobilization of the arsenic is enhanced by accumulation of thick, organic-rich clays in the Himalayan foredeep of Nepal (the Ganges floodplain or Terai). Blanket testing of arsenic in water from shallow tubewells in the Terai districts indicates above-50 ppb arsenic levels are found exclusively south of the Main Frontal Thrust of the Himalaya. These naturally contaminated wells are found in fine grained sediments derived from thrusting-exhumed early floodplain sediments (Lower Siwalik Group). These thick clay areas lie between coarser megafans deposited by antecedent rivers draining the Central and High Himalaya. In the Nawalparasi District, the most heavily affected area in Nepal, arsenic declines southward away from the Siwalik foothills, along with clay fraction in tubewells. Dissolved Na/(Total Cations) ratios in these clay-rich areas increase with depth, indicating progressive cation exchange associated with high-As areas. As(III)/As_total and Fe²⁺/Fe_total increase concurrently with depth, indicating reductive desorption is the principal mobilization mechanism for arsenic. Oxidation of pyrite is contraindicated by minimal concentrations of SO₄⁻ and lack of As-SO₄⁻ correlation. Redox-sensitive species Fe, Mn, NH₄+ , DOC and As peak at ∼20 m depth, and concentrations correlate strongly (linear R² =0.58–0.89). The principal control on arsenic mobilization in this area appears to be formation of a thick clay cap where aquifer materials are derived from Lower Siwalik rocks. This cap impedes percolation of surface water, allowing development of reducing conditions at the depth of the shallowest water-producing zones. Southward toward the India-Nepal border, Increasing near-surface sand fraction limits the distribution of this phenomenon, restricting significant contamination to areas farther from the Indian border, and close to the Siwalik foothills.