Effect of intensive seasonal pumping and recharge on sulfur biogeochemistry in groundwater of agricultural riparian zones.

Physico-chemical characteristics of groundwater are often impacted by agricultural practices such as land use, fertilizer types, and groundwater pumping. This study aimed to identify contaminant sources and redox processes controlling the hydrogeochemistry of groundwater in riparian zones influenced by intensive agricultural activities, focusing on sulfur species. Groundwater samples were collected bimonthly from March 2014 to March 2015 from groundwater wells in two zones in South Korea with different agricultural systems. The water isotopic compositions of the groundwater indicated that all groundwater originated from the same meteoric water. Groundwater samples affected by periodic groundwater pumping exhibited wide variations in Mn²⁺ (47.8 ± 18.2 μM) and Fe²⁺ (123 ± 61.0 μM) and elevated SO₄^2-, while NO₃^- was below the detection limit. Groundwater chemistry was affected by fertilizer and manure, and denitrification. The oxidation of reduced sulfur compounds by oxygen and nitrate did not fully account for the elevated SO₄^2- concentrations and isotopic composition of sulfate (δ³⁴S and δ¹⁸O) in the investigated aquifers. Therefore, we postulate that water level change due to periodic groundwater pumping and recharge enabled oxidants (MnO₂ and Fe³⁺) to also contribute to oxidation of reduced sulfur. Additionally, fertilizers with distinct δ³⁴S values and bacterial sulfate reduction (BSR) affected groundwater chemistry and its sulfur species, including δ³⁴S_SO4 and δ¹⁸O_SO4. Removal of sulfate from the aquifer during pumping limited BSR. Consequently, the agricultural practices may further increase sulfate concentrations in the groundwater. This environmental impact should be thoroughly managed because high sulfate concentrations in drinking water cause ingestion problems in humans.