RESUMEN
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⯱â¯18.2⯵M) and Fe2+ (123⯱â¯61.0⯵M) and elevated SO42-, 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 SO42- 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 δ34SSO4 and δ18OSO4. 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.
RESUMEN
The increase in extreme heavy rain due to climate change is a critical factor in the fate of urban and agricultural pollutants in aquatic system. Nutrients, including NO3- and PO43-, are transported with surface and seepage waters into rivers, lakes and aquifers and can eventually lead to algal blooms. δ15N-NO3-, δ18O-NO3-, and δ11B combined with hydrogeochemical and microbial data for groundwater and surface water samples were interpreted to evaluate the fate of nutrients in a riverside area around weirs in Daegu, South Korea. Most of the ions showed similar concentrations in the groundwater samples before and after heavy rain while concentrations of major ions in surface water samples were diluted after heavy rain. However, Si, PO43-, Zn, Ce, La, Pb, Cu and a number of waterborne pathogens increased in surface water after heavy rain. The interpretation of δ11B, δ15N-NO3-, and δ18O-NO3- values using a Bayesian mixing model revealed that sewage and synthetic fertilizers were the main sources of contaminants in the groundwater and surface water samples. δ18O and SiO2 interpreted using the Bayesian mixing model indicated that the groundwater component in the surface water increased from 4.4 % to 17.9 % during the wet season. This is consistent with numerical simulation results indicating that the direct surface runoff and the groundwater baseflow contributions to the river system had also increased 6.4 times during the wet season. The increase in proteobacteria and decrease of actinobacteria in the surface water samples after heavy rain were also consistent with an increase of surface runoff and an increased groundwater component in the surface water. This study suggests that source apportionment based on chemical and multi-isotope data combined with numerical modeling approaches can be useful for identifying main hydrological and geochemical processes in riverside areas around weirs and can inform suggestions of effective methods for water quality management.