Hydrological buffering during groundwater acidification in rapidly industrializing alluvial plains

Shallow groundwater in alluvial plains is vulnerable to contamination due to infiltration of pollutants from anthropogenic activities. In the alluvial plain of the Yangtze River near Poyang Lake, a silicone monomer industrial park was found to discharge undisclosed amount of effluents containing high levels of hydrochloric acid and total dissolved solids into ponds and ditches that in turn, displayed characteristics of acidic water with high total dissolved solids. However, most shallow groundwater (n = 35, depth 5.5-22 m) collected from the alluvial aquifer downgradient of the industrial park contained low Cl^- (< 59 mg/L). Only 4 groundwater samples showed high Cl^- (59-790 mg/L) with two containing superlative levels (449 mg/L and 790 mg/L) located within a 50 m distance from the polluted ponds and ditches. In addition to hydrochemical data, modeling was used to explain why a highly vulnerable alluvial aquifer was not more contaminated when subjected to such intense point source pollution and to estimate the effluent discharge. Flow and solute transport modeling suggests that a hydrological buffering mechanism resulting from a topography driven groundwater outflow along the boundary between the Pleistocene (Qpw) and the Holocene (Qhl) aquifers where the polluted ponds and ditches are fortuitously located has restricted acid infiltration, with only a few percent of the estimated acid discharge in the amount of 1700-4200 tons per year entering the aquifer. Our model results show that pumping from the upland Qpw aquifer breaks this hydrological buffering much more easily than pumping from the downgradient Qhl aquifer, suggesting the vulnerability of this buffering mechanism thus compromising groundwater utilization in the future.