VULNERABILITY OF HYDROLOGICAL BUFFERING DURING GROUNDWATER ACIDIFICATION: IMPLICATION FOR GROUNDWATER PROTECTION IN RAPIDLY INDUSTRIALIZING ALLUVIAL PLAINS

Shallow groundwater in alluvial plains is vulnerable to contamination due to infiltration of pollutants from intense anthropogenic activities. In the alluvial plain of the Yangtze River near Poyang Lake, a silicon monomer industrial park was found to discharge effluents (n = 3) containing high levels of hydrochloric acid (pH: 3.8, 5.6, 6.0, Cl^-: 328, 529, 742 mg/L) and total dissolved solids (TDS: 528, 917, 1100 mg/L) into several ponds and ditches. Water samples from the ponds and ditches (n = 5) receiving the acidic effluent were characterized by elevated Cl^- (112－2055 mg/L), TDS (189－3122 mg/L), and low pH values (6.2－3.6) in comparison to river water samples (n = 4) with only about 4 mg/L of Cl^-, 53 mg/L of TDS and pH values around 7.7. However, groundwater samples (n = 39, depth 5.5－22 m) collected within 2 km² of the industrial park fall into two categories: the majority low-Cl^- group (n = 35, Cl^-< 59 mg/L, 75th percentile value plus 1.5 times the interquartile range (IQR)) and the high-Cl^- group (n = 4, Cl^-: 59-790 mg/L). Only two groundwater samples are found with superlative Cl^- (449 mg/L and 790 mg/L), which located near the boundary between upland Pleistocene (Qpw) and downgradient Holocene (Qhl) aquifer and within 50 m distance of surface water ponds and ditches contaminated by industrial effluent. The stable isotopes of water (D and ¹⁸O) are consistent with the limited spatial extent of infiltration of acid effluent, as high-Cl^- groundwater isotopic compositions are comparable with that of low-Cl^- groundwater. Groundwater flow and solute transport models were developed to investigate how groundwater flow regulates infiltration of polluted surface water and the evolution of Cl^- plume in aquifers. Only 2－5% of discharged industrial Cl^-entered into groundwater system due to hydrological buffering, a fortuitous feature of the topography-driven groundwater flow system in which groundwater flows out along the boundary between Qpw and Qhl aquifers and thus reduces infiltration. Modeling further shows the pumping from the upland Qpw aquifer breaks the hydrological buffering much more easily than pumping from the downgradient Qhl aquifer, suggesting that groundwater utilization in such alluvial aquifers is compromised.