Assessment of Acid Deposition Effects on Water Quality of the Upper Rio Grande River Section in Texas

Abstract

Airborne pollutants such as SO 4 2- and N O 3 - that cause acid rain may pollute water resources via acid deposition. However, such effects on the water quality of the upper Rio Grande River section in Texas have not been systematically studied. The objective of this study is to collect and analyze field data, and perform hydrological and water chemistry analyses to assess acid deposition effects on the river water quality. The analysis of the precipitation data indicates that the concentrations of ions decrease as the quantity of precipitation increases. The precipitation with higher concentrations of SO 4 2- and N O 3 - has a lower pH while that with higher concentrations of Ca 2+ and Na + has a relatively higher pH value. The analysis of river data demonstrates that the pH value, Dissolved Oxygen (DO), and Total Dissolved Solid (TDS) generally decrease when the flow rate increases immediately following precipitation events. The drop in pH following a precipitation event is due to the low pH in the precipitation. The DO and TDS decrease after the precipitation due to the increased flow rate. The slightly higher pH and lower DO values in the eastern section of the river (where the basin is limestone-dominated) as compared to the western section is due to the limestone erosion caused by the acid deposition. The annual stone loss by the acid deposition is about 72,000 m 3 . The fluctuation between the pH value and the temperature suggests the effect of CaCO 3 solubility on the pH value. The water chemistry analysis using Geochemist’s Work Bench (GWB) has been performed to estimate the effect on the oscillation of CaCO 3 dissolution-precipitation process. The equilibrium pH decreases with decreasing temperature, but increases as the CaCO 3 concentration decreases. The effect of limestone on observed daily pH fluctuations appears to be supported by the simulation.