(188b) Effect of Airborne Deposition On Water Acidity In the Rio Grande River Basin

The Upper Rio Grande River extends from the Texas-New Mexico state line downstream to the International Amistad Dam, a length of 650 miles (1,045 km). With increasing population and urbanization, the water in this segment is becoming high in salts and bacteria. The water quality problem compounded with the concern over water shortage is a serious issue for this Rio Grande River Basin. Airborne pollutants such as SO₄^2- and NO₃^- can reach earth surface via wet deposition or dry deposition, which can pollute the earth’s valuable water resources. The objectives of this study were to collect and analyze data to understand the effect of airborne particulate pollution on the water quality in the Rio Grande Basin and to simulate the acidity of the water in the Amistad Reservoir using the CE-QUAL-W2 under different flow and precipitation conditions. An analysis of the collected data on wet deposition has indicated that the pH values and the concentration of ions in the rainwater vary with the amount of rainfall in each rain event. A higher concentration of SO₄^2- and NO₃^- increases the acidity of the rain water in the ranges  between pH=4.9 and pH=5.4 while a higher concentration of Ca²⁺ and Na⁺ decreases the acidity of the rainwater in the ranges between pH=5.5 and pH=5.8.  A higher amount of rainfall in each rain event tends to decrease the concentration of ions which may increase or decrease the pH of the rainwater depending on the dominating ion species, i.e., SO₄^2-/NO₃^- or Ca²⁺/Na⁺. An analysis of the collected river water data show that the pH in the Rio Grande River water is much higher than the rainwater pH and is in the ranges between pH=7.8 and pH=8.3 due to the buffering effect of Ca²⁺ in the river water.  The pH in the river water has been observed to vary with the water temperature, which decreases with an increase in water temperature.  In addition, the pH decreases when the river flow rate increases due to rainfall, which mainly is because of the decrease in Ca²⁺ concentration due to the rainwater dilution and the delayed dissolution of CaCO₃ into Ca²⁺ in the river water. The results from pH simulations of the Amistad Reservoir water in 2004 based on the two-dimensional water quality and hydrodynamics model, CE-QUAL-W2, have indicated that the precipitation decreases the pH by about 0.1 to 0.15 as compared to the no rain condition. The water pH in the reservoir is observed to vary inversely with water temperature.  However, the pH variation is insignificant when the rates of river flow are significantly reduced from the original flow rates.