THE MOBILE RIVER BASIN WATER-QUALITY ASSESSMENT: RESULTS OF SURFACE-WATER QUALITY MONITORING IN ALABAMA 1999-2000

Surface-water quality data collection for the Mobile River Basin is part of the National Water Quality Assessment (NAWQA) Program of the U.S. Geological Survey. Data collection incorporates nine regularly sampled sites, synoptic studies, and coordinated sampling with the ecological and ground-water components of the study. The sites were selected based on environmental setting and areal coverage and were sampled weekly to monthly. The samples were analyzed for nutrients, pesticides, volatile organic compounds (VOCs), major ions, carbon, and sediment.

Results indicate that concentrations of nutrients and pesticides reflect basin land use. Samples from the Tombigbee and Alabama River sites integrate most of the Mobile River drainage basin; nutrient concentrations and loads are near the median values for rivers draining into the Gulf of Mexico. Nutrient concentrations in Bogue Chitto Creek show effects of intensive agriculture. Cahaba Valley Creek in Birmingham has high nutrients, bacteria, pesticides and VOCs. Three Mile Branch in Montgomery has high nitrate and VOC concentrations, reflecting effects of urbanization. Pintlalla Creek has high carbon concentrations and moderate nutrient concentrations-- probably due to animal grazing. The Chattooga River, which drains row crops and pasture and flows into the nutrient-sensitive Weiss Reservoir, has high phosphorus concentrations.

Over 133 samples have been analyzed for pesticides. Low concentrations of pesticides occurred frequently in urban and agricultural basins. The herbicides atrazine and simazine occurred in over 90 percent of the samples. The herbicides metolachlor and tebuthiuron and the insecticide diazinon occurred in over 50 percent of the samples. The herbicides atrazine, simazine, metolachlor, diuron, 2,4-D, fluometuron, cyanazine, bentazon, MCPA, and picloram were detected at concentrations greater than 1 microgram per liter (µg/L). The highest atrazine concentration measured among the nine sites was in May 1999 at Bogue Chitto Creek (201 µg/L). Three Mile Creek had the highest median concentration of diazinon, and Cahaba Valley Creek had the highest median concentration of simazine.