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The Mobile River basin water-quality assessment—Results of nutrients and pesticide monitoring 1999-2000

Conference Proceedings
By Douglas Harned

Abstract

Data collection for the Mobile River Basin Study Unit (MOBL) of the National Water Quality Assessment (NAWQA) Program of the U.S. Geological Survey began in January 1999. The surface-water sampling design for the MOBL Study Unit includes nine fixed sites selected based on environmental setting, basin aerial coverage, and targeted land uses, that were sampled on a weekly to monthly frequency. Water samples were analyzed for nutrients, major ions, organic carbon, suspended sediment and organic pesticides. Water samples from two sites with considerable upstream urban land use were analyzed for volatile organic compounds (VOCs).

Occurrence, trend, and land-use effect information are obtained from the nine fixed sites. Analytical results through May 2000 include a range of nutrient concentrations and occurrence of pesticides that reflect the different basin characteristics of the sites. The Tombigbee River and Alabama River sites integrate most of the Mobile River Drainage Basin and show nutrient concentrations and loads that are similar to the median values for rivers draining into the Gulf of Mexico. High ammonia and organic nitrogen, phosphorus, and carbon concentrations in Bogue Chitto Creek show effects of intensive agriculture. Cahaba Valley Creek in Birmingham has a characteristically urban mix of high values of nitrate, phosphorus, bacteria, pesticides and VOCs. Three Mile Branch in Montgomery shows evidence of urban effects including high nitrate concentrations and VOC detections. Pintlalla Creek has substantially higher organic carbon concentrations than the other sites except for Bogue Chitto Creek and moderate ammonia and organic nitrogen concentrations probably due to animal grazing. The Chattooga River Basin drains row crop and pasture land uses and flows into the nutrient sensitive Weiss Reservoir. The Chattooga River has shown high phosphorus concentrations.

Pesticides and pesticide degradation products occurred frequently at low concentrations at the study sites. The herbicides atrazine and simazine occurred in over 90 percent of the samples collected. The herbicides metolachlor and tebuthiuron, and the insecticide diazinon occurred in over 50 percent of the samples. Concentrations of the herbicides atrazine, simazine, metolachlor, diuron, 2,4-D, fluometuron, cyanazine, bentazon, MCPA, and picloram were all detected at concentrations greater than one microgram per liter (mg/L). The highest atrazine value measured to date was sampled in May 1999 during a storm event at Bogue Chitto Creek (201 mg/L). This site also had the highest median values for atrazine (0.648 mg/L) and metolachlor (0.067 mg/L). Three Mile Creek showed the highest median value for diazinon (0.023 mg/L), and Cahaba Valley Creek had the highest median value of simazine (0.077 mg/L).

Harned, D.A., 2000, The Mobile River basin water-quality assessment—Results of nutrients and pesticide monitoring 1999-2000 [abs.]: 14th Annual Conference on Water Resources, Symposium Alabama Section of the American Water Resources Association, Sept. 6-8, 2000, Gulf Shores, Ala.