A study of nutrients and pesticides was conducted during February-August 1995 in the west-central part of the Trinity River Basin, where land commonly is used for growing crops. Water and bed-sediment samples were collected at 8 small reservoir sites in the headwaters (known as Natural Resources Conservation Service reservoirs), at 5 stream sites, and at 3 Richland-Chambers Reservoir sites. The analysis included data from the Chambers Creek near Rice site (08064100), which was sampled repeatedly during March 1993-September 1995.
Total nitrogen concentrations in the Natural Resources Conservation Service reservoirs were less than 1.0 milligram per liter, as nitrogen, except in 2 of the 8 reservoirs. For the five stream sites, total nitrogen concentrations at the beginning of the study ranged from 0.5 to 1.8 milligrams per liter. Peaks were noted in all stream sites during either March or April; the greatest peak concentration was 4.8 milligrams per liter, as nitrogen. By the end of the study, concentrations decreased to less than 1.2 milligrams per liter, as nitrogen. In the Richland-Chambers Reservoir, the February-March and June sampling showed total nitrogen concentrations of about 0.6 milligram per liter, as nitrogen.
At the beginning of the study, all five of the stream sites had total phosphorus concentrations less than 0.04 milligram per liter, as phosphorus. Peak concentrations in the streams occurred in the May sampling except at one site. Two sites had concentrations greater than 0.2 milligram per liter, as phosphorus. By the end of the study, concentrations decreased to less than 0.04 milligram per liter, as phosphorus, except at one site where the concentrations were about 0.08 milligram per liter. Concentrations in the Richland-Chambers Reservoir were less than 0.04 milligram per liter, as phosphorus.
Total nitrogen and total phosphorus concentrations generally increased with streamflow and with the percentage of cropland in the drainage area upstream from the sampling site.
Herbicides were detected in the streams much more often than insecticides were. Nineteen herbicides and 9 insecticides were detected at the 08064100 Chambers Creek near Rice site. Atrazine and metolachlor, the most commonly detected herbicides, occurred in all samples at this site. Other herbicides detected in 25 percent or more of the samples were alachlor, fluometuron, prometon, simazine, trifluralin, and 2,4-D.
At the beginning of the study, the number of herbicides detected in the five stream sites was 4 or 5. The greatest number of herbicides detected in the streams occurred in May samples, ranging from 7 to 10. The number of herbicides detected in the Richland-Chambers Reservoir ranged from 6 to 8. Generally, more herbicides were detected in high-streamflow samples than in low-streamflow samples. However, a consistent relation between the number of herbicides in samples and the percentage of cropland in a drainage area was not evident.
At the beginning of the study, atrazine concentrations at the stream sites were less than 0.4 microgram per liter, except at one site. In the streams, concentrations peaked in March and April; the greatest peak concentration was 20 micrograms per liter. By the end of the study, atrazine concentrations decreased to less than 0.4 microgram per liter at all the stream sites. In the Richland-Chambers Reservoir, the concentrations were about 1 microgram per liter during February-March and about 3 micrograms per liter in June. Atrazine concentrations tended to increase with increasing streamflow. A consistent relation between atrazine concentrations and the percentage of cropland in a drainage area was not evident.
The greatest number of insecticides detected in water samples was two. Diazinon, the most frequently detected insecticide, had slightly greater concentrations in May and June - between 0.01 and 0.02 microgram per liter.
The only organochlorine insecticides detected in bed-sedime
Additional publication details
USGS Numbered Series
Water-Quality Assessment of the Trinity River Basin, Texas - Nutrients and Pesticides in the Watersheds of Richland and Chambers Creeks, 1993-95