Nutrients and organic compounds in Deer Creek and south branch Plum Creek in southwestern Pennsylvania, April 1996 through September 1998
This report presents results of an analysis of nutrient and pesticide data from two surface-water sites and volatile organic compound (VOC) data from one of the sites that are within the Allegheny and Monongahela River Basins study unit of the National Water-Quality Assessment Program of the U.S. Geological Survey. The Deer Creek site was located in a 27.0 square-mile basin within the Allegheny River Basin in Allegheny County. The primary land uses consist of small urban areas, large areas of residential housing, and some agricultural land in the upper part of the basin. The South Branch Plum Creek site was located in a 33.3 square-mile basin within the Allegheny River Basin in Indiana County. The primary land uses throughout this basin are mostly agriculture and forestland. Water samples for analysis of nutrients were collected monthly and during high-flow events from April 1996 through September 1998. Concentrations of dissolved nitrite, dissolved ammonia plus organic nitrogen, and dissolved phosphorus were less than the method detection limits in more than one-half of the samples collected. The median concentration of dissolved nitrite plus nitrate in South Branch Plum Creek was 0.937 mg/L and 0.597 mg/L in Deer Creek. The median concentration of dissolved orthophosphate was 0.01 mg/L in both streams. High loads of nitrate were measured in both streams from March to June. Concentrations of dissolved ammonia nitrogen, dissolved nitrate, and total phosphorus were lower during the summer months. Measured concentrations of nitrate nitrogen in both streams were well below the U.S. Environmental Protection Agency (USEPA) maximum contaminant level (MCL) of 10 mg/L. Water samples for analysis of pesticides were collected throughout 1997 in both streams and during a storm event on August 25-26, 1998, in Deer Creek. Samples were collected monthly at both sites and more frequently during the spring and early summer months to coincide with application of pesticides. Seventy-eight pesticides and 7 pesticide metabolites were analyzed in 31 samples collected in Deer Creek and in 18 samples collected in South Branch Plum Creek. Of the 85 pesticides and pesticide metabolites analyzed, 25 of the pesticides were detected at least once in Deer Creek, and 20 of the pesticides were detected at least once in South Branch Plum Creek. Atrazine was the most commonly detected pesticide in both streams. There was a distinct seasonal pattern of atrazine, simazine, and metolachlor concentrations measured at both sites. Prometon was detected in 3 of the 18 samples collected in South Branch Plum Creek in 1997 and in 28 of the 31 samples collected in Deer Creek in both 1997 and 1998. Prometon generally is applied in conjunction with asphalt paving projects and is commonly used in residential areas. The highest measured concentrations of prometon detected in Deer Creek were in the five storm samples collected on August 25-26, 1998. At the Deer Creek site, 9 of the 25 pesticides detected throughout the study were detected only in the sample collected on June 13, 1997. Those nine pesticides included acifluorfen, bentazon, bromoxynil, dicamba, dichlorprop, fenuron, linuron, MCPA, and neburon. Nine other pesticides also were detected in that sample. All concentrations of pesticides were well below established drinking-water guidelines. The maximum measured concentration of diazinon in Deer Creek (0.097 µg/L) and South Branch Plum Creek (0.974 µg/L) exceeded the aquatic life guideline of 0.009 µg/L established by the National Academy of Sciences/National Academy of Engineers. The maximum measured concentration of azinphos-methyl in South Branch Plum Creek (an estimated value of 0.033 µg/L) exceeded the chronic aquatic-life guideline of 0.01 µg/L established by the USEPA. Twenty-five samples were collected from Deer Creek and analyzed for volatile organic compounds (VOCs). Of 87 VOCs analyzed for, 22 were detected at least once, and 12 were gasoline-related compounds. Acetone, benzene, carbon disulfide, meta/paraxylene, methyl chloride, MTBE, p-isopropyl toluene, toluene, and 1,2,4-trimethylbenzene were each detected in five or more samples. VOCs generally were detected during the colder winter months and not frequently during the summer months. The maximum measured concentrations of benzene, ethylbenzene, o-dichlorobenzene, styrene, and toluene were two or more orders of magnitude lower than the MCLs established by the USEPA.