Evaluation of agricultural best-management practices in the Conestoga River headwaters, Pennsylvania: Hydrology of a small carbonate site near Ephrata, Pennsylvania, prior to implementation of nutrient management
The U.S. Geological Survey, in cooperation with the U.S. Department of Agriculture and Pennsylvania Department of Environmental Protection, investigated the effects of agricultural best-management practices on water quality in the Conestoga River headwaters watershed. This report describes environmental factors and the surface-water and ground-water quality of one 47.5-acre field site, Field-Site 2, from October 1984 through September 1986, prior to implementation of nutrient management. The site is partially terraced agricultural cropland underlain by carbonate rock. Twenty-seven acres are terraced, pipe-drained, and are under no-till cultivation. The remaining acreage is under minimum-till cultivation. Corn is the primary crop. The average annual rate of fertilization at the site was 480 pounds per acre of nitrogen and 110 pounds per acre of phosphorus. An unconfined limestone and dolomitic aquifer underlies the site, Depth to bedrock ranges from 5 to 30 feet below land surface. Estimated specific yields range from 0.05 to 0.10, specific capacities of wells range from less than 1 to about 20 gallons per minute per foot of drawdown, and estimates of transmissivities range from 10 to 10,000 square feet per day. Average ground-water recharge was estimated to be about 23 inches per year. The specific capacity and transmissivity data indicate that two aquifer regimes are present at the site. Wells drilled into dolomites in the eastern part of the site have larger specific capacities (averaging 20 gallons per minute per foot of drawdown) relative to specific capacities (averaging less than 1 gallon per minute per foot of drawdown) of wells drilled into limestones in the western part of the site. Median concentrations of soil-soluble nitrate and soluble phosphorus in the top 4 feet of silt- or silty-clay-loam soil ranged from 177 to 329 and 8.5 to 35 pounds per acre, respectively. Measured runoff from the pipe-drained terraces ranged from 10 to 48,000 cubic feet and was 1.7 and 0.8 percent, respectively, of the 1985 and 1986 annual precipitation. An estimated 90,700 cubic feet of surface runoff carried 87 pounds to total nitrogen and 37 pounds of total phosphorus, or less that 0.65 percent of the amount of either nutrient applied during the study period. Rainfall on the snow-covered, frozen ground produced more that half of the runoff and nitrogen and phosphorus loads measured in pipe-drained runoff. Graphical and regression analyses of surface runoff suggest that (1) mean-storm concentrations of total nitrogen species and total phosphorus decreased with increasing time between a runoff event and the last previous nutrient application, and (2) mean total-phosphorus concentrations approached a baseline value (estimated at 2 to 5 milligrams per liter for total-phosphorus concentrations) after several months without nutrient applications. Dissolved nitrate concentrations in ground water in wells unaffected by an on-site ammonia spill ranged from 7.4 to 100 milligrams per liter. Average annual additions and removals of nitrogen were estimated. Nitrogen was added to the site by applications of manure and commercial fertilizer nitrogen, as well as by precipitation and ground water entering across the western site boundary. These sources of nitrogen accounted for 95, 3, 1, and 1 percent, respectively, of estimated additions. Nitrogen was removed from the site in harvested crops, by ground-water discharge, by volatilization, and in surface runoff, which accounted for 42, 28, 29, and less than 1 percent, respectively, of estimated removals.