Water samples were collected from four streams in Lancaster County from 1992 through 1995 and analyzed for selected herbicides. Samples were collected from the Little Conestoga Creek near Churchtown, Mill Creek (a tributary to the Conestoga River) at Elshelman Mill Road near Lyndon, the Conestoga River at Conestoga, and Pequea Creek at Martic Forge. Most samples were collected from stormflow that occurred during the growing season. Samples were analyzed for alachlor, aldrin, atrazine, chlordane, cyanazine, dieldrin, malathion, metolachlor, propazine, simazine, and toxaphene. Most samples had detectable concentrations of alachlor, atrazine, metolachlor, and simazine, and the loads of these constituents that were transported during each of the 4 years were computed. Of the samples collected from each of the streams-Little Conestoga Creek, Mill Creek, Conestoga River, and Pequea Creek-10, 12, 15, and 18 percent, respectively, had atrazine concentrations greater than 3.0 micrograms per liter, the U.S. Environmental Protection Agency maximum contaminant level. Loads of atrazine, metolochlor, and simazine were greater than loads of any other herbicides. The largest loads were transported during 1994. Loads of atrazine transported by the four streams during periods of stormflow from May to September 1994 totaled 3.46, 28.3, 263, and 46.8 pounds, respectively. The total loads of atrazine transported by the four streams-Little Conestoga Creek, Mill Creek, Conestoga River, and Pequea Creek-during calendar year 1994 were 6.48, 54.1, 498, and 102 pounds, respectively. A little less than half the atrazine load transported by each stream-45, 39, 42, and 42 percent, respectively-was transported during storms that occurred from May through September. Average annual yields of atrazine for the period 1992-95 were 0.59, 0.64, 0.68, and 0.51 pounds per square mile from the Little Conestoga Creek, Mill Creek, Conestoga River, and Pequea Creek, respectively. Average annual yields of simazine were 0.36, 1.2, 0.54, and 0.48 pounds per square mile, respectively, and average annual yields of metolachlor were 0.46, 0.49, 0.54, and 0.31 pounds per square mile, respectively. Less than 1 percent of both the atrazine and metolachlor that was applied to all basins was transported by streamflow.
The Susquehanna River, which drains an area of 27,510 square miles in New York, Pennsylvania, and Maryland, is the largest tributary to Chesapeake Bay. Nutrients and sediments from the river are suspected of contributing to a marked decrease in the productivity of the bay. A disproportionately large percentage of these nutrients and sediments originate in the lower part of the Susquehanna River Basin. The lower Susquehanna River Basin is a 5,439-square-mile area between the confluence of the West Branch Susquehanna River at Northumberland, Pa., and the mouth of the river below Conowingo, Md., excluding the Juniata River Basin. Three large hydroelectric dams span the river, at Safe Harbor (Lake Clarke) and Holtwood (Lake Aldred) in southern Pennsylvania, and at Conowingo (Conowingo Reservoir) in northern Maryland. The drainage area of the river at Conowingo Dam is 27,100 square miles. The annual loads (1985-89) of total nitrogen, total phosphorus, and suspended sediment transported from the upper Susquehanna (the area above Northumberland including the West Branch Susquehanna River Basin) and the Juniata River Basins combined averaged 86.6 million, 5.11 million, and 3,170 million pounds. Total nitrogen, total phosphorus, and suspended-sediment loads entering the reservoir system averaged 135 million, 7.71 million, and 5,040 million pounds per year. Most of the nitrogen (131 million pounds), about 60 percent of the phosphorus (4.40 million pounds), and about 25 percent of the sediment (1,400 million pounds) that entered the reservoirs were discharged to Chesapeake Bay. Average yields (1985-89) for total nitrogen, total phosphorus, and suspended sediments in the upper Susquehanna and Juniata River Basins combined averaged 6.1 Ib/acre, 0.36 Ib/acre, and 224 Ib/acre (pounds per acre) per year and yields in the lower basin averaged 15.1 Ib/acre, 0.81 Ib/acre, and 582 Ib/acre per year. Multiple-regression equations developed from relations between land use and measured loads predict annual yields of total nitrogen for a 100-percent urban basin to be 51.6 Ib/acre, for a 100-percent agricultural basin to be 37.1 Ib/acre, and for a 100-percent forested basin to be 1.78 Ib/acre. The annual sediment yield predicted for a 100-percent urban area is 1,960 Ib/acre; for a 100-percent agricultural area, 1,100 Ib/acre; and for a 100-percent forested area, 91 Ib/acre. Yields of total nitrogen, total phosphorus, and suspended sediment for the Susquehanna River Basin upstream from the reservoir system are probably about 3 to 4.5 times the yields prior to the clearing of forests and development in the basin during the 1800's. In addition, the total nitrogen, total phosphorus, and suspended-sediment yields for the upper basin are probably about two to four times the yields when the basin was forested, and yields for the lower basin are probably about seven to eight times the yields prior to early development.
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