Agricultural activity on the land surface is one important factor affecting water quality in the Lower Susquehanna River Basin.
In the Lower Susquehanna River Basin Study Unit, land use is 47 percent agricultural, 47 percent forested, and 4 percent urban; 2 percent of the area is water bodies or barren land (Mitchell and others, 1977; see map above). The well-drained areas with rolling hills and valleys in the southern part of the basin contain most of the population and some of the most productive agricultural land in the Nation. These agricultural and urban areas commonly are areas underlain by carbonate (limestone) bedrock.
The study area was subdivided on the basis of land use, physiography, and bedrock type to assess the effect of these characteristics on water quality. Major water issues in the Study Unit include the effects of agricultural land use on water quality and ground-water contamination in areas underlain by limestone bedrock. These issues were used to prioritize the selection of major environmental settings for study (Risser and Siwiec, 1996).
Water used for public supply is largely from surface water, and only about 25 percent of the water used for public supply comes from ground-water sources (see graph at left). In 1990, more than 1.2 million people used public-supplied water. In addition, 800,000 rural homeowners depended on water from wells for domestic (household) supply. Thermoelectric cooling, industrial and mining, and public-supply withdrawals are the major uses of water. Withdrawals for thermoelectric cooling are much greater than withdrawals for other uses. The water-quality degradation in return flows from water used for cooling primarily involves increases in water temperature.
Thermoelectric, public supply, and industrial and mining are the largest water uses in the Lower Susquehanna River Basin. Rural homeowners depend chiefly on ground water for domestic supply.
Mean annual precipitation in the Lower Susquehanna River Basin Study Unit ranges from 38 to 48 inches (see figure). The precipitation is generally less in the center of the basin because of storm patterns; however, the entire Study Unit receives enough rainfall so that irrigation is not common except in periods of drought. The precipitation is distributed fairly evenly throughout the year. About 45 percent of the precipitation is contributed by storms in May through September during the growing season, and much of this precipitation is used by plants. Most of the remaining 55 percent occurs when vegetation is dormant; therefore, this precipitation is more available to infiltrate into bedrock and enter ground-water systems.
The Lower Susquehanna River Basin has a temperate climate, with adequate precipitation for the crops grown in the area. (Modified from Risser and Siwiec, 1996.)
The hydrologic conditions during the study were variable, including wet and dry years. This information is important in the interpretation of the water-quality data collected. For example, the nitrate loads were calculated for 1994, a wet year, and may provide a higher estimate for loads than would have been calculated during a year with more normal flow. The spring of 1993 and 1994 and the summer of 1994 were periods of greater than normal precipitation and streamflow (see figure below). Heavy winter snowstorms in these years caused high flows in the spring. The snowmelt from the winter of 1993 caused a record high average streamflow of 250,100 cubic feet per second from the Susquehanna River to the Chesapeake Bay during April of that year. Higher than normal flows were observed throughout 1994, when streamflow in the Susquehanna River at Harrisburg was 42 percent greater than normal. The summer of 1993 and most of 1995 were dry periods. Drought declarations were in effect for most of the counties in the Study Unit in September 1995.
Higher than normal flows were observed throughout 1994, when streamflow in the Susquehanna River at Harrisburg was 42 percent greater than normal. Drought declarations were in effect for most of the counties in the Study Unit in September 1995.