Water from 30 percent of the wells sampled and about 20 percent of the streams sampled would exceed the U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level (MCL) for nitrate-nitrogen of 10 mg/L if not properly treated before use as drinking water. Most of the samples representing sources of drinking water were the samples collected from wells used for household supply. Ground waters in agricultural areas were most likely to have nitrate concentrations that exceeded drinking-water standards, though not all agricultural areas were the same. Land use and bedrock type accounted for most of the variation in nitrate concentrations in ground water (see graph on page 9). Ground-water nitrate concentrations were highest in agricultural areas underlain by limestone, where 45 percent of the samples exceeded the MCL. Waters from 36 percent of the wells in agricultural areas underlain by crystalline bedrock also had nitrate concentrations greater than 10 mg/L. Water from wells in urban areas underlain by limestone and in forested and agricultural areas underlain by sandstone and shale had nitrate concentrations that seldom exceeded the MCL.
Streams in agricultural areas underlain by limestone had nitrate concentrations that, if not lessened by appropriate treatment before use as drinking water, commonly would exceed the USEPA MCL. Streams in other areas did not. Some streams in limestone areas have nitrate concentrations of 10 mg/L or more year round. In limestone areas, streams were commonly fed by springs that discharged ground water containing high concentrations of nitrate. Water from tributaries in limestone areas, such as Mill Creek (see graph on bottom of page 10), had nitrate concentrations near 10 mg/L with some seasonal fluctuation. In limestone and other areas, the highest nitrate concentrations were generally in the winter and spring. Seasonally high concentrations of nitrate are an issue for some water suppliers. Suppliers of drinking water regularly monitor nitrate concentration.
Ground-water and stream-water quality are related to manure management and application rates. Best-management practices, like the manure-storage structure (concrete structure with chain-link fence) on this farm in Lancaster County, Pa. (left), help keep manure from being applied to the fields in the winter when nitrate is more likely to enter streams and ground water (above, photograph by Dennis W. Risser, U.S. Geological Survey).
Nitrate concentrations in the Susquehanna River at Harrisburg were generally less than 2 mg/L, which is considerably below the MCL for nitrate in drinking water of 10 mg/L. Concentrations of nitrate at these levels, when multiplied by the large flows of the Susquehanna River, contributed large amounts of nitrate to the Chesapeake Bay when compared to other rivers entering the bay. Although safe for human consumption after filtration and water treatment, the water flowing from the Susquehanna River into the Chesapeake Bay still contained enough nitrate to stimulate algae growth and affect the bay ecosystem. Estimates of loads and yields of nitrate for 1994 from samples collected when the flow was dominated by ground water (base flow) showed that streams from agricultural areas underlain by limestone bedrock contributed large amounts of nitrate per unit area to the Lower Susquehanna River when compared to streams in areas with other land uses and bedrock types. However, streams in agricultural areas underlain by sandstone and shale and crystalline bedrock also provide large amounts of nitrate to the Susquehanna River.
Nitrate concentrations in ground water are highest in agricultural areas underlain by limestone bedrock, where almost half of the samples collected exceed 10 mg/L of nitrate. Shallow bedrock depth and highly fractured bedrock in valleys underlain by limestone can allow nitrate from manure and fertilizer to infiltrate rapidly into the ground water.
For agricultural areas underlain by crystalline or limestone bedrock, nitrate concentrations generally were higher in ground water than stream water. In urban areas and areas underlain by sandstone and shale, nitrate concentrations generally were higher in stream water than in ground water.
The main nitrogen source in the Study Unit is animal manure used as an agricultural fertilizer. High manure-application rates showed a strong association with elevated nitrate concentrations in areas of specific land uses and bedrock types. Nitrogen in manure and fertilizers added to agricultural land is essential to plant growth; however, concentrated animal operations can produce more manure than the crops grown on that farm can use. The number of concentrated animal operations is increasing in some parts of the basin. Improper or excess application can cause nitrate and other forms of nitrogen to enter the ground water or streams. Recently, through the efforts of the Chesapeake Bay Restoration Program, many agricultural operations have voluntarily taken advantage of new technologies to manage manure more efficiently. The data collected in this study provide a baseline to evaluate the effectiveness of the Pennsylvania Nutrient Management law, which requires concentrated animal operations to develop and have approved nutrient-management plans by 1998.
The nitrate data from the seven subunits were compared to determine factors that affect nitrate movement and concentration. Nitrate concentrations were higher in stream water in areas underlain by limestone than in areas underlain by other bedrock types; however, these areas generally had the highest manure-application rates. When an agricultural area underlain by limestone and an agricultural area underlain by sandstone and shale with similar manure-application rates were compared, nitrate concentrations in streams were not significantly different. Manure-application rate may be the most important factor controlling nitrate concentrations in streams in agricultural basins underlain by limestone.graph below).
A comparison of samples from 41 streams in agricultural areas underlain by limestone (subunits 2, 3, and 5) shows a strong relation between manure-application rate and nitrate concentration.
Nitrate concentrations were higher in ground water than in streams in agricultural areas underlain by limestone bedrock and in agricultural areas underlain by crystalline bedrock. Conditions in these areas allow much of the agriculturally applied nitrogen to enter the ground water. Under certain conditions, forested areas near streams (riparian buffers) can remove nitrate from the ground water before it flows into the stream.
Nitrate concentrations were higher in streams than in ground water in urban areas underlain by limestone. Streams in urban areas were affected by point-source discharges. Nitrate concentrations were also higher in stream water than ground water in both agricultural and forested areas underlain by sandstone and shale. The conditions in the sandstone and shale aquifers are not favorable for the movement of nitrate into the ground water. Conditions in these aquifers also were such that nitrate could change chemically to other forms of nitrogen (denitrification) and potentially leave the water system and enter the atmosphere. The sandstone and shale subunits had the lowest median nitrate concentrations in streams and ground water compared to the other subunits.
The relation of topography and land use is important in understanding nitrate occurrence. In the Ridge and Valley Physiographic Province, where agricultural areas are in the valleys and forested areas are on the ridges, the ground water under the valleys may be mixed with water from the ridges, which dilutes agricultural contaminants. In areas of the Piedmont underlain by crystalline bedrock, agricultural land use is commonly on the hilltops, and the steep hillsides are forested. There, the ground water under the agricultural land contains contaminants from the agricultural land use, but contaminants may be absorbed by vegetation or diluted as the water passes under the forested areas on the way to the stream.
Temporal variation in nitrate concentrations in streams during periods when storm runoff is absent (base flow) and flow is dominated by ground-water flow was determined from the analyses of samples collected throughout the year at seven long-term monitoring sites (see map on page 7). Nitrate concentrations were commonly higher in the winter months than in the summer months. An example plot for Mill Creek is shown below.
Nitrate concentrations in stream base flow decline through the summer and increase through the fall, reaching the highest concentrations in the winter months.
Statistical analysis showed that high flows in the streams were related to high nitrate concentrations. This variation may have been caused by the seasonal change in the amount of water that flows through the ground and carries nitrate to the streams (more water transports more nitrate). Other possible explanations for this variation include the seasonal cycle in plant uptake of nitrogen and seasonal fluctuations in uptake of nitrate by algae in streams. Because no information was available about the time for ground water to travel from land surface to streams, interpretation of this temporal variation was not conclusive. Nitrate concentrations in stream base flow are unlikely to change quickly in response to land-management practices because it may take years for ground water now in aquifers underlying the basin to flow into streams.
