A recent study reveals that 1.37 million pounds of nitrogen and .94 million pounds of phosphorus enter the Chesapeake Bay's ecosystem each year through tidal shoreline erosion.
These substances, identified as a primary cause of nonpoint source pollution, account for more than 5 percent of the total nitrogen and 23 percent of the total phosphorus deposited into the Chesapeake Bay annually. The one year, $24,472 study was conducted by the Department of Conservation and Recreation, Division of Soil and Water Conservation, Shoreline Programs Bureau. The project was funded through a grant from the Council on the Environment and matching funds from the Department of Conservation and Recreation.
Nonpoint source pollution, or NPS pollution, has been identified as a major threat to the bay. It doesn't come from a single source, like a sewage treatment plant or industrial charge pipe. It comes from croplands, city streets, pasturelands, and suburban lawns. When it rains, excess nutrients, pesticides, sediment, heavy metals and toxic substances run off these areas and eventually find their way to the bay.
The nutrients, such as nitrogen and phosphorus and sediments are of particular concern. They are responsible for the overproduction of algae in the bay. When the algae die, they deplete the bay's dissolved oxygen levels, thus robbing other bay aquatic life of necessary dissolved oxygen.
In 1987, representatives from Virginia, Pennsylvania, Maryland and the District of Columbia met to discuss the bay's pollution and how to clean up the bay. These representatives set as a goal the reduction of phosphorus and nitrogen contributions to the bay by 40 percent by the year 2000.
Since the beginning of the cleanup program, questions have been asked about the role of tidal shoreline erosion in nonpoint source pollution. Although research has been conducted on the agricultural, atmospheric and groundwater contributions of NPS pollution, the role of sediment and nutrients in tidal shoreline erosion, and their amounts, has not been addressed.
Lee Hill, environmental engineer consultant, says, "We recognized the presence of tremendous amounts of sediment and nutrients from tidal shoreline erosion, but from this study we can identify those banks with high sediment volumes and target them for programs to control the nutrient loads."
Fourteen eroding bank sites along the Chesapeake Bay and the Potomac, Rappahannock, York and James rivers were selected for study. Site selection was based on historical erosion rates of greater than two feet per yard and erosion volumes of greater than one cubic yard per foot per year. Most sites were also located within 1,400 feet of living marine resources.
Random soil samples were collected and analyzed for grain size, total nitrogen, total phosphorus and inorganic phosphorus. The results of the grain size analysis indicated a large difference between the sediment samples from the beach sites and those taken from the adjacent banks.
As expected, bank samples had higher percentages of silt and clay than adjacent beach sediments. Approximately 70 percent of the bank samples were more than 10 percent silt and clay. Only 7 percent of the beach samples contained this level of silt and clay. The lower silt and clay content of beach samples results from the sorting action of waves and currents that carry the finer grained particles offshore and leave the coarser sediments on the beach.
Bank sample nitrogen and phosphorus concentrations were not found to differ significantly among the sites. Nitrogen concentrations showed a more consistent relationship with grain size and bank heights than phosphorus concentrations did. Nutrient loading rates differed among the sites because of the influence of bank height and erosion rate. Sediments and nutrients from shoreline erosion are deposited directly into neighboring waters.
Further research is planned to determine the overall magnitude of nutrient contributions from shoreline erosion and the influence of this contribution on the 40 percent nutrient reduction goal.