Journal of Conference Abstracts

Hydrology and Chemistry of Coastal Plain Streams in the Chesapeake Bay Drainage

Thomas R. Fisher (fisher@hpel.cees.edu), Margaret Mayers Norton (mmnorton@wam.umd.edu), Kuang-Yao Lee (kylee@hpel.cees.edu) & Jorge A. Benitez (jabet@wam.umd.edu)

Horn Point Environmental Labouratories, University of Maryland-CEES, Cambridge, Maryland 21613, U.S.A.

The Choptank and Chester River basins are low relief (less than 30 m), coastal plain catchments dominated by agriculture (about 50 % of land use). Shallow, nearly continuous aquicludes, a high stream density of 27 m ha^-1, and extensive channelization result in groundwater flow paths of less than 2 km. Rainfall is about 1000 mm yr^-1; streams export less than 20 % of rainfall during summer and annually during dry years, but about 50 % of rainfall during winter and wet years. Nitrate concentrations are high (50-400 µmol L^-1) and dominate total N (100-600 µmol L^-1) due to groundwater enrichment by agricultural fertilisers. Phosphate concentrations are low (0.5-2.0 µmol L^-1) and represent a small fraction of total P (2-6 µmol L^-1) due to strong soil sorption and particulate retention. From 1980 to the present, volume-weighted P concentrations have declined, while N concentrations have increased, probably as a result of recovery of channelised stream sections, increasing fertiliser use, and basin development.

Base flow occurs more than half of the time, but brief periods of stormflow dominate export of N and P. During storm events, NO₃^--N and total N concentrations decrease due to dilution of groundwater by low-N overland flow, and P concentrations increase due to by-passing of sorption sites and soil erosion. This hydrochemical behaviour focuses P export on events but diffuses N export between storms. Export coefficients for N and P vary from 2.22 kg ha^-1 yr^-1 and 0.05-0.35 kg ha^-1 yr^-1, respectively, and vary primarily with agricultural and suburban land use, particularly on well-drained soils close to streams. While P export is low and generally less than bulk precipitation inputs, N export is approximately equivalent to atmospheric inputs. In spite of considerable anthropogenic disturbance and loss of forest land cover, these basins are in or near steady state with regard to atmospheric inputs of N and P, although anthropogenic N and P leaks into their respective estuaries. Control of N and P export from these basins should focus primarily on losses of agricultural NO₃^--N to groundwater, soil erosion from disturbed land, and terrestrial retention of N and P from human wastes.