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HYDROGEOLOGIC FOUNDATION IN SUPPORT OF ECOSYSTEM RESTORATION: BASE-FLOW LOADINGS OF NITRATE IN MID-ATLANTIC AGRICULTURAL WATERSHEDS (EPA/600/R-99/104)
Citation:
Hantush, M. AND J. Cruz. HYDROGEOLOGIC FOUNDATION IN SUPPORT OF ECOSYSTEM RESTORATION: BASE-FLOW LOADINGS OF NITRATE IN MID-ATLANTIC AGRICULTURAL WATERSHEDS (EPA/600/R-99/104). U.S. Environmental Protection Agency, Washington, D.C., 1999.
Description:
Field evidence suggests that deep denitrification in the subsurface has the potential for removal of nitrate from ground water. Two adjacent agricultural watersheds in the mid- Atlantic coastal plain display remarkable differences in their ground-water nitrate discharges. It is believed that a combination of denitrification in reducing subsurface sediments and historical nitrogen input variations are responsible for the observed behavior. The process of deep denitrification may be influenced by geohydrologic factors, including the regional dip of strata in the mid-Atlantic coastal plain. A multi-scale-modeling framework is being developed for analysis and assessment of nitrate removal by denitrification in reducing subsurface sediments in riparian zones. Analytical characteristic solutions are derived for general transects in watersheds, and regional ground-water and nitrate transport and fate models are being developed. Preliminary assessment based on the characteristic solutions predicts a potential for more than 60 percent reduction of ground-water nitrate at the discharge to Morgan Creek and 20 percent reduction at the Chesterville Branch. This may be attributed to a shallower zone of reducing sediments (redox zone) below the former, where a significant fraction of ground-water fluxes that enter this zone are assumed to be denitrified, and a relatively deeper redox zone below the latter, whereby a greater portion of groundwater flux may escape denitrification before discharging to surface water. The analysis may have overpredicted ground-water nitrate loadings to the Chesterville Branch, possibly due to overlooking potential denitrification in the peat-rich riparian stream-valley sediments. Although predictions are consistent with field findings, more intensive sampling would be required to resolve deep denitrification from potential removal within riparian stream-valley sediments. Simple indices are derived on the basis of steady-state mass balance, which describe the removal capacity for ground-water nitrate in agricultural watersheds. The indices relate the reduction in nitrate base-flow loading to potential denitrification in the subsurface in agricultural catenments and stream riparian zones. A regional ground-water surface water flow model has been developed in order to investigate the impact of denitrification in ground water at the watershed scale. The simulated ground-water levels and stream discharges compared very well with quasi-steady synoptic measurements of heads in the surficial Columbia-Aquia aquifer and the lower Hornerstown aquifer. The measured stream flows in Morgan Creek are reproduced by the simulated values. The regional flow model is a prelude to analysis of regional nitrate transport and fate and assessment of the potential impact of deep denitrification on nitrate budget in the paired watersheds, including total mass discharge to the Chester River. This report covers a period from 10/01/97 to 08/01/99 and work was completed as of 08/01/99.