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HYDROGEOLOGIC FOUNDATIONS IN SUPPORT OF ECOSYSTEM RESTORATION: BASE-FLOW LOADINGS OF NITRATE IN MID-ATLANTIC AGRICULTURAL WATERSHEDS
Citation:
Hantush, M. AND J. Cruz. HYDROGEOLOGIC FOUNDATIONS IN SUPPORT OF ECOSYSTEM RESTORATION: BASE-FLOW LOADINGS OF NITRATE IN MID-ATLANTIC AGRICULTURAL WATERSHEDS. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-99/104, 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.