Numeric modeling of subsurface groundwater flow for the study of chemical transport from agricultural fields toward estuaries and tidal marshes on Wadmalaw Island, South Carolina

Chemical runoff from agricultural fields into nearby tidal estuaries is a potential danger to many marine species in the estuaries. Although surface runoff is the most obvious means by which chemicals might travel from a field into an adjacent estuary, it is not the only mode of transport. Subsurface water flow and associated transport of chemicals derived from fertilizer have been documented to be a component of recharge from land into offshore waters.; A numeric single-layer hydrologic flow model and a model of advective chemical transport were developed for part of Wadmalaw Island near Charleston, SC, to examine subsurface migration of chemicals in the groundwater under agricultural fields and in ponds.; Large irrigation ditches adjacent to fields were found to affect subsurface chemical transport. Such transport directly from fields to marshes requires about 400 days, but subsurface transport from fields to large drainage ditches occurs within a week, and the ditches drain to adjacent marshes as point sources within hours after heavy rain. Of the total chemical assumed initially in the water table under irrigated fields, only 0%--3% migrated directly to the marshes in three years, while 33%--46% migrated to the ditches. This study predicts that if one pound of a pesticide is applied per acre of land and 10% of that pesticide reaches the groundwater, then advective transport (neglecting adsorption, dispersion, and decay) will result in a maximum concentration of 10.6 ug/L in the groundwater migrating to the drainage ditches.; Chemicals were also found to migrate from irrigation ponds to adjacent deep drainage ditches. In one pond located only 75 feet from a parallel ditch, most of the chemical initially in the pond migrates to the ditch within three years. It was found that when the pond is perpendicular to the ditch, chemical transport to the ditch over three years is reduced by about half. At a separation distance of 400 feet, three-year total transport is reduced by more than 90% for both orientations.; The location of an irrigation pond was found to be a factor in chemical transport from fields to marshes. Locating a pond between an agricultural field and an adjacent marsh reduces three-year total chemical transport to the marsh by 30% compared with a location inland of the field. The orientation of a pond near the western marsh was found to have little effect on the steady-state size of its capture zone, but does affect the rate at which water migrates from the pond to the marsh. When the pond is perpendicular to the marsh (and thereby to the iso-head contours), the channel of pond-to-marsh water flow is narrower than when the pond is parallel to the marsh. Total pond-to-marsh water flow is reduced somewhat in the perpendicular orientation, but the linear flow speed is greater in the narrower channel, reducing total migration time.; Finally, despite periodic drawdown from irrigation pumping, water leaks from the southern tip of the pond to the western marsh in about 11 years over a distance of about 1,500 feet.; These findings suggest that nonpersistent chemicals should be applied in areas adjacent to estuarine tidal creeks.