A multiyear study of the effect on ground water of land disposal of sewage sludge (biosolids) provides insight into the risk of nitrate pollution of ground water in a south-eastern U.S. Piedmont Province setting. Two fields used for biosolid disposal and growth of row crops provided information about potential rates of nitrate movement to shallow ground water and possibly to ground water in the crystalline rocks underlying the sapro-lite. Initial studies based on semiannual sampling of a limited number of monitoring wells suggested that nitrate from a given biosolids application might take 200 to 300 days to reach the water table once a minimum cumulative nitrate loading had been reached after several years of biosolids application. In the longer used of two fields studied nitrate-nitrogen (NO3-N) concentrations of greater than 50 mg/l extended downward from the water table 6.1 m (20 ft) or more nearly to the crystalline bedrock surface.

Ground-water monitoring at a field to which biosolids were first applied during the study suggests that in some cases only 90 to 100 days may elapse before biosolids-derived nitrate reaches the water table. Water-table depths ranged from less than 3.05 m (10 ft) near a stream to over 9.2 m (30 ft) in the higher portions of the field.

Stable nitrogen isotopes were used to identify the appearance of nitrate in the ground water. These were also used to determine that nitrate from biosolids was entering a stream through baseflow and thereby contributing to nitrate buildup in surface waters.

The study also demonstrates that during biosolid application to fields nitrate accumulates to form a nitrate reservoir in the soil and saprolite. Nitrate in this reservoir can be moved to the saturated zone during periods of precipitation when precipitation exceeds evapotranspiration and use of the nitrate by crops. The ground water is especially vulnerable to nitrate buildup when a field lies fallow during the groundwater recharge period during winter and spring months. Evidence from this study as well as others suggests that nitrate concentrations in ground water will decline once biosolids application ceases.

In considering use of land disposal and cropping for handling municipal sewage sludge, decision-makers need to contemplate future possible uses of the land and whether or not nitrate-enriched ground water may be a limiting factor in future uses. Trade offs must be made between the risk of significant ground-water pollution and additional costs for nitrate removal.