20 - Failure of agricultural riparian buffers to protect surface waters from groundwater nitrate contamination

Summary

ABSTRACT For two years we studied the flux of nitrogen moving in shallow groundwater from double row-cropped uplands through a flood plain and into a second order stream in Maryland. Two floodplain sites were compared: one forested and the other vegetated by grass. At both sites, the soil layer through which the groundwater moved was very sandy. The nitrate concentrations leaving the crop fields were 20–30 mg N1⁻¹ and averaged 25 mg N⁻¹. Nitrate concentrations declined about 32% on average from the field edge to 48 m into the forest and this decrease was about 44% on average in the grassed buffer. These decreases were greater in the winter than in the summer. Nitrate to chloride ratios declined about 43% across the riparian forest transect. Declines in nitrate concentration were not accompanied by offsetting increases in dissolved organic N or ammonium. Soil E_h averaged 191 mV and 263 mV at 33 m and 48 m into the forest, respectively. While nitrate removal rates were the highest of three study sites we have investigated in the Maryland Coastal Plain, nitrate concentrations entering the stream channel were still high (12–18 mg N⁻¹). The flux of nitrate in groundwater from the farm fields at this site clearly exceeded the nitrate removal capacity of these riparian buffers.

INTRODUCTION

Coastal receiving waters are often overenriched with nutrients, especially in cases where the drainage basins are intensively farmed or support large populations of humans (Beaulac & Reckow, 1982; Turner & Rabalais, 1991).