Nutrient management implications of patterns in groundwater chemistry and flow paths in impaired streams

Background/Question/Methods: Historic land-use change can reduce water quality by impairing the ability of stream ecosystems to efficiently process nutrients such as nitrogen.  We present study results from streams affected by urbanization, quarrying, agriculture, and impoundments in the Chesapeake Bay watershed in Maryland and Pennsylvania, USA.  Our objectives were to identify patterns among biogeochemistry, microbiology, geology, and hydrology in order to identify effective nutrient management practices for impaired streams.

Results/Conclusions: Results showed that chemistry and hydrology were related spatially and temporally at the groundwater/surface-water interface.  Water table fluctuation controlled subsurface redox conditions which dictated nitrogen dynamics.  Low water tables due to reduced stream flow created redox conditions that were more conducive to microbial removal of nitrogen.  Multiple, corroborating measurements of microbial activity confirmed that subsurface sediments were actively removing nitrate nitrogen, especially when more organic carbon was available for microbial respiration.  Mass spectrometry results suggested that removal of nitrate in ground water via denitrification was limited by carbon availability and that relatively small inputs of organic carbon corresponded to large reductions in ground water nitrate, especially where agricultural inputs of nitrogen were high.  Prehistoric wetland sediments buried due to historic land use and mill dam impoundments, were significantly better able to support denitrification.  Collectively, our results suggest that management practices that can increase organic carbon availability to microbes, increase ground water residence times, and expose buried wetland sediments may improve the nitrogen removal capacity of impaired streams.