SPATIAL VARIATION IN NUTRIENT CONCENTRATIONS IN GROUNDWATER AND SURFACE WATER OF A SMALL HEADWATER AGRICULTURAL BASIN, LANCASTER COUNTY, PENNSYLVANIA

Agricultural land use can affect the nitrogen and phosphorus cycles in headwater streams. Excess nitrogen and phosphorus, from animal and anthropogenic sources, could be transported to groundwater, assimilated into organisms, absorbed to soil or sediment particle exchange sites, or flow downstream.

This study analyzed the spatial effects of pasture and dairy operations on nutrient concentrations in surface water and shallow groundwater in the 15 km² Big Spring Run watershed, Lancaster County, PA. Water samples from six transects perpendicular to the stream channel were collected between September 2008 and September 2010 and analyzed for nitrogen and phosphorous concentrations, specific conductance , ORP, dissolved oxygen, and pH. The main channel had two transects above and two transects below the confluence with a small tributary also having two transects. Surface water samples were collected in the thalweg, and groundwater samples were collected from shallow piezometers in the thalweg and left and right banks. Temperature and stage height were collected at three USGS gaging stations.

Surface water and groundwater concentrations of phosphorus, pH, and ORP showed little spatial variability. The tributary had higher surface water concentrations of TDN, nitrate, and ammonium than the main channel above the confluence, but had lower specific conductance. The main channel below the confluence reflected a mixture of water from the main channel and the tributary. Groundwater right bank and left bank ammonium, nitrate, and TDN concentrations showed great spatial heterogeneity, varying from greater than to less than stream concentrations. Groundwater samples from the thalweg reflected a mixture of left and right bank groundwater with a few exceptions in the tributary transects above the confluence. Discharge data collected by the USGS indicated a groundwater influx of ~20%. TDN concentrations in groundwater were higher during the spring while TDP did not show seasonal trends.

Agriculture had a major impact on both groundwater and surface water chemical compositions. The low groundwater influx, combined with great spatial heterogeneity, resulted in little impact on the surface water composition. The source of heterogeneity in groundwater composition is not clear.