Using Spatial Analysis to Assess Nitrate Sources in Two Headwater Agricultural Streams in Central Pennsylvania.

Elevated nitrate (NO₃) concentrations in groundwater represent an important source of nitrogen to agricultural streams, particularly during baseflow conditions. Identifying spatial and temporal patterns in stream water chemistry may elucidate landscape features and hydrological processes within a watershed that either contribute to or mitigate these NO₃ additions. To test this idea, we used spatial statistics to analyze the longitudinal variation of stream water NO₃ concentrations in two adjacent agricultural streams (FD36 and RS) located in central Pennsylvania. We sampled each stream at baseflow on 14 dates from April 2009 to January 2012. On each date, water samples were collected every 10 m over a 500 m stream reach. All major groundwater seeps within each reach were also sampled on the same dates. Nitrate concentrations varied seasonally in FD36 and were significantly lower during the summer compared to the winter (1.1 vs. 6.0 mg L^-1). No seasonal patterns were observed in RS (mean, 13.3 mg L^-1). Spatial analysis, however, showed that NO₃ concentrations in both watersheds were longitudinally dependent on all dates, but that this dependency varied by different degrees and distances. The distance over which NO₃ concentrations were dependent averaged 88 and 149 m for FD36 and RS, respectively, suggesting that concentrations in FD36 were more spatially variable. The number and location of seeps on each sampling date were positively correlated (R²=0.86) to the spatial patterns observed in stream water NO₃ concentrations; thus, seeps are likely a major source of NO₃ in these watersheds. Spatial analysis of stream water chemistry can potentially be a valuable tool for assessing potential sources of NO₃ to headwater streams, which is essential for the development of effective and viable remedial strategies in agricultural streams.