Comparing sampling schemes for monitoring pollutant export from a dairy pasture

Dairy cow pastures and feeding areas around barns can be a significant sour ce of nonpoint source pollutants to nearby streams. To help document the si gnificance of these sources, nutrient export in streamflow from a 56.7-ha, mostly agricultural, watershed located in southwestern North Carolina was m onitored from August 1994 to January 1996. Total nitrogen and phosphorus ex port rates from the upper, predominantly pasture, part of the watershed wer e 18.0 and 1.4 kg/ha/yr, respectively, as measured by weekly grab sampling and 18.7 and 4.9 kg/ha/yr, respectively, as measured from storm event monit oring. Nitrogen and phosphorus export rates for the area between the monito ring sites, which included overgrazed cow holding and feeding areas and far m buildings, were 376 and 86 kg/ha/yr, respectively for grab sampling and 3 51 and 160 kg/ha/yr, respectively, for storm event monitoring. To estimate the amount of reduction from nonpoint source controls necessary to effect a significant reduction in pollutant loading, statistical analyses of the lo ad data were conducted. The analyses for the five pollutants monitored show ed that total suspended solids would require the greatest reduction (34.6 p ercent for weekly grab and 33.6 percent for storm) in loading after the imp lementation of controls for statistical significance. Nitrate plus nitrite was found to require the least reduction (12.6 percent for weekly grab). Po llutant export rates computed from weekly grab samples and storm event samp les used separately were compared to corresponding export rates computed fr om combining grab and storm event samples to assess the differences in moni toring schemes.