FLOW-PROPORTIONAL, TIME-COMPOSITED, AND GRAB SAMPLE ESTIMATION OF NITROGEN EXPORT FROM AN EASTERN COASTAL PLAIN WATERSHED

The balance between resources expended and information obtained is an integral aspect of water quality investigations. As part of a Water Quality Demonstration Project in the eastern Coastal Plain, we monitored stream water quality at the watershed outlet. Four methods of assessing stream water quality were compared. These methods were time-composite sampling with continuous flow measurements (TC), flow-proportional sampling with independent measurement of flow (FP), grab sampling with instantaneous flow measurements (IG), and grab sampling for quality assurance/quality control checks using daily USGS flow measurements (UG). Flow measurements using the TC and IG methods were highly correlated (r 2 = 0.97). Because of more intensive measurements during high flow, the FP method sampled greater flow rates during the sampling period. For all four methods, nitrate-N, ammonia-N and total Kjeldahl nitrogen (TKN) concentrations were not correlated to stream flow. Because of the significantly greater flow sampled, the FP method predicted significantly greater mass loading rates for both nitrate-N, ammonia-N, and TKN. Grab sampling (IG and UG) and the TC methods were not significantly different for the entire study period; however, a few monthly differences were significant. These results suggest that an appropriate sampling method should adequately weight sampling of both storm and base flows.