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United States Department of Agriculture

Agricultural Research Service

Title: Flow-Proportional, Time-Composited, and Grab Samples Estimation of Nitrogenexport from An Eastern Coastal Plain Watershed

Authors
item Stone, Kenneth
item Hunt, Patrick
item Novak, Jeffrey
item Johnson, Melvin
item Watts, Donald

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 26, 1999
Publication Date: N/A

Interpretive Summary: Water quality investigations must be conducted in a manner to ensure a balance between the amount of financial resources spent and information obtained. As part of a Water Quality Demonstration Project in the eastern Coastal Plain, stream water quality was monitored and compared using four methods. These methods automatically sampled the stream at a fixed time and at a fixed flow volume. The stream was also sampled every 2-3 weeks with grab samples. Flow measurements using these methods were very similar. When there was higher than normal flow, the method using a fixed volume sampled more frequently and gave higher flow measurements. Because of its higher flow measurements, the fixed volume method predicted significantly higher nutrient losses for both nitrate-N,ammonia-N, and TKN. Sampling with grab samples and at fixed times predicted similar findings for the entire study period; however, in a few months, there were significant differences between the two. These results suggest that an appropriate sampling method should adequately weight sampling of both storm and base flows.

Technical Abstract: 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 samplin 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 (r2=0.97). Because of more intensive measurements during high flow, the FP method measured higher flow rates during the sampling period. For all four methods, nitrate-N and ammonia-N concentrations were not correlated to stream flow. Because of the significantly higher flow, the FP method predicted significantly highe 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.

Last Modified: 4/17/2014
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