Document Type

Technical Report

Publication Date



Hydrology, TMDL, Water sampling


In order to determine the impacts of non point source (NPS) pollution and to develop Total Maximum Daily Loads (Tr\tfDLs), accurate measurements of pollution loads in streams are critical. The objectives of this study were to accurately detemline pollutant loads at two sites by intensive storm sampling, to develop sub-sampling and other data analysis techniques, to detemline the effect of sample interval on load calculation accuracy, and to find the minimum sample interval required to determine storm loads at a required accuracy. The two stream sites used were a 1st order and a 3rd order stream in the Illinois River basin in Arkansas. The samples were analyzed for NO3-N, ~-N, TKN, Ortho-P, Total-P, and TSS. Storm loads were calculated by multiplying discharged volume by concentration for each sampling interval and summing over the storm. The loads calculated using the 30 minute interval data were termed the "best estimate" load. Loads were also calculated for 60, 120, and 240 minute sampling intervals using subsets of the data. The load estimates for the longer sampling intervals were expressed as a percentage of the best estimate load. The results showed that as sampling interval increased the error of the load estimate increased. For example, the Moores Creek data indicated that if we desire that the calculated TSS load is within 5% of the best estimate load with a 95% confidence level, we need to sample approximately every 50 minutes during a storm. This optimum sampling interval varies with the parameter measured and with the stream order. The information gained from this study should help water quality investigators develop sampling schemes to meet their goals of accuracy, precision, efficiency, and cost.

Report Number

PUB 182