Publication Summary

This report compares two independent water quality data sets from the Palouse and Yakima rivers. One set was collected by USGS using integrated sampling techniques and the other by Ecology using a grab sample method.

In general, single point grab samples provided results similar to the more intensive and expensive integrated sampling when comparing concentrations. However, for sediment and total phosphorus, loads calculated from grab sample results were much lower than loads calculated from integrated sampling.

If grab sample data are used to determine loads, the analyst should fully understand the potential limitations of this sampling method.

Results collected by integrated sampling (USGS) and by single grab sampling (Washington State Department of Ecology) were compared at two stations, the Palouse River at Hooper and the Yakima River at Kiona. About 10 years of matching data exist at each station for each monitoring program. The purpose of this analysis is to compare these two independent data sets.

In general, single point grab samples provided results similar to the more intensive and expensive integrated sampling method for the Palouse and Yakima rivers when comparing concentrations of water quality constituents. Sampling methodology should not significantly affect conclusions based on concentrations at these stations. However, for sediment and total phosphorus, loads calculated from grab sample results were much lower than loads calculated from integrated sampling results. This significantly low bias could lead to incorrect conclusions. If grab sample data are used to determine loads, the analyst should fully understand the potential limitations of this sampling method.