STATISTICALLY-BASED SAMPLING DESIGN FOR A LARGE RIVERS WATER QUALITY MONITORING PROTOCOL

Suzanne Magdalene, St. Croix Watershed Research Station
Daniel Engstrom, St. Croix Watershed Research Station
Joan Elias, National Park Service
Erik Beever, National Park Service

The National Park Service (NPS) Great Lakes Network (GLKN) is developing a monitoring protocol for the water quality of large, non-wadeable rivers, including the St. Croix National Wild and Scenic Riverway. This is one of the first NPS protocols of its kind in the nation, so the monitoring protocols of the U.S. Geological Survey National Water Quality Assessment (NAWQA) program and the U.S. Environmental Protection Agency Environmental Monitoring and Assessment Program (EMAP) were used as guidelines. In particular, NAWQA's nonrandom site selection method and EMAP's random site selection method were both deemed as important. A sizable database of historical water quality data, provided by 30 years of weekly to quarterly monitoring at six locations on the St. Croix River, was analyzed to determine the background variability of various water quality parameters. The Riverway was stratified into an upper portion dominated by forests and wetlands, and a lower portion dominated by urban and agricultural development. Three parameters (pH, dissolved oxygen, and total phosphorus) were selected to represent very low, moderate, and very high variability, respectively. Statistical power analyses indicated that historical monitoring efforts involving parameters with very low to moderate variability had sufficient power to detect change. However, the statistical power of historical monitoring for total phosphorus was very low. The observed variabilities were used to determine the statistical sample size (number of randomly-selected monitoring stations) that would be required to detect a 20% change from background. Based on the mean variability of pH and dissolved oxygen, the statistical sample size should be three stations in the upper portion and three stations in the lower portion of the Riverway. In contrast, the high observed variability of total phosphorus would require over 20 stations in each portion of the Riverway - far more than the GLKN budget would allow. Therefore, the decision was made to seek to detect 20% change in very low to moderate variability parameters. In addition to the six randomly-selected stations, several nonrandomly-selected stations will be chosen to monitor key tributaries, to expand monitoring of Lake St. Croix, and to increase the sampling frequency at stations monitored by other agencies.