Monitoring in the Tualatin River Basin to assess the effectiveness of the Oregon Forest Practicers Program: Preliminary results of phosphorus monitoring

The Oregon Department of Forestry is a designated management agency to provide a forestry program to assist in bringing the Tualatin River into complicance with pH and dissolved oxygen water quality standards. The Oregon Environmental Quality Commission set a total maximum daily load (TMDL) of 70 µg/l total phosphorus to control algae growth and meet these standards. The Department of Forestry has a basin effectiveness monitoring plan to determine that the Oregon Forest Practice Rules' BMPs are maintaining adequate control of phosphorus loadings from forest operations. Three sites monitored during May to October of 1990 were augmented by eight more sites in 1991. As laboratory methods were refined, the results became more accurate. The 1991 monitoring showed lower phosphorus levels that were consistent for each tributary. Mean total phosphorus levels ranged from 17 to 65 µg/l. Preliminary field reconnaissance suggests a correlation between phosphorus levels and underlying geology. Concentrations were lowest at sites underlain by tertiary intrusive basalts, next higher for sites with terrestrial basalt, next higher for one site with a basalt-sandstone bedrock mix, and highest for sites underlain predominately by sandstone. These results may be modified by 1992 monitoring and further analysis.