Water Quality in Watersheds Thurston County monitors various measures of water quality including water temperature, dissolved oxygen, pH, and fecal coliform bacteria. The County began using a Water Quality Index (WQI) for the 2016-2017 water year. The WQI is designed to rate general water quality relative to expected water quality and uses a scale from 0 to 100. The higher the number, the better the water quality. Scores less than 40 suggest that water quality did not meet expectations and indicate such sites are of the highest concern for impairment. Because the WQI measures expected water quality, the WQI score cannot be used to compare two watersheds. For example, if water quality is expected to be poor in watershed A and excellent in watershed B, and the expectations were met for each watershed, then both would be rated of low concern because the expected water quality rang true.
Of the nine watersheds Thurston County tracks, the Totten Inlet watershed was of the least concern with a score of 85. Budd Inlet watershed had the lowest score (51).
Water Quality in Lakes Thurston County began monitoring lake water quality in 1988. Sampling sites for lakes are typically located in the deepest part of the lake, and lakes sampled may change from year to year based on a number of factors. Monitoring on Deep Lake (in Millersylvania State Park) and Capitol Lake (owned by the State of Washington) ceased in 2011.
Of the eight lakes Thurston County currently monitors, Summit Lake has the best water quality while Black Lake has the worst.