River water quality in the County of Ventura is generally good, with the overall Index showing A to C grades at the mass emission stations during 2019/20 in both wet and dry weather. Mass emission stations are located in the lower reaches of the three major Ventura County watersheds: Calleguas Creek (ME-CC), Ventura River (ME-VR2), and Santa Clara River (ME-SCR). Mass Emission drainage areas are large and include multiple sources of discharge, including natural sources, wastewater treatment plants, municipal and agricultural sources, and groundwater discharges.
Figure 1. WQI Rating System
Figure 2. 2019/20 WQI Grades
The Index shows 2019/20 scores to be similar to 2018/19 in all watersheds, with slight improvements at ME-SCR in both wet (improved bacteria, nutrients, organics and metals scores) and dry (improved organics and metals scores) weather. As in 2018/19, ME-CC minimally exceeded the bacteria WQO in dry weather but the effect of the exceedance on the score is large because the bacteria score is based on only one sample and therefore had a 100% exceedance rate, but overall the ME-CC scores were similar to the previous three years. ME-VR held steady for dry weather but dropped a little in wet weather due to a decrease in bacteria and metals scores, although the wet weather nutrients score improved. Lower scores are typically affected by salts in dry weather, and salts, metals and bacteria in wet weather.
Figure 3. Combined Wet and Dry WQI Trends for Each Receiving Water Station
Fluctuations in grades are usually associated with changes in salts and bacteria in dry weather, and salts and metals in wet weather. Overall water quality improved in Ventura County from 2003/04 to 2011/12, but then began dropping through 2016/17 coinciding with increasing (severe) drought conditions in Ventura County and a corresponding decrease in scores for salts and metals. A return to closer to average rainfall in 2017/18 through 2019/20 coincided with an improvement in salts scores, however, the heavy impact of the Thomas Fire on the Ventura River Watershed likely dragged down the 2017/18 scores at ME-VR2 (metals and toxicity in wet weather and bacteria in dry weather), reducing the scores overall. The Ventura River Watershed appears to have rebounded significantly since then. A more detailed discussion is available in the monitoring section of our annual reports, beginning with the 2012/13 monitoring year (December 2013 report).
Figure 4. WQI Trends for All Locations Combined