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 2020/21 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. 2020/21 WQI Grades
As in 2018/19 and 2019/20, 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. Salts improved at ME-CC in dry weather but worsened in wet weather. The effect of the salts score on the decrease in overall score for ME-CC in wet weather was reduced by an improvement in the metals score. These changes were the main factors behind the respective increase/decrease in overall score for ME-CC in dry/wet weather for 2020/21. ME-VR held steady for dry weather but increased a little in wet weather due to improved bacteria and metals scores. ME-SCR dry weather score dropped due to a decrease in salts score, but the effect was lessened due to an improved organics score. ME-SCR wet weather score dropped for all subindices except metals, which improved but not enough to counteract the other subindices, resulting in a decrease in overall wet weather score and overall score for ME-SCR. 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