In 2017, the City and County of Napa entered into an agreement to study surface water quality within the municipal watersheds of the reservoirs that provide the City with the majority of its drinking water. The Study provides detailed and comprehensive data that can be used by both the City and the County to improve reservoir and land use management to better protect the public’s water supply.
The Study calls for collecting water quality samples from 20 locations along streams that flow into Milliken and Hennessey reservoirs. Samples are collected during storm events throughout the winter months and analyzed for nearly 30 different constituents. The voluntary 2017 agreement commits both the City and County to each spend up to $200,000 annually over three years to conduct this Study, for a total cost of $1.2 million.
Samples were collected over three years, although the last two years were very dry and there were not many storms where measurements could be collected, so data was limited. However, no significant contamination has been detected to date.
The study does not directly relate to the GSP, as it analyzes water quality in the upper watersheds, outside of the Napa Valley Subbasin. But it does complement the GSP, as it provides valuable information on the water quality of surface flows in the tributaries that flow into the subbasin.