Napa County and other public agencies have been monitoring local groundwater resources since the mid 1900s.
Based on recent studies and bi-annual monitoring of groundwater levels in over 100 volunteered wells (map (PDF)), level trends in the Napa Valley Sub-basins of the Napa-Sonoma Valley Groundwater Basin are stable in the majority of wells with long-term records. Although some wells show a response to drought conditions, levels in recent drought years are generally higher than those during the 1976 to 1977 drought. Elsewhere in the County long-term groundwater level records are more limited, with the exception of the Milliken-Sarco-Tulucay (MST) Subarea. Groundwater level declines observed in the MST Subarea as early as the 1960s and 1970s have stabilized since about 2008. The observation that groundwater level responses differ within the MST Subarea and even within the north, central, and southern sections of this subarea indicate that localized conditions, whether geologic or anthropogenic in nature, might be the primary influence on conditions in the subarea.
While there is limited long-term groundwater quality data (map (PDF)), overall quality appears to be good except in select areas in the most northern and southern parts of Napa County. Areas near Calistoga exhibit geothermal influences and the southern lowlands of the County exhibit elevated levels of naturally occurring dissolved solids and chlorides, likely due to their proximity to San Pablo Bay.