Groundwater Basics

Groundwater is water located below ground where the voids or pore spaces between sediment grains or rocks are fully saturated. Groundwater originates from a variety of sources, including precipitation, irrigation, infiltration from surface waters, and injection wells to name a few. Groundwater can flow from one part of the subsurface to another; it can also leave the subsurface as outflow to streams or other surface waters, spring flow, evaporation, and discharges from wells. In a sense, all groundwater starts as some form of surface water. The two types of water, surface water and groundwater, are inherently connected.

Groundwater is part of the water cycle

Depending on location, slope, geology, soil type and vegetation, the ground can receive and retain groundwater similar to the way a sponge soaks up water. The percolation process that moves water from the surface to the subsurface is often a relatively slow process that occurs over several years to over a millennium in some cases. 

Napa County and other public agencies have been monitoring and studying groundwater resources since the mid-1900s. This work has improved our understanding of groundwater conditions. Based on these efforts, the County has been able to:

  • Expand voluntary groundwater monitoring in key locations to provide better data and fill data gaps
  • Develop better groundwater data collection procedures
  • Report on annual groundwater trends and conditions
  • Estimate the rates of aquifer replenishment and study groundwater and surface water interaction

Groundwater data availability in Napa County varies widely among the subareas. The bulk of the historical and current groundwater level and quality data is located in the Napa Valley area with limited data elsewhere.

Groundwater level trends in most of the Napa Valley floor appear to be stable for the majority of wells monitored. While many wells have shown at least some degree of response to recent drought conditions, levels do recover in the spring after winter rains. Elsewhere in the County long-term groundwater level records are limited, with the exception of the MST Subarea. Groundwater level declines observed in the MST as early as the 1960s and 1970s have stabilized since 2009.

Infiltration of precipitation has been shown to provide significant groundwater recharge in Napa County, particularly in unconsolidated geologic settings like much of the Napa Valley floor (Kunkel and Upson 1960, LSCE and MBK 2013). Recently, long term precipitation records in Napa County have been used to develop a water year rating system that defines Very Dry, Dry, Normal, Wet, and Very Wet water year types (LSCE 2016, 2NDNATURE 2013).

Napa State Hospital Annual Precipitation graph

Napa State Hospital Annual Precipitation - Click image to view larger

Since the mid-1900s consecutive Dry or Very Dry years have occurred five times. Groundwater levels have shown greater sensitivity to shorter periods of well below-average precipitation, such as the consecutive Very Dry years of 1976 and 1977. Many monitored wells recorded their lowest groundwater levels in 1977, although the subsequent period from 1987 to 1994 saw a long period of below-average precipitation. Nevertheless, for those areas of the county where groundwater data are available, groundwater levels typically recover to previous levels with the return of average rainfall, despite experiencing some decline during times of drought.