Between 1979 and 2010, groundwater pumping in the Santa Rosa Plain watershed (an area larger than the groundwater basin) exceeded the water flowing into the watershed’s aquifer by about 3,000 acre feet annually. The Groundwater Sustainability Plan will refine and update this model to reflect the smaller basin area, but generally, an imbalance can lead to dropping groundwater levels and a reduction in creek and stream flows. Climate change models predict longer, hotter summers and more erratic precipitation patterns, a combination which is likely to increase demands on groundwater during dry periods. Careful monitoring and management of groundwater and surface water is needed to ensure a sustainable, reliable supply of water in our wells, creeks and streams.
Groundwater levels in the Santa Rosa Plain’s shallow aquifer have been relatively stable over time. Near the Laguna de Santa Rosa, water levels are close to the surface; along the eastern basin boundary, levels are about 15 to 30 feet below ground surface; near the southern edge of the plain, water levels are as much as 50 feet below the surface.
In intermediate and deeper wells in the southern Santa Rosa Plain, groundwater levels declined in the late 1970s and 1980s as pumpage increased. The decline peaked in the early 1990s and began to recover in the early 2000s when water use efficiency and increased reliance on the Russian River reduced groundwater use.
Groundwater quality is generally high in the Santa Rosa Plain basin, but naturally occurring elements such as iron, manganese, boron, and arsenic are widely variable in groundwater and can pose problems in some areas. Areas in southern Santa Rosa Plain also exhibit increasing chloride concentrations.
Groundwater Sources and Usage
The 78,720-acre Santa Rosa Plain groundwater basin is located within the larger 167,680-acre Santa Rosa Plain watershed (generally corresponding to the Laguna de Santa Rosa and Mark West Creek watersheds). A 2014 US Geological Survey study estimated that 13.7 billion gallons of groundwater were used in the Santa Rosa Plain watershed each year between 2004 and 2012, representing nearly 50% of the basin’s water supply. On average, about 96% of the water used is replenished annually.