To ensure that California's groundwater is sustainably managed in the future and over the long-term, current state definitions of what constitutes groundwater may need to be revised, according to research published this week in PNAS. A McGill University-led research team has analyzed big data of more than 200,000 groundwater samples taken from across the state and found that there are problems with the guidelines used for groundwater management. Known as the 'Base of Fresh Water', the guidelines are close to fifty years old and don't reflect current uses, knowledge, concerns or technologies related to managing groundwater in this coastal state with a multi-billion-dollar agricultural industry.

The research shows that existing groundwater wells already penetrate and encroach upon the bases of fresh water that are used to define basin bottoms. In addition, brackish waters exist within current groundwater basins, and fresh water exists outside delineated groundwater basins. Brackish water, which was once deemed unusable, can now be used, thanks to technological advances. Finally, there are concerns about regulating groundwater on the basis of its quality rather than its usage, as is currently the case, since this provides a loophole for potential groundwater users who could drill deeper and skirt existing restrictions on freshwater pumping.

Together, these findings suggest that groundwater may already be poorly safeguarded in some places and that the 'Base of Fresh Water' concept may need to be reconsidered as a means to define and sustainably manage groundwater in future.