Deep groundwater is poorly safeguarded in California

Groundwater is a vital resource for California, providing 34 million Californians with water [1] and irrigating a multi-billion dollar agricultural industry. To ensure we sustainably manage groundwater for current and future societal, economic, and environmental water needs, protecting groundwater quantity and quality is crucial.

Under California’s Sustainable Groundwater Management Act (SGMA), local groundwater sustainability agencies (GSAs) are tasked with bringing their groundwater basins into balance and to achieve groundwater sustainability by 2040. While the state has taken the lead in delineating the horizontal extent of groundwater basins (under “Bulletin 118” recently rebranded as “California’s Groundwater”), GSAs are tasked with delineating the vertical extent of their groundwater basins (“basin bottom”). There are various methods for finding the basin bottom, such as finding the depth-to-bedrock or base of freshwater (defined as “the depth in a well where the water has less than 3,000mg/L of total dissolved solids (TDS)” [2]). And, California’s Department of Water Resources (DWR) gives a lot of discretion to GSAs on how they define their basin bottom[3].

The base of freshwater methodology for defining the basin bottom is premised on the assumption that salinity monotonically increases with depth and that brackish/saline groundwater is unusable. This past week, a new publication testing these assumptions was released in PNAS. The key findings from the study are:

• Fresh groundwater exists below the base of fresh water. This deeper freshwater is currently not protected under SGMA and at risk of contamination by oil and gas operations.

• Groundwater-pumping wells (all types - domestic, agricultural, industrial) in some areas are deeper than or encroaching upon bases of freshwater.
  This means that using base of freshwater data without taking into account well depths can provide a potential loophole for groundwater pumpers seeking to evade pumping restrictions within SGMA basins by pumping below bases of freshwater.

• Decades-old bases of freshwater maps are currently informing sustainable groundwater management in California. More the 60% of groundwater sustainability plans submitted under SGMA this past January, use outdated USGS base of freshwater maps that do not reflect current conditions.

• Deep groundwater is being increasingly accessed. As fresh water becomes increasingly scarce, brackish water (especially those that are being beneficially used) need to be sustainably managed. Brackish groundwater is an increasingly valuable resource for Californians now that it is economically viable to use and cheaper than other water supply options.

Water supply cost comparison, with the cost per unit of water increasing left to right (Modified from [4]. Data Sources: Dynamic Recharge [5]; On-Farm Recharge [6]; Recharge Facilities [7]; Conservation [8]; Groundwater Desalination [9]; Dams and Reservoirs [10]; Ocean Desalination [11].

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References and Notes

1. https://www.ppic.org/publication/groundwater-in-california/

2. T. Davis, M. K. Landon, V. G. L. Bennett. 2018. Prioritization of oil and gas fields for regional groundwater monitoring based on a preliminary assessment of petroleum resource development and proximity to California’s groundwater resource. Scientific Investigations Rep. 2018-5065, US Geological Survey.

3. See pages 8-9; California Department of Water Resources Best Practices Guidance for Hydrologic Conceptual Models. Available at: https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/Groundwater-Management/Sustainable-Groundwater-Management/Best-Management-Practices-and-Guidance-Documents/Files/BMP-3-Hydrogeologic-Conceptual-Model_ay_19.pdf

4. Matsumoto, S. M.M. Rohde, S. Heard. 2019. Policy Note: Economic tools to achieve groundwater sustainability for nature: Two experimental case studies from California. Water Economics and Policy, 5(4).

5. The Nature Conservancy, preliminary data.

6. Sustainable Conservation. 2014. On-farm infrastructure needs assessment and costs to implement groundwater recharge using flood flows on cropland. Sustainable Conservation: San Francisco, CA.

7. Perrone, D. and M.M. Rohde. 2017. Benefits and economic costs of managed aquifer recharge in California. San Francisco Estuary and Watershed Science, 14(2), https://escholarship.org/uc/item/7sb7440w#main.

8. Hanak, E., B. Gray, J. Lund, D. Mitchell, C. Chapelle, A. Fahlund, K. Jessoe, J. Medellín-Azuara, D. Misczynski, J. Nachbaur, and R. Suddeth. 2014. Paying for water in California. Public Policy Institute of California: San Francisco, CA.

9. McCann, H., A. Escriva-Bou, and K.Schwabe. 2018. Alternative Water Supplies. Public Policy Institute of California: San Francisco, CA. Available at: https://www.ppic.org/publication/alternative-watersupplies/

10. Proposed Proposition 1 CALFED project costs for San Luis Reservoir, Los Vaqueros Reservoir, Shasta Reservoir, Temperance Flat, Sites Reservoir.

11. Cooley, H. and R. Phurisamban. 2016. The cost of alternative water supply and efficiency options in California. Pacific Institute: Oakland, CA.