Warren Subbasin Groundwater Recharge

Science Center Objects

The Hi Desert Water District (HDWD) provides water service to about 10,000 customers in the areas of Yucca Valley, Yucca Mesa, and some unincorporated areas of San Bernardino County. The study area is the 19 square mile Warren subbasin of the Morongo groundwater basin (approximately 100 miles east of Los Angeles, California).

Artificial Recharge

Water being released into a recharge pond, Yucca Valley, California

Water being released into a recharge pond, Yucca Valley, California. (Public domain.)

The Hi Desert Water District (HDWD) implemented an artificial groundwater recharge program in early 1995 to reverse the decline of groundwater levels in the Warren subbasin. Groundwater levels declined as much as 300 feet from 1940 to1994, owing to increased pumping. 

The artificial groundwater recharge program introduced imported surface water to recharge ponds to replenish ground water in the underlying aquifer and, in response, water levels recovered as much as 250 feet in the vicinity of the recharge ponds. However, since the implementation of the artificial recharge program, groundwater nitrate concentrations have shown a marked increase from 10 milligrams per liter to more than 110 milligrams per liter. Prior to the water-level rise, nitrate concentrations were near background levels, indicating that the nitrates had not reached the water table or that denitrification was occurring in the unsaturated zone. The increase in nitrate concentrations in Warren subbasin ground waters is attributed to the entrainment of septage by rising water levels.


  1. Identify the vertical distribution of nitrogen species in the unsaturated zone within the west hydrogeologic unit
  2. Determine the potential for denitrification in the unsaturated zone
  3. Provide a means of monitoring artificial recharge
  4. Develop water-management strategies that control water levels and nitrate concentrations

Principle Tasks

  1. Characterize the hydraulic, chemical, and microbiological properties of the unsaturated zone
  2. Monitor changes in water levels and water quality in response to the artificial-recharge program
  3. Use a groundwater flow and solute-transport model previously developed by the USGS to allow HDWD to manage their water-resource facilities


Wastewater Treatment

Map of study area, Yucca Valley, California.

Map of study area, Yucca Valley, California. (Public domain.)

In 2011, the California Regional Water Quality Control Board passed a resolution prohibiting the discharge of septic systems in parts of the Yucca Valley. In response, the HDWD implemented a Groundwater Reuse and Replenishment Project (GRRP). When completed, this project will return about 2,000 acre-feet of recycled water to the aquifer.

Residents and businesses in Yucca Valley, CA rely primarily on septic systems to treat their wastewater. The Hi-Desert Water District (HDWD) is constructing a sewer system and wastewater treatment plant (WWTP) as part of the GRRP, initially serving the West, Midwest, Mideast, Northeast, and East hydrogeologic units of the Warren subbasin. Monitoring of groundwater with high nitrate is needed to help evaluate the rate of movement of recharged water away from the GRRP and to help mitigate existing nitrogen plumes. Title 22 regulations stipulate that the fate of organic carbon discharged from the GRRP spreading ponds be monitored.



  1. Characterize the geohydrology of the saturated- and unsaturated-zone in the vicinity of the proposed WWTP (East hydrogeologic unit, Warren subbasin) by monitoring water-quality parameters including dissolved organic carbon (DOC) using new and existing monitoring wells.
  2. Monitor the vertical and horizonal flow of recharge water applied to the ponds in the vicinity of the GRRP using baseline geophysical data obtained prior to water application in the ponds and repeat geophysical surveys approximately monthly after water is applied.
  3. Continue to sample production and monitoring wells to evaluate nitrogen concentrations within the basin by collecting stable isotopes, major and minor ions, DOC, nitrates and nitrogen isotopes. Selected wells will be sampled for wastewater compounds and pharmaceuticals to identify if these compounds are in the applied wastewater and if they move downgradient in the groundwater.

Principle Tasks

  1. Drill and construct three monitoring sites in the vicinity of the GRRP recharge ponds.
  2. Install three lysimeters in the new monitoring sites to collect water chemistry.
  3. Monitor the vertical and horizontal movement of recharge water using electromagnetic logs, surface direct-current resistivity surveys, and relative gravity.