The Effects of Artificial Recharge on Nitrate Concentrations in Groundwater in the Joshua Tree Subbasin, California

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The Joshua Basin Water District (JBWD) is implementing an artificial groundwater recharge program to reverse the decline of groundwater levels and to store water in the Joshua Tree groundwater subbasin of the Morongo groundwater basin (~100 mi east of Los Angeles, CA).

Map of Joshua Tree study area.

Map of study area

The artificial recharge program involves the application of imported surface water to recharge ponds to replenish groundwater in the underlying aquifer. In addition, septic tanks are currently the primary form of wastewater treatment, although small-scale package treatment plants may be mandated for future developments. JBWD is concerned that an increase in nitrate concentrations may result from septage from existing developments with high septic-tank densities reaching the water table and from the possible entrainment of septage by the rising groundwater levels.

The objectives of the proposed continuation of the USGS-JBWD Cooperative Study are: 1) monitor the movement of the wetting front and changes in water levels and nitrate concentrations in response to the artificial-recharge program; 2) continue an existing basin-wide water quality monitoring program; and 3) use the existing groundwater flow model to calculate inflows and outflows from subbasins within the model using ZoneBudget.

This study will address Goal 1, Objectives 2/3 (advancement of monitoring networks and techniques for determining water-quality/assessment of water resources and their suitability to meet human and ecosystem needs) and Goal 2, Objective 4 (comprehensive understanding of human interactions with water availability) (U.S. Geological Survey Water Science Strategy, 2012. This proposed work will provide water managers with more detailed information with which to address important issues, such as artificial recharge and nitrate contamination.

The study consists of three primary tasks: 1) data collection at existing unsaturated-zone monitoring sites at the recharge facility (JTUZ-3 and -4); 2) continuation of the basin-wide water-quality monitoring program; and 3) model updates.