Evaluation of groundwater resources of the Anza-Terwilliger area, Anza, California

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It is important to better understand these characteristics of the aquifer system so that water managers in the area can develop a groundwater management plan and monitoring plans for the area to allow for prudent future management of water resources. 

Map of study area in  Anza-Terwilliger area, Anza, California

Map of study area.

(Public domain.)

Population growth and water use continue to increase in the Anza-Terwilliger area. Groundwater continues to be the sole source of water supply. However, the size and characteristics, current conditions, vertical changes in water quality, and water balance of the aquifer system in the Anza-Terwilliger area are poorly understood. It is important to better understand these characteristics of the aquifer system so that water managers in the area can develop a groundwater management plan and monitoring plans for the area to allow for prudent future management of water resources. The USGS proposal (CA07E) was not funded due to changes in economic conditions that limited available funds. Since that time, local water management entities have continued to compile existing hydrologic information, including from USGS studies in the 1970s, 1980s, and early 2000s, in preparation for developing a groundwater management plan for the groundwater basin. The High Country Conservancy (THCC), a non-profit, has been awarded funds through the California Department of Water Resources (DWR) Integrated Regional Water Management (IRWM) plan for the Santa Margarita River watershed to conduct a study of groundwater resources; THCC is working with Rancho California Water District (RCWD), who would be the direct partner on a Joint Funding Agreement. The THCC contacted USGS regarding selected groundwater assessment tasks. These tasks fit within the scope of the previously approved proposal but with a substantially reduced scope. Staff from THCC will be compiling, analyzing, and describing existing hydrologic and water-quality data in a report delivered to DWR early in 2014 to fulfill the terms of the grant. The USGS will complete selected technical analyses, listed below, that will provide additional critical information to THCC to advance the state of understanding of the basin; the cooperator would like work to begin by about May 1, 2013. The work completed by USGS will be documented in a short USGS report published in FY2014.

The objective of this work is to

  1. Better define basin shape and lithology,
  2. Resume groundwater-level monitoring (inactive since end of previous USGS study in 2008),
  3. Collect depth-dependent water quality and flow data,
  4. Synthesis of new information with previously published USGS information on the Anza area.

In FY13, USGS will update and refine analysis/modeling of existing gravity data and collect new data to fill gaps and constrain models used to interpret the gravity data and estimate aquifer thickness across the study area. This updated analysis will include evaluation of driller’s logs to map spatial variation in geologic units of different density; a better understanding of variations in subsurface density improves estimates of depths to bedrock. The interpretation of the gravity data will also be facilitated by updated estimates of the position of the water-table. Water-level data will be collected in a network of approximately 70 wells twice per year, spring and fall. Data from this network was previously collected by USGS through 2007. USGS will take the lead on collecting the data in the water level network during the spring 2013 measurement set. During this data collection, USGS staff will train local personnel to collect and quality-assure field water-level data; beginning in fall 2013 (FY14), THCC staff will take the lead on field measurements with quality-assurance, oversight, and database support from USGS. USGS will collect flow and water-quality data under pumping conditions from one or more production wells as part of this study. USGS will also measure vertical flow in the wellbore under ambient (unpumped) conditions if possible. These depth-dependent data will be used to begin to fill critical gaps in understanding of the connections between relatively deeper and shallower portions of the aquifer system and the depths at which contaminants of concern enter production wells. The newly collected data will be placed in the context of previously published USGS information and summarized in a manuscript early in FY14. The analysis will include compiling estimates of groundwater recharge from previous water-balance modeling that includes the Anza area. Reconnaissance estimates of groundwater discharge will be compiled from analysis of land use data and water-balance calculations.

The THCC and USGS analyses and reports will serve as the foundation for future proposals for cooperative studies to develop quantitative hydrologic models of the area that will support groundwater management. The proposed study will address a Science Direction of the USGS Science Plan for 2007-2017 to conduct a water census of freshwater in the U.S.

This project is related to project CA07E