Groundwater Availability Within the Salton Sea Basin Final Report

Abstract

It is widely recognized that increasing demands for water in Southern California are being affected by actions to reduce and redirect the amount of water imported from the Colorado River. In the Imperial Valley region, for example, import reductions will not only affect agricultural users but also could produce significant collateral impacts on the level and quality of water in the Salton Sea, its regional ecology, or even the long term air quality in the greater basin. The notion of using groundwater in the Imperial Valley as an additional source for agricultural or domestic needs, energy production, or Salton Sea restoration efforts, so as to offset reductions in imported water, is not a new concept. Even though it has been discussed recently (e.g., LLNL, 2002), the idea goes back, in part, to several studies performed by the US Department of Interior and other agencies that have indicated that there may be substantial, usable amounts of groundwater in some portions of the Imperial Valley. It has been estimated, for example, that between 1.1 and 3 billion acre-feet (AF) of groundwater lie within the extended, deep basin underlying the valley and Salton Sea region, even though much of it may be unrecoverable or too poor in its quality (Imperial County, 1997). This is a significant volume with respect to the total annual precipitation volume received in California, whose average is close to 200 million (or 0.2 billion) AF per year (DWR, 1998), and especially with respect to the total annual precipitation received in the Salton Sea watershed itself, which we estimate (Appendix A) to be approximately 2.5 million acre feet (MAF) per year. Clearly, a thorough appraisal of the groundwater resources in the Imperial Valley and Salton Sea region--i.e., an assessment of their overall physical availability--will be needed to determine how they can be used and managed to suit new or redirected demands in the region. Development of an improved or updated groundwater assessment in the Salton Sea Basin is the subject of the project described in this report. Much of the project work was done in cooperation with the US Bureau of Reclamation, Lower Colorado Region Office ('Reclamation'), which manages the Salton Sea Restoration project for the US Department of the Interior, and complements other recent assessment efforts (e.g., Imperial County, 1995). In this context, the notion of groundwater availability is defined by four separate, but interrelated concepts or components: (1) Volume and Capacity--This refers to the volume of groundwater available in storage in (or the related storage capacity of) the sediments and geologic media that comprise a groundwater basin. The volume of groundwater in a basin will vary in time as a function of recharge, well production, and land subsidence. (2) Producibility--This refers to the ease or difficulty of extracting groundwater in a basin from wells. Groundwater producibility will be affected by well depth and the formation permeability surrounding the open intervals in wells. (3) Quality--This refers to the extent that water produced from wells is potable or otherwise suitable for domestic or other uses. It may also refer to the chemical compositions of groundwater that are unrelated to potability or suitability issues. Groundwater quality will be affected by its residence time and flow pathway in the formation and will also be influenced by the quality of its original source before entering the groundwater regime. (4) Renewability and Recharge--This refers to the extent that groundwater is recharged to the basin as part of the natural hydrologic cycle or other artificial means. Groundwater renewability is normally a function of recharge derived from precipitation (and thus a function of regional climate), but may also be affected in local areas by irrigation, leaking canals, aquifer storage and recovery operations, and so forth. Along with the other factors, renewability will strongly affect how much water can be safely produced from a basin from one year to the next. In this report, we specifically consider two categories of information that comprise, are pertinent to, or facilitate such a groundwater assessment. The first category includes the vast assortment of descriptive geologic, hydrologic, chemical, and climatic data related to the regional groundwater aquifers in the Salton Sea area and the occurrence, movement, production, and quality of groundwater. We will refer to these as Hard Data. They may include, for example, spatially distributed geologic or lithologic information, operational wells, water level monitoring data, and well production reports, groundwater quality information, other land use information, historical precipitation and climatic records, and so forth. The second category includes more interpretive or analytic information based upon, or derived from these data and knowledge of related geologic, hydrologic, chemical, or climatic processes.