Two recent California Department of Water Resources (DWR) reports1 on Borrego Water District (BWD) Monitoring Wells MW-4, just east of the Borrego airport, and MW-5, near the Borrego Sink, are now available on the DWR web site:http://wwwdpla.water.ca.gov/sd/groundwater/groundwater_pubs.html
MW-4 at the east end of the Borrego airport was primarily intended to define geologic formations and strata; but the Technical Information Record for MW-4 contains worrisome disclosures about the future availability of groundwater in the Borrego basin:
MW-5 is located 1.2 miles northeast of the Borrego Sink. The purpose of the MW-5 study was to increase knowledge of the geology and groundwater levels in the area of the Borrego Sink. The following is an excerpt from the TIR for MW-5:
There are several indications of highly indurated to lithified materials deeper than about 200 feet bgs [below ground surface]... these rocks are highly indurated at relatively shallow depths. The level of induration implies that the specific yield2 of these rocks is probably 5 or less percent... whereas previous investigators have interpreted about 10 or higher percent specific yield in the Borrego Valley groundwater basin...3 based on previous models and estimates of specific yield for the basin, the southern part of the Borrego Valley groundwater basin may have a significantly lower storage capacity than previously proposed.
...Because this project produced the first core available from the Borrego Valleygroundwater basin, it provides an important check on the traditional methods used to interpret specific yield and other aquifer characteristics in this groundwater basin and other groundwater basins in the state. The interpretation of observations made during the drilling of MW-5 imply that there may be substantially less water in storage in the Borrego Valley groundwater basin than previously interpreted. Additional collecting of core samples and careful logging of wells is necessary to re-evaluate the distribution of specific yield and other characteristics in the southern portion of Borrego Valley.4
In sum, core samples form MW-5 indicate:
- The sub-surface materials in the vicinity of the well are tightly compacted, very hard, and rock-like, i.e., "highly indurated to to the ground surface. These dense materials contain relatively little water and make pumping what water there is difficult.
- The above conditions result in a specific yield of 5% or less; at most only 43 percent of the average of historical estimates for the valley, or less than half what was previously estimated;
- Water drawn from the monitoring well is high in TDS and sulfates and not potable without treatment;
- The southern part of the Borrego Valley groundwater basin may have a significantly lower storage capacity and at least some of the water may be of lower quality than previously proposed, resulting in substantially less usable water than was heretofore assumed.
TIRs bear the following caveat: "This TIR is primarily a working paper and is subject to revision or replacement;" so it is difficult to say with certainty what these disclosures mean for Borrego's future water supply and estimates of aquifer (and community) longevity. Safe to say, however, it is not encouraging news; and no one should be complacent about Borrego's groundwater situation given these results, however preliminary they may be.
It is worth pointing out that the history of estimates of aquifer capacity, as distinct from the amount of water actually in storage, for the Borrego Basin shows a strong tendency to diminish with each successive estimate.
These latest findings, based on the best science available, are further confirmation of that tendency.
Timothy M. Ross, Engineering Geologist with DWR and author of the TIR on MW-5, will attend the BWD board meeting 26 September 2007, and may be able to give more definitive answers about the future of Borrego's sole-source aquifer at that time. Anyone who thinks they may be in Borrego for more than a few years should be there.
See also Red Flags Over Borrego
1 California Department of Water Resources, Southern District, Resources Assessment Branch, Groundwater Section, 2007, An Interpretation of Geologic Materials Encountered in the Boring of Borrego Water District Monitoring Well MW-5: Technical Information Record SD-07-02.
California Department of Water Resources, Southern District, Resources Assessment Branch, Groundwater Section, 2007, An Interpretation of Geologic Materials Encountered in the Boring of Borrego Water District Monitoring Well MW-4 and Geophysical Observations and Initial Interpretations: Technical Information Record SD-07-02.
2 "Specific Yield" is defined as "the ratio of the volume of water a rock or soil will yield by gravity drainage to the total volume of the rock or soil." Generally speaking, it is a measure of how much water can be extracted from a specific matrix by pumping expressed as a percentage. The lower the percentage, the less extractable water. This explanatory footnote is not part of the TIR for MW-5.
3 A table accompanying the above text indicates that the average of five specific yield estimates made since 1968 is 11.6 percent. If the actual specific yield is, in fact, five percent, or the high end of the range, then it is only 43 percent of the average of historical estimates or less than half what was previously estimated. This explanatory footnote is not part of the TIR for MW-5.4An Interpretation of Geologic Materials Encountered in the Boring of Borrego Water District Monitoring Well MW-5, pages 6 &7.
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