GrokSurf's San Diego

Local observations on water, environment, technology, law & politics

What’s up with the Chollas Creek wells?

Posted by George J Janczyn on July 27, 2011

Wes Danskin, USGS Project Chief for the San Diego Hydrogeology Project, shares this update from the Chollas Park Monitoring Well installed under contract with the City of San Diego to study groundwater in that area of the San Diego Formation aquifer (for a page collecting all the project updates as well as other groundwater news click this link).

WHAT WE FINALLY INSTALLED
Depth of well screen (SC) and sand are listed

Well #6: SC: 29.5′-49.5′; Sand: 16′-56′
Well #5: SC: 140′-160′; Sand: 180′-119.5′
Well #4: SC: 330′-350′; Sand: 310′-372′
Well #3: SC: 520′-540′; Sand: 432′-585′
Well #2: SC: 760′-780′; Sand: 739′-814′
Well #1: SC: 1040′-1060′, 980′-1000′, & 920′-940′ w/a 40′ sump; Sand: 887′-1100′

DEVELOPING THE WELL
The 6 piezometers at the site were developed, meaning the drilling fluid was removed. The process involves pumping air down each piezometer, which bubbles the water mixed with drilling fluid up and out. The aquifer then refills the piezometer with water, and the process continues until water-quality parameters (conductance, pH, and turbidity) stabilize, indicating that we are extracting only native water from the aquifer. The small amount of water in #5 means that it was not well developed. The low yield of #1 means that it took quite a while and some artful use of air, hose, and patience to get it developed. After winter rains next year, we will go back and see if we can develop and sample #5.

SAMPLING THE WELL
All piezometers have been sampled, except #5 which is dry, and #1, which is taking longer because of the low yield of the fractured bedrock. Sampling of #1 will be completed later this week. A broad range of water-quality constituents will be sampled including major and minor ions, trace elements, stable isotopes of hydrogen and oxygen and radioactive isotopes of hydrogen (tritium) and carbon (C-14), and volatile organics. By analyzing these data we can infer the source of the original recharge, when the recharge occurred, and whether human actions have affected the water. Because the general chemistry of water in the San Diego area like most basins is fairly similar, dependent on the rocks and derived sediment that the water is flowing through, we find it helpful to analyze trace elements and other minor constituents of water to determine of groundwater flow paths. It is rare for water districts or individuals to test for these constituents because they are commonly not viewed as a health or water-treatment hazard. Note, the radioactive isotopes are used for dating the time since recharge and are many, many times below a health hazard.

We also sampled a shallow well downslope from SDCP in order to compare our shallow piezometer data with it.

WATER TABLE
Water table appears to be at about 240 feet; Piezometers #5, #6 are both perched water tables.

PHOTO
Thought you might like to see samples of the water from each of the 5 piezometers we developed. You can tell #6 needs some more development; its a bit cloudy.

But we got great water out of #1, yeah! Getting water out of that fractured bedrock had me a bit worried for awhile.

–wes

 

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