What’s percolating beneath San Diego?
Posted by George J Janczyn on May 17, 2011
Wes Danskin, the Project Chief for the USGS San Diego Hydrogeology Project has been busy studying what’s going on underneath San Diego. Late last year he updated us with work underway at that time. Now he’s working on a new project with a deep USGS monitoring well:
That time again. Another deep USGS monitoring well is being installed. This is the first in a series of updates.
The United States Geological Survey (USGS) is partnering with the City of San Diego. Many thanks to Greg Cross, George Adrian, and Marsi Steirer of the City Water Department and to Keith Selby and Mike Morrow of the City Park Department for their critically important help in getting this project going.
A deep multiple-completion well is being installed to help define the geology and groundwater resources in the coastal San Diego area. This and prior USGS wells are described on the project website, http://ca.water.usgs.gov/sandiego. This well is referred to SDCP (San Diego Chollas Park).
Started May 10, will continue through about June 10.
Chollas Community Park, toward the westernmost end, just east of 54th Street. Drilling in the North Chollas Community Park parking lot, in a median between parking stalls, about 50 feet from a drainage from Chollas Lake.
To define the groundwater flow paths from the eastern part of San Diego County, to the western part where additional groundwater extraction likely would occur. The geochemical water samples that we have collected previously from coastal wells (stable isotopes of hydrogen and oxygen) suggest that the source of the groundwater is recharge from precipitation not on the alluvial plain of the coast, but further east, such as near El Cajon, or further east. This means that the groundwater would need to flow through hard rock (granites, and Santiago Peak Volcanics) in order to reach the coastal sediments west of about Interstate 805.
This well site in Chollas Park (SDCP) was chosen to be east of the La Nacion fault which is approximately 54th street. This site means the drilling will penetrate some unconsolidated sediment (maybe 500 feet), and then penetrate the hard rock. Ideally, the total depth of the drilling will be about 1500 feet. Completion of the well with about five 2-inch PVC piezometers will allow us to sample groundwater from different depths and better understand the groundwater flow system.
Another well site, referred to as SDHF is located at the intersection of Home Avenue and Federal Blvd, and is on the down-dropped west side of the La Nacion fault. These paired wells (SDCP and SDHF) will aid us in understanding groundwater flow from east to west across this major structural feature in San Diego.
Are possible by contacting me.
PROGRESS TO DATE
Will give more details in a subsequent email, but the short answer is that at about 2 pm on Sunday, we were at 410 feet in a sandy, clay, ready to take another core.
Rough stratigraphy, from my memory:
Depth — Geologic characteristics
10 feet — Sandy fill.
20 feet — Santiago Peak gravel. Scared us, thinking we maybe already hit bedrock, but it probably was gravel eroded from a construction project upslope.
40 feet — Stadium Conglomerate, or redeposited San Diego Formation. Hard drilling
100 feet — Really hard and slow drilling, scared us again; thought we’d hit bedrock, was just a hard boulder and a worn out bit.
150 feet — Stadium Conglomerate, wore out two bits so far (button carbide and tipped iron).
234 feet — Formational change to sand; obtained cores. Friar’s formation? Seaward facies of Stadium?
280 feet — Still in oxidized (red streaks) of sand with variably clay.
410 feet — Sandy clay, preparing to obtain another core. I lost my Depth to Bedrock pool; thought we’d hit bedrock by 312 feet. Monte’s gravity suggests about 480 feet.
May 16, 2011:
We hit bedrock. We’re through the sediment, and drilling into bedrock (Santiago Peak Volcanics). Hit bedrock below 398 feet though admittedly it is a bit of a blur between zones of weathered bedrock and hard rock. We are down to 578 feet. We’ll take a core Tuesday morning to see what the material actually looks like. Water flowing into the well is limited so we are not finding highly pressurized bedrock, yet, with too much water to readily get rid of.
I may never live this depth-to-bedrock-predicted-by-gravity thing down.
DEPTH TO BEDROCK
Seems we found bedrock at roughly the depth predicted by Monte Marshall’s gravity map processed by Carolyn Glockhoff in our office. While I agree that gravity measurements are helpful at defining the general shape of a basin, which why I was eager to use Monte’s work, I’ve never been under the geophysists’ illusion that the gravity measurements are sufficiently precise to predict anything particular, much less depth to bedrock at a particular drill site.
But then when you have no alternatives, gravity makes sense to use; so we did. And it worked.
Wesley R. Danskin
United States Geological Survey (USGS)
California Water Science Center
4165 Spruance Road, Suite 200
San Diego, CA 92101 USA