USGS drilling new groundwater measurement well in San Pasqual Valley
Posted by George J Janczyn on February 1, 2013
As part of its wide-ranging groundwater studies in San Diego County, the United States Geological Survey (USGS) San Diego Hydrogeology Project is drilling a new well in the San Pasqual Valley.
Under the direction of project chief Wes Danskin and in coordination with the City of San Diego, the drilling activity is located along SR78 just past Cloverdale Road (at the yellow pushpin in the photo below):
Or, as seen in a broader context with two other existing wells at Santa Ysabel SDSY and Lake Hodges SDLH (near I-15) marked with green pushpins. Click images for larger version:
The Cloverdale well design is more complex than other San Pasqual sites. Mr. Danskin explains:
The well was sited to measure the groundwater flowing out of the Cloverdale tributary valley into the San Pasqual Valley. The well was situated as close as possible to Cloverdale Creek so that well data can also be related to surface water leaving Cloverdale and entering the main San Pasqual Valley.
The well needed to be situated so that it was between two hard rock outcrops to ensure the shallow well would sense the alluvium and have enough alluvium to sense; we got a bit lucky in this regard.
The well needed to be as far from the road and bridge as possible, just in case Caltrans decides to widen the bridge or road some decade in the future.
We needed to get a core in the weathered and hard rock zones to prove we are in them, respectively. Then to provide undisturbed samples that can be used later for mineralogy and testing of chemical properties. This aids in understanding water quality data, and in predicting the effect of water-quality changes.
The basic ideas are:
Grout the upper 20 feet so that no water goes down the annulus.
#3 well – In the alluvium, senses the water table
#2 well – In the weathered hard rock (granodiorite), spans the zone between the more weathered and less weathered zones. The weathered granite should have less transmissivity and a lower storage coefficient than the alluvium.
Pumping wells likely affect the weathered zone as much or more than the alluvium. This is what we see from the SDSY well.
#1 well – In the hard rock; split into a 1a and 1b, for upper and lower zones, each located near fractures. They are separated by blank casing and grout in the annulus, so that we later can go in and sample each zone, and install a separate water-level monitoring device (via a packer) in the lower zone to see if the pressures are similar to zone 1b.
Data from SDSY suggests that the alluvium, weathered, and hard rock all are depressurized by the pumping. I would have expected the deep, hard rock zone to have elevated pressures and be less affected than the weathered zone. This is only slightly true. It is possible that this impression is partly caused by our well construction design, so Cloverdale SDCD is to test that concept.
The fractured hard rock may have different water transmitting properties than the weathered granite.
Well #1 is a 3-inch diameter PVC well so that we can later go in with the Electromagnetic (EM) tool and measure changes in salinity with depth. I suspect that San Pasqual Valley over the next decades will become more and more saline, and that eventually the water will become too salty for most agriculture. The valley then might evolve to more of an animal based area.* The EM testing will allow us to quantify this salination. The SDSY well shows that the salination so far is mostly in the upper zone.
Photos of shaker and sieve cuttings:
Most of the well installation will be done by Friday (today), well development will be Saturday, and equipment will likely move offsite Sunday.
Then, more well development via a compressor; water quality sampling for a large number of constituents; slug tests to measure the hydraulic properties of the different formations; description of cuttings for the CA well report and for San Diego well data report #2; installation of a vault, water-level sensing equipment, and satellite link; updating website with data and photos; and taking a nap.
Photos and project details are courtesy of Wes Danskin.
Wesley R. Danskin
United States Geological Survey
4165 Spruance Road, Suite 200
San Diego, CA 92101
* On the subject of San Pasqual Valley gradually becoming more saline, I asked Mr. Danskin “You said you suspect the valley may eventually become too saline for most agriculture. Would that be because of agricultural irrigation using imported water that’s quite salty?” His response (please note these are his observations and not an official USGS conclusion):
I would say the issues are:
1. Irrigation return flow; recycling water and accumulating salts in the upper soil; groundwater system. Farmers have indicated to me that they are aware of the increasing salt content and it is affecting their agricultural decisions.
2. Lack of outflow. Near-surface ground water would otherwise have the ability to cleanse the system occasionally by removing salts and discharging to streams that would flow to the ocean.
The natural hydraulic gradient in the lower part of the basin, near Interstate 15, would have been from bottom to top, pushing salts to the surface, then into a stream, then to the ocean. A system without a natural outlet to the ocean (e.g. Owens Valley), tends to accumulate salts in the downstream area, and they just accumulate, eventually looking like a salt lake. The discharge from Lake Hodges mitigates this to some degree, but not as much as an un-dammed system.