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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
Research Hydrologist
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.


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San Diego Formation: new USGS deep monitoring well progress report November 2011

Posted by George J Janczyn on November 7, 2011

On October 20, USGS under contract with the City of San Diego began operations on a new deep monitoring well in the Chollas Creek vicinity near the intersection of Home Avenue and Federal Blvd. The well is the latest in a series of multiple-depth wells being drilled in selected areas of four coastal river basins for the San Diego Hydrogeology Project.

The primary objectives of the project are to develop an integrated, comprehensive understanding of the geology and hydrology of the San Diego area, focusing on the San Diego Formation and the overlying alluvial deposits, and use this understanding to evaluate expanded use of the alluvial deposits and the San Diego Formation for recharge and extraction.

Project Chief Wes Danskin writes about the latest developments:

November 1:

1. Hit the Otay formation, at 648 feet. Check out the photos of cuttings below; bet you can pick it out too. And we hit it within about 60 feet of where I thought.

And very pleased with my newest idea: poker chips to identify the formations; I can write on them, and they don’t dissolve or fly away like post-its.

The poker-picks you see here are: the San Diego formation identified via shells; and the Otay formation looks like Otay.

Each square sample box is 10 feet of drilling. The rectangular boxes indicate where we took a core.

Photo 1 below is 0-240 feet.

Read the rest of this entry »

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USGS set to drill next deep monitoring well in San Diego

Posted by George J Janczyn on October 15, 2011

The United States Geological Survey (USGS), in cooperation with the City of San Diego, will begin installation of the next deep monitoring well site. The drilling and well installation is being done by USGS Research Drilling Unit, which aids USGS scientists throughout the western United States. The USGS is part of the Department of the Interior, and is the primary federal agency charged with investigation of earth resources. The USGS does not have a regulatory function, so as our drillers say, “It’s all about the science.” It is common for the USGS to partner with other governmental agencies, in this case the City of San Diego, sharing scientific questions, skills, and finances.

The next deep monitoring well site in the San Diego area will be drilled to a depth of about 1500 feet, with 4-6 piezometers installed, each to a different depth. Water-level and water-quality information can be obtained from the piezometers. Pressure transducers will be installed to continuously monitor water levels; this real-time data is routed via satellite to a public website. The well site is designed to be a long-term investment for the next 50-100 years. Initially, it will provide information about the local geology and hydrology; later, it can be used to manage groundwater operations.

The well location is at the intersection of Home and Federal streets, just off SR94, just west of Interstate 805. Can’t miss us. Look for the big yellow drill rig with USGS on it. You are welcome to stop by anytime. The well site was chosen to be on the west side of the La Nacion fault, paired with the recently completed Chollas Park well site (SDCP), and about halfway in between the El Toyon (SDEP) and Aquaculture (SDAQ) well sites.

The drilling operations will begin on Oct 20, 2011, and will continue for about a month. The first 2+ weeks will involve drilling and coring. The 3rd week will focus on geophysical logging, interpretation to design construction of the 4-6 piezometers, and reeming the hole larger to accommodate the piezometers. The 4th week will focus on installing the piezometers. We drill 7 am to 7 pm, everyday, holidays included.

An additional source of water in the San Diego area may come from the San Diego Formation and the overlying, and possibly underlying, sedimentary deposits of sand, silt, and gravel. Prior to the USGS study, which began in 2001, the location, thickness, and water-yielding character of these deposits was poorly known. Since 2001, the USGS has installed 10 deep monitoring well sites throughout the coastal San Diego area; the sites are a key part of creating a three-dimensional view of the San Diego underground. This geologic, water-level, and water-quality information will help guide public agencies and consultants in how to best utilize the limited, mostly brackish, groundwater resources in the coastal San Diego area.

We’ll use mud rotary drilling technique, along with wire-line coring to collect undisturbed samples of sediment from selected depths. The piezometers are mostly 2-inch and one 3-inch PVC pipes which are installed to different depths. Each PVC pipe has 20 feet of perforations at the bottom to allow water to enter the pipe only at that depth. Sand is installed around the perforations and a thick clay-like grout is installed elsewhere. The end result is that we will have a nest of 4-6 PVC pipes that we can measure water levels in, and collect water quality samples from. The 3-inch PVC pipe extends to the bottom of the well bore and allows us to conduct a variety of future measurements, such as identifying changes in salinity throughout the entire 1,500 feet section of aquifer.

As usual, you are more than welcome to visit the site; bring your friends, Cub Scout packs, university classes, cameras. Just stop by; we will have an educational outreach area to help us synthesize the information and help communicate it to you. Or call me at the mobile phone below and we’ll set up a time to meet at the site.

Visit the project website

Wesley R. Danskin
Research Hydrologist
United States Geological Survey (USGS)
California Water Science Center
4165 Spruance Road, Suite 200
San Diego, CA 92101 USA
619-225-6100 office
858-663-6832 mobile

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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).

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′

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.

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

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.



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A brief talk about the USGS San Diego Hydrogeology Project

Posted by George J Janczyn on July 12, 2011

A few weeks ago I visited the site of the USGS monitoring well being drilled at Chollas Park. The well is part of the USGS San Diego Hydrogeology Project studying water quality, quantity, and flow characteristics in the San Diego Formation aquifer (click here for the story from the visit).

Here’s Project Chief Wes Danskin discussing the program with San Diego KGTV Channel 10 reporter Joe Little (apologies for the less-than-perfect camera work):


Also, here’s Joe Little’s report on 10 News.


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Chollas Park groundwater study: pictures tell the story

Posted by George J Janczyn on June 21, 2011

Here’s the latest from Wes Danskin’s log on the USGS groundwater study at San Diego Chollas Park (previous log entries on this study are here).

Done drilling, chose depths for wells (piezometers–pressure sensing wells), installed 2-inch PVC piezometers, and are developing of them as I write this. Decided to include photos of each activity so you don’t have to read all this, just view the photos.

Hit bedrock (Santiago Peak volcanics at 877 feet). Core at 895 feet is shown below. Many fractures are present

Based on geophysical logs, drill cuttings, cores, pore water chemistry, geologic mapping, evolving concepts of groundwater flow, likely constraints to developing a water supply, defining the yuck factor of Chollas water, taking advantage of evaporating Chollas Lake water … one of the more difficult things I do. You definitely get your money’s worth from the Chief Scientist on the project (me).

Well #1: 1040′-1060′, 980′-1000′, & 920′-940′
Well #2: 760′-780′
Well #3: 520′-540′
Well #4: 330′-350′
Well #5: 140′-160′
Well #6: 30′-50′

Send air down the wells to force water and left-over drilling fluid up and out. In the photo you’ll see water being ejected from one of the wells.

As we extract water/drilling fluid, the aquifer replenishes the well water, which eventually ends up cleaning the well of our drilling effects. We monitor clarity of the water, pH, conductance, and temperature to known when we have removed drilling-related water and fluids and when we have native ground water. Great news is that the fractured well shown in the photo above makes water, not much, but makes water via those fractures.

Sampling the wells for water quality.
Installing a vault to protect the wells.
Installing water-level monitoring equipment, and satellite link.
Putting the information on our website.

Whew, I’m exhausted; almost done.



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Site visit: USGS groundwater study at San Diego Chollas Park

Posted by George J Janczyn on June 7, 2011

Recently Wes Danskin, Project Chief for the U.S. Geological Survey (USGS) San Diego Hydrogeology Project, shared notes from his log (here and here) regarding installation of a monitoring well at Chollas Park that will be used as part of a study to learn about water quality, quantity, and flow characteristics in the San Diego Formation aquifer. Funding for the well comes from the City of San Diego.

Last Thursday I received an email from Danskin: “Stop by if you’d like, we’ll be on site for about another week completing the piezometer installation.” No need for a second invitation; the next morning I grabbed my camera and headed over to the project site.

The well is just west of Chollas Lake which itself is just west of the College Grove shopping center near the SR-94 freeway at College Avenue.

Several other visitors were already there, including a few members of the San Diego Association of Geologists (SDAG). 10News reporter Joe Little was there, preparing to interview Danskin for the evening news.

(all photos can be clicked for enlargement)

10News reporter Joe Little (left) prepares for the interview with Wes Danskin.

Wes animatedly talked about a new three-dimensional geologic map of the region that he’s been working on. No previous geologic studies in the San Diego/Tijuana area have produced such a map. According to the SDAG website (“Mapping the San Diego Underground”):

“A total of 91 wells, which showed stratigraphy older than Quaternary age, helped provide depth information to produce this 3D hydrogeologic framework model. This study relied on pre-existing GIS (geographic information systems) datasets including DEM (digital elevation model), surface geologic maps, drilling and e-logs, and literature references to wells or outcrops. Direct examination of USGS multi-depth wells provided the most reliable “ground truth” for geologic boundaries used in the model.”

Danskin explained there are three types of wells: 1) monitoring wells to identify water levels and quality and geology of the groundwater basin; 2) pilot production wells to determine the quantity of water flowing through the ground; 3) full scale production wells. For the San Diego study, the first two types are being used.

Continuing, Danskin said the San Diego Formation extends north-south from La Jolla to south of the border, and west-east from the ocean to the vicinity of the I-805 freeway. Groundwater has been extracted from the San Diego Formation for over 50 years. Sweetwater Authority has been distributing it to National City and Chula Vista.

“The important part about this well is that we’re actually able to get down into the hard rock,” said Danskin. “None of the other wells, with the exception of one near Qualcomm Stadium, were we able to identify this important part of the geologic story,” he said, noting that “the critical part of what we’re doing is defining how the geologic layers are arranged and that allows us to understand how the water moves through them.”

An interesting fact: using carbon dating they found that it could be up to 30,000 years since that groundwater was last in the atmosphere.

Little asked whether pumping and treating groundwater can be cost-effective. Danskin replied that Sweetwater Authority’s pumping of groundwater shows it is already cost-effective. It will gradually become even more competitive because the price of imported water continues to increase. It is becoming more attractive from a reliability point of view too, because we import up to 90% of our water from the Colorado River and the Sacramento-San Joaquin Delta, and both of those sources are climatologically, environmentally, and politically at risk.

Danskin said chances are that most of the water in the Formation will be salty and require reverse osmosis treatment…not as salty as the ocean but it may contain the same salt level as V-8 juice. Still he has an optimistic outlook about the project finding a reliable groundwater flow. “It’s taken about ten years,” he said, “but things are finally starting to make sense.”

A typical tricone drill bit.

Numerous soil samples are taken at regular checkpoints as the drill goes deeper.

Danskin discussing the various well locations in San Diego.


Here’s video I shot while Danskin discussed the project:


See also Joe Little’s report on Channel 10 News:


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A rocky search for water beneath San Diego

Posted by George J Janczyn on May 26, 2011

Last week Wes Danskin treated us to news (What’s percolating beneath San Diego?) on the new USGS deep monitoring well at San Diego Chollas Park (SDCP), part of a study of the geology and groundwater resources in the coastal San Diego area.

Here’s the latest from Wes’s log:


May 17: Every picture tells a story

Picture of our core at 580 feet depth

May 20: Sounds of silence

Drill rig broke yesterday; frayed the cable supporting the kelly head; dangerous not to replace; so we will.

A core we obtained from 665 feet; looks like Friars Formation to me

May 23: 875 feet, no, not yet

We all underestimated the depth to bedrock, even the drillers.

Sunday night, at 875 feet deep. Maybe in the top of the weathered bedrock. Cuttings appear to be ground-up rock; drill times are longer, but not so long to indicate hard rock, I think. Taken cores along the way, every 150 feet or so, most recent: 440, 580, 665, 860 feet.

Will continue drilling, until we once again think we are in hard rock, probably the Santiago Peak Volcanics, then take a core, to make sure it is what we think.

I’d like to get 1 or 2 piezometers deeper than the production zone of National City well field, which has been producing fresh water for 50+ years. Bottom of those wells is about 700 feet below sea level; presently we are at 550 feet below sea level. Good news is we have covered the majority of the production zone, so if groundwater is flowing roughly horizontally to the coast, we will be able to sample the upgradient side of the flow field, or at least a similar flow field a bit north of the National City wells. Bad news is that drilling another 450 feet (150 to bottom of zone + 300 feet) will not be easy. But our drilling is about patience and flexibility. So we’ll see how it goes.

May 25: Bedrock and beyond

Yes, we hit bedrock, finally, at 877 feet. And we have proof, as shown in the photo below (core of the Santiago Peak Volcanics, … same rock as what the coastal dams are anchored in). Note the fractures and related water deposits, such as calcite shown at the point of the pen. This deposit demonstrates water flow through the fractures, albeit, we don’t know when.

In the process of squeezing the sediment that we obtained via coring. Short answer I think is that the pore water is more saline (ec = 1800; tds = 1000) than I would have thought/hoped. A bit confused by why we are not seeing any 600 tds water like National City wellfield. Darn. Answer may change a bit when we install the piezometers in the more transmissive zones.

We’ll continue drilling in the bedrock with the goal of getting deep enough (200+ feet) to install a well, hopefully with sufficient fractures to yield water. I have a plan for 5 piezometers: 3 inch to the bottom to monitor changes in salinity and temperature; 2-inch for the other 4. Estimated completion without geophyscial logs, etc are:

	          5. Water table at 50+ feet, 
	          4. Below water table at maybe 250 foot depth in stadium conglomerate, 
	          3. Something in middle 400-500 foot zone, 
	          2. One in Friars formation at 700 feet, and
	          1. One at bottom in Santiago Peak Volcanics at 1000+ feet.

Core of the Santiago Peak Volcanics, same rock as what the coastal dams are anchored in

PHOTO BELOW. Bill Elliott, SDSU professor and local gravity/geology guru; and Adam Kjos, our local USGS man on the spot, supervising the drilling. Both are trying to figure out from the drill cuttings what might be going on, e.g.,

  1. What formation are we in, where
  2. Why the 600 feet thickness of stadium-like coarse deposits are present
  3. Whether sand at 700 feet might be the Friars formation, or something like it
  4. The weathered zone above the volcanics appears to be about 60 feet thick
  5. Do any of these geologic units really correspond to geologic units mapped at the land surface?

Thanks to Bill Elliott for coming to the drill site and sharing his expertise in local rocks to help us understand what we might have been drilling through, and to Dave Schug from URS for similarly stopping by, though I was not on site then to take photos and quiz him.


Postscript May 31, 2011

Reached the end of the line, 1100 foot depth. Every well sort of tells you when it is done giving up secrets. When drilling took more than 3 hours 20 minutes for each 20 feet, we decided to call it a good effort. We have obtained much new information, and whatever else we could gain from drilling deeper into the Santiago Peak Volcanics, probably is better done at another location, starting in bedrock and using air rotary technique.

After reaching 1100 feet, based on my request to get a good 100+ feet into the Santiago Peak Volcanics so that we can complete a well that will reliably yield water, we stopped, cleaned the well, and called in our geophysical logging experts over the weekend (never a holiday for anyone associated with drilling; got done logging at 3 am; thanks Tony and Mike).

The geophysical logs are attached so that you can see for yourself the variation in material. Don’t worry; you don’t have to understand the logs or what they mean to get the basic idea. When the squiggly line changes, that means the earth or the water quality has changed. So at a first level, big picture analysis, look to see where the lines change once or twice or three times in the span of 1100 feet. Bet you can pick out where the bedrock starts. Say 877 feet. Another package of material is from about 440 to 600; I’m guessing alluvial fan based on cuttings and drilling notes; and another unit, mostly sand, from 600 to the weathered bedrock at about 800. But the SP log shows two zones within that last zone, so again this is not so precise as to become boring, or a single answer wins. Will know more, or think we do, after several hours of analysis by several of us.

We’ll be starring at the geophysical logs, the cuttings, the drilling notes, the locations of core samples, and using our Ouija board to divine where to place the piezometers. Let you know in the next couple of days the precise well design.

In the meantime we are reeming the 7 7/8-inch pilot hole, first with a 13-inch bit down to about 350 feet to accommodate two piezometers above that level; then with a 10-inch bit down to 820 feet to accommodate 2 more piezometers above that depth, then with the 7 7/8 inch bit to clean out the debris that fell in the hole down to the total depth of 1100 feet.

The rationale for drilling a pilot hole first is that a smaller diameter hole can be drilled faster, circulation time is faster, the geophysical logs will be better, and there may be fewer problems with caving. The bad part is we will spend 4-6 days redrilling the well to make it the size needed to install piezometers. If we knew the geology ahead of time, then perhaps we could design the general size of the well bore ahead of time and only drill the well once. But in San Diego that knowledge does not yet exist. Getting there though. The geologic framework model that Carolyn and Claudia are preparing shows that we are getting to where drilling will be less wild-catting and more predictable engineering.


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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:


May 15, 2011:

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, 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.

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.

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
Research Hydrologist
United States Geological Survey (USGS)
California Water Science Center
4165 Spruance Road, Suite 200
San Diego, CA 92101 USA
619-225-6100 office


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