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
May 20: Sounds of silence
Drill rig broke yesterday; frayed the cable supporting the kelly head; dangerous not to replace; so we will.
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.
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.,
- What formation are we in, where
- Why the 600 feet thickness of stadium-like coarse deposits are present
- Whether sand at 700 feet might be the Friars formation, or something like it
- The weathered zone above the volcanics appears to be about 60 feet thick
- 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.