I recently had the opportunity to sit down with Timothy Parker of Parker Groundwater Consulting in Sacramento, CA. 

Loni Lyttle: So, I wanted to talk to you today about groundwater recharge. California, along with the rest of the western United States has been in a long drought and is facing serious issues with groundwater depletion not to mention subsidence, where the land is actually sinking due to overdrafting the aquifer. But then we get these two wet years in 2023 and 2024 (so far) with above average rainfall. I think this lulls people into a sense of security about California’s water situation and I’m not sure it’s that simple. I’m not a hydrogeologist so I wanted to get in touch with someone who knows more on the subject. 

Tim Parker: You are right – the surface water reservoir drought is over but our groundwater reservoirs are still in drought conditions. So there’s a lot of recharge going on and we don’t always know how successful it is and whether or not it’s causing potentially adverse impacts. And I hate to start off on that point, but it is something we have to be concerned about.

There’s a lot of good information out there. Just in the last few years, the state has started putting a lot more information out on groundwater level information, including changes over a year that you can see visually. 

One of the things in my research that I was surprised about was on subsidence in the Central Valley. We’re aware of the subsidence there. But when you look at the change in subsidence that’s being monitored on an annual basis, the change during those wetter years was much less. I thought that was pretty interesting because there is a significant lag time with subsidence. Once you have subsidence and you shut the pumping off, It doesn’t just stop altogether. It continues to subside for some period of time.

LL: That is interesting. I mean when I think of subsidence, I think immediately of the Central Valley just because I know it’s so severe. I mean the water that people are pumping there is ancient water, right? That basin filled up a long time ago?

TP: Yes and no. One of the things we found with the statewide aerial electromagnetic project is the way the sediments were deposited in the Central Valley, especially the San Joaquin Valley. So during the Pleistocene you had periodic glaciers, lots of ice and then melts. That action created these incised alluvial fans. (read more about this topic here.) 

So that’s very coarse grain sediments coming off of the Sierra Nevada mountains. And actually Stanford’s done some work tracking subsidence data, groundwater levels, and precipitation. Through that you can see the water coming into the basin especially along those kind of superhighways for recharge coming off of the mountains. Now that’s fresh, young water coming in. But you’re right, most of the deeper water being pumped is old. Back in the 60s, there were some studies done and found that groundwater levels have dropped several hundred feet. And again, that’s where we are today. Groundwater levels have have continued dropping significantly, and that means you’re getting some recharge of fresher, younger water up on top, the pumping is mostly deeper, typically where the groundwater gets older.

Parts of the San Joaquin Valley has huge problems with depletion and associated subsidence. The economic impacts are big in these situations.

Subsidence, i.e. sinking land, can damage water-moving infrastructure that relies on gravity. 

LL: So in the San Joaquin Valley for instance, how many years of water do they have left if they continue to pump as they have been?

TP: I haven’t looked into that. I do know it’s dropped several hundred feet over the decades. It’s about a 2 million acre-feet overdraft per year. That’s one of the estimates that’s out there. If pumping continues at the current rate, the basin has a limited lifetime, whether that’s a few decades or many decades.

The other issue in all of that is water quality. The deeper you go, you’re going to run into higher total dissolved solids in addition to the problem of land subsidence. Subsidence is a big economic driver because it affects infrastructure. The Central Valley and the state as a whole relies on moving water from where there’s more precipitation in the north to where there’s much less precipitation in the south. And these aqueducts and irrigation ditches, et cetera, are gravity drains. When the land sinks, that interrupts the flow, and so they’re having to do a lot of repairs. The other thing is that you have floods in different areas where you didn’t before. So that’s another risk and liability.

LL: So can you go more into where our groundwater comes from? How variable is groundwater within a given region or even within a given basin? I’ve seen it where we are. Even within Paso Robles, there’s so much variation. You have eastern Paso, which looks like an old western movie, but then the water is really good. There are good wells out there where you hit water at less than 100ft. Then growers in western Paso have to get their water out of fractured rock and it’s barely a trickle.

TP: It has to do with the geology and California has a very interesting geology. It was formed by tectonic plates pushing against one another. That formed the Central Valley as a structure. One plate underneath the ocean (oceanic plate) was subducted underneath the continental plate. That caused the subducted plate to melt and form the Sierra Nevadas. The Central Valley was what they call a forearc basin, where sediments were formed as part of an inland sea. And then marine sediments that were on top of the ocean plate were scraped up and formed into the coastal ranges. So you have these marine sandstone shales all folded up and deformed. And that’s the bedrock that you’re talking about in the Paso area on the west. And then the other thing that happened 20 to 30 million years ago, was that subduction zone changed to a transform fault, which you probably know the name of…the San Andreas fault. When that happened, there was this sort of northwest structural, tensional and extensional change that occurred.

As a result of the transition to San Andreas transform fault tectonics, you have all these northwest trending small basins in the coast ranges that are faulted as a result. And those filled with alluvium or sand, silt and gravel. And so those are the kind of the two types of basins you’re talking about. In the coast ranges, it’s typically more limited unconsolidated alluvial deposits, and fractured bedrock that doesn’t produce much water. It’s okay for a residence where you need five to ten gallons per minute. And then in the alluvial systems, you get a lot better production, typically. But anyway, that’s kind of the difference in that is whether you’ve got an alluvial aquifer or bedrock aquifer that you’re getting water from.

LL: These small coastal basins, is that what we have in northern California wine country. Are Sonoma and Napa in those little coastal basin areas?

TP: There’s so much diversity up there. I’ve actually worked in the Sonoma Valley for almost 20 years as a groundwater management consultant. Sonoma is one of the more complex basins because you have also volcanics and multiple faults there. What we like to say is that it’s “compartmentalized”. It’s got all these separate compartments. You can drill a well in the Sonoma Valley and move 25ft over and you might get water in one and nothing in the other. That’s how quickly things can change. It’s very complicated geology. The San Andreas is a fault zone that’s several miles wide. Then there’s a lot of other faults that are the same trend that run through these coastal basins. And then you have other faults that run to 90 degrees to that. So there’s all this different movement that’s occurring, and that’s what makes the hydrogeology complex. Some of those faults can be conduits and a lot of them can be barriers, so you can move from one side of a fault to another and get completely different geology and groundwater results.

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