The groundwater crisis in the San Joaquin Valley is especially critical. In Stanislaus County, tens of thousands of acres of new almond orchards have put a tremendous demand on groundwater, causing concern for the future of the aquifer. Many residential wells have run dry, yet thus far county supervisors have failed to take action.
On June 28, Stanislaus County Supervisor Terry Withrow was quoted as saying, “We’ve got to stick with the science and get past the emotions.” We agree. Therefore, The Valley Citizen will be publishing a series of articles based on the science of groundwater and groundwater use in the northern San Joaquin Valley.
Dr. Vance Kennedy has a PhD in Hydrology. He is retired from the United States Geological Survey. While at the USGS, Dr. Kennedy received the Distinguished Service Award, the highest possible such award bestowed by the Department of Interior. At the age of 91, Dr. Kennedy still farms eight acres of citrus north of Modesto. Here is Dr. Kennedy’s list of facts about groundwater in Stanislaus County, especially the eastern portion:
- Geology in the valley and foothills is composed of alternating layers of shale, sand, and gravel.
- Shale is virtually impermeable in either the vertical or horizontal direction. Hence, it acts as a major barrier to vertical movement of groundwater in the geologic column. However, shale occurs as localized lenses, except in the case of the Corcoran clay, which extends westward from about the middle of Modesto.
- Sediments are laid down in essentially horizontal layers with inter-fingering shale, silt, sand and gravels.
- Horizontal permeability or the sediment layers varies widely, with clean gravels thousands of times more permeable than silts.
- The overall effect of these facts is that the horizontal permeability of a rock column is commonly 10 to 100 times that of the vertical permeability.
- In the case of a deep well tapping into a widespread permeable aquifer, the water removed will easily come from thousands of feet around the well.
- The shale layers will cause a zigzag vertical movement of water downward to replace the water removed by the deep well. That movement can be slow, but it must happen because of the required mass balance. In other words, there won’t be big open holes where the water was removed.
- There has been much talk about “confined” aquifers as though they have no connection with the water above them and that one can remove water from them with little or no effect on shallow aquifers. Physically, THAT WATER MUST BE REPLACED (caps are Dr. Kennedy’s) or the sediments will shrink, causing subsidence. There is no “free lunch.”
- Conversations with some other professional hydrologists indicate that east of the Corcoran clay there may be “semi-confined” aquifers in the sense that vertical water movement is very slow, so most replacement of well water must come laterally.
There are no extensive clay layers acting as a seal between shallow and deep aquifers east of middle Modesto. Hence, water removed from deep aquifers must inevitably affect shallow aquifers and, therefore, streams and reservoirs.
Both drought and large deep wells account for drying up of shallow wells, with responsibility varying with the area and amount of water from large deep wells. That responsibility can only be determined if volumes of water being removed by wells is open information.
The Stanislaus County Board of Supervisors can and should require information publicized on large well production by anyone. The argument from one official against publication is that it will encourage lawsuits. That won’t happen unless there is reason to believe that what is being done is illegal. Do the Supervisors want to permit illegal actions?
John Wagner says
An interesting article, although Dr. Kennedy’s fairly technical review makes it a bit hard for a layman to grasp the subtleties. However, your summary is most helpful.
Can anyone define “deep wells” and “shallow wells” and estimate the amount of the water use from each in the area? If Big Ag is pumping huge volumes from deep wells and, as Dr. Kennedy says, the water available in shallow wells will ultimately trickle down to refill the deeper aquifers, it almost looks like the likely incidence of shallow wells going dry could be estimated over time in some areas. Maybe providing a dry well forecast could help the soon-to-be-waterless get some action from the politicos.
Eric Caine says
The summary is Dr. Kennedy’s. We’ll be following up with more science and some definitions. As I understand it, shallow wells are usually residential and depth depends on the water table. Most around here are well under 100 feet. The dry well forecast is “now,” as more go out everyday; again, most are residential.
Bruce Frohman says
I would presume that a shallow well is a well in the upper layers of valley sediment, above the highest level of impermeable shale. A deep well would be one drilled below the first level of impermeable shale.
A friend of mine in Madera County told me that when his shallow well went dry last year, he had a deep well drilled. He told me that a hydrologist predicted that his new deep well will go dry within 5 years.
This illustrates how rapidly we are presently depleting ground water resources.