Vance Kennedy: Yes, groundwater is interconnected

The widespread belief that deep agricultural wells are not affecting nearby shallower wells east of Modesto is simply not true. Deep wells depleting groundwater can have devastating effects on neighbors.

It’s very likely we will see many more shallower wells begin to go dry as the drought continues, many as a result of the great increase in deep, high-capacity wells pumping nearby. To understand why this will happen, one must know a little about deep wells and our region’s geology. The basics are fairly simple if one remembers that water always flows downhill, whether on the surface or underground.

Consider the characteristics of any highly productive well. To produce a large volume of water continuously, the water being pumped out of the ground must be replaced by more water flowing to the bottom of the well underground. That underground water moves both vertically and laterally. That means the aquifer or aquifers being pumped must allow water to flow freely to the well. In other words, the aquifers must be permeable. The well also must draw from a large area.

Over thousands of years, rivers flowing out of the mountains carried various types of particles, ranging from tiny bits of clay to larger silts to even larger granules of sand and then gravels. As those ancient rivers began to reach flatter ground, they lost velocity and carrying power. The heavier and coarser particles dropped out of the flow first, forming beds of gravel; the smaller sands flowed farther into the flatter ground, spreading out in large fans. Finally the silts and clays were put down as the water stilled.

All of this is beneath us today.

Clays, when compressed, become shale, and water cannot move through shale or great deposits of clay. Silt is not much different. However, as the size of the grain increases, so does permeability, perhaps by a factor of 100 or 1,000 times. It is these coarser deposits that easily transmit water laterally that we call aquifers. They were originally deposited as horizontal layers, but the rising Sierra has tilted them up in the foothills.

The strength of these ancient storms varied, sometimes moving around sediment that had been put down before. This resulted in “lenses” of clay and silt being interlayered with coarser materials. Water moving laterally, or from side to side, can move easily through these beds, but water moving down must zig-zag slowly around the layers of clay and silt.

So, when a well taps an aquifer it can pull water laterally immediately, but water being pulled from aquifers above or below can be delayed in arriving at the well. That means you might not see the effects of a large well’s pumping until well after it begins pumping.

Millions of years ago, the Valley floor was a vast lake or inland sea. Clays accumulated in considerable thickness, forming what we now call the Corcoran Clay. This clay bed is thick enough to act as a barrier to any vertical flow of water. The Corcoran Clay is present beneath much of western Stanislaus County, but thins out to nothing along a line running northwest to southeast and passing through the middle of Modesto. From that point east, there is no impact on the movement of groundwater due to Corcoran Clay.

In eastern Stanislaus County, there are clay and silt deposits that slow the vertical movement of groundwater. But this only delays the movement, it doesn’t prevent it.

A single deep, high-capacity well can drain water away from shallow wells from a wide area.

Without water, property has little value. So, as deep wells pull water away from shallower wells, there is potential for widespread loss of property value for the land above.

California law does not help the shallow well owners – at least not yet. A law passed last year will require groundwater sustainability, but not for several years into the future.

This situation could have been anticipated and steps could have been taken to control the drilling of deep wells and the depletion of shallower aquifers. But our political process did not address it and is unquestionably at fault.

None of this is a secret to anyone. Our county supervisors can act even now, if they will.

Public awareness and political pressure have to provide the answers.

We have two ongoing disasters. The first is the drought that has reached its fourth year, one we can do little about. The second is the result of large, deep wells draining water from nearby shallower wells. We can do something about that, but action is needed very soon.

This story was originally published May 9, 2015 at 11:49 PM with the headline "Vance Kennedy: Yes, groundwater is interconnected."