Deep well drilling in Paso Robles raises contamination concerns

Wells being drilled as deep as 1,500 feet in the Paso Robles groundwater basin are raising concerns that poor-quality water pumped from deep aquifers could contaminate upper aquifers where most wells draw water.

The issue came to the San Luis Obispo County Board of Supervisors on Tuesday after a deep well in Creston was found to be contaminating upper aquifers.

There are 320 deep wells in the county, with 230 of those in the Paso Robles basin, said Elizabeth Pozzebon, director of county Environmental Health Services. Almost all are agricultural wells, particularly vineyards, and the majority were drilled between 2012 and 2015.

The county defines a deep well as one that is more than 600 feet deep and penetrates more than one aquifer. Some go as deep as 1,500 feet.

County Supervisor Frank Mecham, whose district includes much of the Paso Robles basin, said during the supervisors’ meeting Tuesday that basin residents are increasingly worried that the deep wells are a threat to their well water. The board heard a report on the subject of deep well drilling but took no action.

Deep wells can cause a number of problems, said Tim Cleath, a geologist with the San Luis Obispo-based Cleath-Harris Geologists. The biggest threat is that water from deeper aquifers is often of much-poorer quality and can have high levels of salt, sulfur and other dissolved minerals.

Water from these deep wells is often of such poor quality that it must be mixed with better-quality water before it can be used for crop irrigation.

If a well is improperly drilled or its seal leaks, the upper aquifer can be contaminated by the lower aquifer. Pumping large amounts of water from a deep well also raises the risk of the ground sinking, Cleath said.

All deep wells must be reviewed by a geologist to ensure that the well will not contaminate an upper aquifer, county Health Officer Dr. Penny Borenstein said.

“Additionally, concrete seals must be placed in the borehole to preclude mixing between multiple aquifers when such mixing is determined by EHS to likely result in significant deterioration of water quality,” she said in a staff report to county supervisors.

County Environmental Health Services recently investigated four wells in the Creston area that were drilled to depths ranging from 1,010 feet to 1,485 feet after complaints were received about them.

A geologist determined that three of the wells did not pose a threat to the water quality of the upper aquifer.

However, the fourth well, known as the Cotta well, was determined by county officials to be contaminating upper aquifers.

The county required that a concrete plug be installed to close off the lower aquifer.

A follow-up water sampling plan is underway to determine whether the concrete plug has been effective.

Neighboring wells have also been tested to make sure their water quality has not been degraded, Borenstein said.

This story was originally published March 8, 2016 at 1:28 PM with the headline "Deep well drilling in Paso Robles raises contamination concerns."