* Editor’s note: This is the sixth in a series of “Ask the expert” columns appearing in advance of the Measure I election, regarding various technical aspects of the city’s water utility and the proposed Woodland-Davis surface water project.
The expert: Ed Schroeder, UC Davis professor emeritus of civil and environmental engineering. Schroeder specializes in biological process engineering and has authored more than 150 publications and three books on wastewater treatment, waste gas treatment and water quality.
The question: If the city elected to continue to supply its residents with only well water, what would it have to do to ensure that the supply meets all regulatory standards — including those effluent standards coming up in 2017? Or in other words, what upgrades would it have to make to the system? Can you estimate how long those upgrades would last? And how much it would cost the city to do?
The answer: A combination of wells from the intermediate and deep aquifers probably can provide water that meets both drinking water and treated wastewater discharge standards for some time. The length of time and cost are impossible to predict for several reasons.
New wells will be needed to replace aging ones and increase the draw from the deep aquifer. The intermediate aquifer quality is deteriorating, particularly with respect to nitrates, and wells already have been taken out of service that exceed the U.S. Environmental Protection Agency’s primary drinking water standard.
Increased reliance on the deep aquifer raises a number of questions because we cannot predict the rate of transmission of constituents from the intermediate aquifer due to increased pumping or the rate of replenishment of the deep aquifer.
At present, the quality of water delivered to our homes varies throughout Davis. In general terms, the water will be similar to the well closest to our home and we can see the very large differences by looking at the data provided by the city for individual wells. How this situation will change over time if deep aquifer pumping is increased is unknown.
Subsidence is a possibility if deep aquifer pumping exceeds the sustained yield and would cause damage to underground utilities, foundations, sidewalks and streets. Committing to a groundwater-only policy risks subsidence but the extent is unknowable without further information about the sustained yield of the deep aquifer. If subsidence were significant, the cost to repair the resulting damage would be very high, however.
Meeting the treated wastewater discharge standards will require greater use of the deep aquifer. Selenium is the current mineral of greatest concern but others, including the total dissolved solids (TDS or total salts), may become issues. We should note that drinking water standards for many minerals are much less stringent than environmental standards and selenium provides an example.
The primary drinking water limit for selenium is 50 micrograms per liter while the discharge limit for Davis is 5 micrograms per liter. Treatment to the 5 micrograms per liter is very problematic without going to reverse osmosis, which in turn raises problems with energy requirements, cost and brine disposal.