City government

Ask the expert: Davis’ groundwater future is unknown

By From page A1 | February 07, 2013

* Editor’s note: This is the third in a series of “Ask the expert” columns that will appear in the coming days, 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: Graham Fogg, UC Davis professor of hydrogeology. Fogg’s academic interests include groundwater contaminant transport, groundwater basin characterization and management, geologic/geostatistical characterization of subsurface heterogeneity for improved pollutant transport modeling and numerical modeling of groundwater flow and contaminant transport.

The question: How long can Davis survive with the current well water system and have safe, drinkable water? Can you briefly describe the problems that could arise with the city’s drinking water, in terms of supply and quality, if Davis remained on a well-only system, and when those problems might occur?

The answer: Research indicates that downward movement of contaminated groundwater is a long-term and ongoing process and the deeper groundwater quality potentially will continue to degrade in the coming decades even in the unlikely event that anthropogenic contamination at the surface is eliminated.

In other words, the current well water system, which does include a number of aged wells that need to be replaced, is sufficiently vulnerable to ongoing contamination from surrounding land uses that it would be prudent to consider alternative or supplementary sources of water.

One alternative is to construct more wells in the so-called deep aquifer, which is perhaps more protected from surface contamination because of its depth beneath still more clay and silt layers.

Unfortunately, the capacity of the deep aquifer to satisfy city water demand is unknown at this time because there is not yet enough information on the lateral extent of that aquifer and on how and at what rate it is recharged.

Even if such information becomes available, however, concentrating more groundwater pumpage in the deep aquifer will only accelerate the downward movement of contaminated groundwater toward the deep aquifer. In other words, just because the deep aquifer is deep does not mean it is invulnerable to contamination. When I arrived in Davis in 1989, the intermediate depth aquifer zones that Davis has tapped also were considered invulnerable to contamination. Twenty-four years later it is a different story.

How long can Davis survive with the current well water system? Perhaps one to three decades. Can Davis survive significantly longer if it taps into the deep aquifer? Answer: Unknown, and unlikely to be known until substantially more development of the deep aquifer occurs. The costs of different options must be weighed, but one should also weigh the different degrees of uncertainty inherent to these options.

The main problem with staying on wells likely would be significant uncertainty regarding future capacity of the aquifer system to deliver sufficient volumes of water that meet drinking water standards, both with respect to anthropogenic contaminants (e.g., nitrate and salinity from irrigation) and natural contaminants such as selenium and chrome.

Because the amount of water the aquifer can safely yield is unknown, it is also possible that development of that aquifer will lead to overdraft conditions. In the case of groundwater overdraft, the possible detrimental effects are land subsidence, higher energy costs of pumping from deeper static water levels and intrusion of deep saline groundwater that lies below the deep aquifer.

Enterprise staff

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