Thursday, January 31, 2013
In November 2011 I wrote offficials of Walla Walla County about my concerns regarding protection of our shallow aquifer in the airport area. I recommended that a more thorough hydrogeologic assessment be made before the airport was designated as being outside of the Critical Aquifer Recharge Area (CARA).
After the Growth Management Hearings Board remanded the county’s aquifer recharge regulations for further consideration of the shallow gravel aquifer in the vicinity of the airport, its consultant completed further analysis. It is interesting not only that the shallow aquifer in the airport area is potentially subject to pollution but also that our groundwater has been contaminated there — twice.
Two more factors should be considered:
Based on data in the county soil survey manual, the vertical hydraulic conductivity there is 0.8-2.5 inches per hour. This level of conductivity is generally accepted as sufficient permeability for septic drainfields. And if permeable enough for drainfields, it is permeable enough for contaminants to pollute our shallow aquifer.
The earlier assessment that the airport area need not be in the CARA may have been based, in part, on the presence of caliche (calcium carbonate)-cemented gravels in the subsurface. If a contaminant spill were acidic, the acid might dissolve the caliche, making the gravels more permeable, thereby increasing the chances of aquifer contamination.
In summary, contaminants can migrate down vertically and have polluted the shallow aquifer in the airport area. In addition, contaminants could and would move laterally on the surface and/or through permeable beds to streams like Mill Creek and Mud Creek, potentially polluting them. Considering that the airport area lies over our shallow aquifer, and that it is topographically uphill and hydrologically upvalley relative to our community, it should be in the CARA.
Robert J. Carson