The number of Hill Air Force Base monitoring wells will double in Clinton, as a plume of contaminants in the groundwater stream below the city slowly spreads to the west.
The solvents, which were dumped from a Hill railroad engine repair shop, were discovered in the groundwater below Sunset and Clinton in 1990. The base stopped using the solvents during the mid-1970s.
About nine new 4-inch diameter wells will be drilled in Clinton, making a total of about 15 wells that Hill staff periodically monitors to gauge the concentration of the spreading plume.
The wells are about 20 feet deep, said Mel Wood, public works director. Wood said that the contaminant level is very, very low and that the solvent evaporates when exposed to oxygen.
Wood also said that the plume of contaminants spreads very slowly in the groundwater, at a rate of an inch or an inch and a half a year.
Hill has built trenches that allow oxygen to reach the groundwater and evaporate some of the contaminants.
The Clinton City Council voted to renew its agreement with the base on a year-by-year basis for the next five years. The agreement is about three to four years old, Wood said.