Geophysical Well Logging And Pumping Water Column Sampling For Radionuclide Reduction Studies In Municlpal Wells

Several hundred municipal wells producing from the Cambro-Ordovician aquifer in the upper<br>midwest exceed the USEPA interim standard of 5 pCi/l for combined Ra(226) and Ra(228).<br>Numerous geophysical logging studies were performed to determine if the Radium<br>concentration could be reduced in these wells by isolating discrete portions of the aquifer with<br>casings or liners. This paper presents the results of a study of two municipal wells in Ottawa,<br>Illinois that demonstrate some of the methods tested.<br>Well 12 was drilled in 1989 to a depth of 1200 feet with surface casing to 427 feet. The<br>well was initially tested at 1200 gpm with 7.1 pCi/l of total Radium. A suite of well logs,<br>consisting of dual induction, neutron density, gamma density, and spectral gamma logs, was<br>run to identify water bearing formations which may contain high levels of Uranium or Thorium,<br>the precursors of Ra(226) and Ra(228). A zone of the Oneota Dolomite between 530 to 580<br>feet was found to have relatively high effective porosity with moderately elevated levels of<br>Uranium and Thorium. A surface casing with packer was installed to a depth of 600 feet to<br>isolate the unit. The well was test pumped at 500 gpm for 20 hours with a 4% reduction in<br>specific capacity and a total Radium level of 4.8 pCi/l, a reduction of 32%.