Changes in the saltwater interface corresponding to the installation of a seepage barrier near Lake Okeechobee, Florida

Abstract

In 2011, the U.S. Geological Survey and the U.S. Army Corps of Engineers began monitoring the saltwater interface near Lake Okeechobee to evaluate changes in interface depth that could possibly be related to the repair of the Herbert Hoover Dike. A seepage barrier (or cut-off wall), installed by the U.S. Army Corps of Engineers, is a wall of grout designed to protect the Herbert Hoover Dike from internal erosion caused by the piping of water. The seepage barrier prevents water from flowing through or immediately under the dike by diverting the flow below the dike, into the surficial aquifer system. The seepage barrier extends below the saltwater interface in some areas. Monitoring consisted of collecting water samples and time series electromagnetic-induction log (TSEMIL) datasets from 10 well clusters, each of which have 1 shallow and 1 deep monitoring well, with 5- to 10-foot- (ft) long-screened intervals. The deep wells are 120 to 187 ft deep, and the shallow wells are 44 to 100 ft deep. Changes in the depth of the saltwater interface were identified that correspond closely to the depth of the bottom of the seepage barrier. These changes may have been the consequence of changes in groundwater flow initiated by the seepage barrier installation. In areas of the dike where a seepage barrier had not been installed, or where the bottom of the seepage barrier is well above the saltwater interface, monitoring detected no changes in the depth of the saltwater interface. At five of the monitoring-well cluster locations, a long-screened well was also installed for monitoring and comparison purposes. These long-screened wells are 160 to 200 ft deep, and have open intervals ranging from 145 to 185 ft in length. Water samples were collected at depth intervals of about 5 to 10 ft, using 3-ft-long straddle packers to isolate each sampling interval. The results of monitoring conducted using these long-screened interval wells were generally too variable to identify any changes that might be associated with the seepage barrier. Samples from one of these long-screened interval wells failed to detect the saltwater interface evident in samples and TSEMIL datasets from a collocated well cluster. This failure may have been caused by downward flow of freshwater from above the saltwater interface in the well bore.