Robert Valderrama | Geology Reports

USGS2016

Collection, processing, and quality assurance of time-series electromagnetic-induction log datasets, 1995–2016, south Florida

Time-series electromagnetic-induction log (TSEMIL) datasets are collected from polyvinyl-chloride cased or uncased monitoring wells to evaluate changes in water conductivity over time. TSEMIL datasets consist of a series of individual electromagnetic-induction logs, generally collected at a frequency of once per month or once per year that have been compiled into a dataset by eliminating small uniform offsets in bulk conductivity between logs probably caused by minor variations in calibration. These offsets are removed by selecting a depth at which no changes are apparent from year to year, and by adjusting individual logs to the median of all logs at the selected depth. Generally, the selected depths are within the freshwater saturated part of the aquifer, well below the water table. TSEMIL datasets can be used to monitor changes in water conductivity throughout the full thickness of an aquifer, without the need for long open-interval wells which have, in some instances, allowed vertical water flow within the well bore that has biased water conductivity profiles. The TSEMIL dataset compilation process enhances the ability to identify small differences between logs that were otherwise obscured by the offsets. As a result of TSEMIL dataset compilation, the root mean squared error of the linear regression between bulk conductivity of the electromagnetic-induction log measurements and the chloride concentration of water samples decreased from 17.4 to 1.7 millisiemens per meter in well G–3611 and from 3.7 to 2.2 millisiemens per meter in well G–3609. The primary use of the TSEMIL datasets in south Florida is to detect temporal changes in bulk conductivity associated with saltwater intrusion in the aquifer; however, other commonly observed changes include (1) variations in bulk conductivity near the water table where water saturation of pore spaces might vary and water temperature might be more variable, and (2) dissipation of conductive water in high-porosity rock layers, which might have entered these layers during drilling. Although TSEMIL dataset processing of even a few logs improves evaluations of the differences between the logs that are related to changes in the salinity, about 16 logs are needed to estimate the bulk conductivity within ±2 millisiemens per meter. Unlike many other types of data published by the U.S. Geological Survey, the median of TSEMIL datasets should not be considered final until 16 logs are collected and the median of the dataset is stable.

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USGS2015

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

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.

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