Estimation of Water Quality

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The figure to the (left) shows the formation resistivity measured in 23 wells in the study area plotted as a function of the specific conductance (SC) of the pore water. The formation resistivity was obtained by averaging induction logs over the screened interval of the wells (typically 10 ft). The wells were then pumped and a water sample collected after sufficient time for the well bore to be purged. The specific conductance of the sample was measured.

For shallow wells the conductivity was measured directly in the well. The straight line shows the correlation between the estimate water quality and the formation resistivity. A chloride scale has been added based on a relationship between SC and chloride content that is valid for surface and ground waters found in this area. This graph has been used to convert geophysically determined formation resistivity into estimated water quality in the depth-slice maps shown on previous page.

TEM Survey Results

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Two TEM soundings and their interpretation are shown to the right. EG111 is landward of the FWSWI and EG108 seaward of the FWSWI. (See location map.) The measured data are the apparent resistivity-time functions. A nonlinear parameter estimation technique was used to determine the resistivity-depth function which produces the best match to the measured data.

The scatter plot at the lower right is the layer resistivity for the first and second layers from the interpreted TEM data as a function of depth to the bottom of the layer. The data separate into those with values less than 10 ohm-m and those greater than 15 ohm-m. Based on the water quality estimates (see graph) a formation resistivity of less than 10 ohm-m corresponds to a chloride concentration of more than 2000 ppm. Using this criterion we classify interpreted resistivity values of less than 10 ohm-m as saltwater saturated and construct the location of the freshwater/saltwater interface (FWSWI) (see map below, left).

The TEM data have also been used to estimate the depth to the base of the Biscayne aquifer (below, right). The TEM contours show detail not seen in the drilling results of Fish and Stewart (1991).

TEM Derived FWSWI	Depth to Base of Biscayne Aquifer
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Summary and Future Plans

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Geophysical measurements have been successful in locating the FWSWI, estimating aquifer water quality, and estimating the depth to the base of the Biscayne aquifer. These results are being used to construct and refine ground-water flow models of the area. Airborne measurements avoid problems of access common to ground based geophysical methods and provide much higher sampling density than is possible from drilling.

During FY2001 we will extend our geophysical coverage further to the northwest in the direction of Everglades City (see map to right). A 1000 sq. km (400 sq. mi) HEM survey will be flown, as well as ground-based geophysical measurements. These data will be used in constructing a regional ground-water flow model than includes solute transport.

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For more information contact:

David Fitterman
U.S. Geological Survey
Box 25046 MS 964
Denver, CO 80225
303-236-1382
e-mail: fitter@usgs.gov

Related information:

SOFIA Project: Geophysical Studies of the Southwest Florida Coast

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