Geophysical Constraints on Fresh Ground-water Flows to Florida Bay
Topical Area: Water Quality
David V. Fitterman and Maryla Deszcz-Pan, U.S. Geological Survey; Denver, CO
Recent studies of Florida Bay have raised questions about the possibility of fresh ground-water flows into the Bay. Such subsurface flows could have a significant influence on the quality of water in the Bay. Investigations aimed at locating possible flows have focused on the use of seepage meters installed on the Bay bottom. We have been involved in a series of airborne geophysical surveys and ground-based geophysical measurements aimed at mapping saltwater intrusion in the Everglades National Park north of Florida Bay. These measurements can indirectly provide constraints on possible fresh ground-water flows to the Bay.
The electromagnetic geophysical techniques, which we have used, map variations in electrical resistivity of the subsurface as a function of position and depth. Because the electrical properties of water saturated geologic materials are highly dependent upon the resistivity of the pore water, the electrical resistivity of the rock-water system can be used to estimate the pore water resistivity, which is in turn related to water quality.
Helicopter electromagnetic (HEM) surveys flown inland of Florida Bay collected data every 10 meters along flight lines, which are spaced 400 meters apart and run north-south. Resistivity-depth models are computed at every data point and displayed as resistivity-depth slices. These maps show a fairly uniform interpreted resistivity of 1-2 ohm-meters in the region from Florida Bay landward for distances of 5-10 km. Such low resistivities are caused by saltwater saturated rocks in the near-surface Biscayne aquifer. There is no indication of a freshwater saturated zone at depth, which, if present, is expected to have resistivities of greater than 30 ohm-meters.
It is geophysically possible to have a more resistive zone, which our measurements can not detect, below the saltwater saturated zone. To estimate the depth of such a zone we postulated a fresh-water zone of infinite thickness with a resistivity of 35 ohm-meters underlying a 2 ohmmeter zone representing salt-water saturated rock and compared the expected HEM signals from models with and without a fresh-water zone. Our HEM data could detect an infinitely thick, resistive fresh-water zone if the overlying 2-ohm-meter salt-water zone were less than 16 meters thick. We also considered models where the freshwater saturated material was confined to a 16meter-thick zone with 2-ohm-meter material above and below it. This resistive layer could be detected if it were within 8 meters of the surface.
As the simulated, fresh-water saturated zone is made thinner, the depth at which it is detectable decreases. Thus thin, high resistivity zones could exist which are not detectable geophysically. If such zones are freshwater saturated, then there are hydrologic questions which need to be answered: 1) how could such a thin zone be isolated from salt-water saturated zones above and below it so that there is no mixing of the water, 2) could such a zone exist over extended distances(1 0's of kilometers), 3) could such a thin zone carry sign)ficant quantities of water to Florida Bay, and 4) if such a zone carries freshwater to Florida Bay, why have there been no observations of freshwater seeps in the Bay?
In addition to the HEM surveys we have made about 60 time-domain electromagnetic (TEM) soundings in our study area. These measurements are able to see deeper and with greater subsurface resolution than the HEM surveys and provide more precise estimates of formation resistivity as a function of depth. A line of soundings stretching from Flamingo, to Nine Mile Pond, across Taylor Slough towards the intersection of highway U.S 1 and the C-111 canal show a general decrease in interpreted resistivity with depth. With resistivities of from 1 to 10 ohmmeters at depths of 10 meters and greater, these areas are most certainly saltwater saturated. However, it is possible to embed thin ( 1-5 meter thick) fresh-water saturated zones which are not detectable. Once again this possibility brings to mind the questions stated previously as to whether or not such thin zones are hydrologically realistic and sign)ficant.
One exception to the pattern of resistivity decreasing with depth is seen to the south and east of Taylor Slough. At locations north of Little Madeira Bay and Joe Bay, the interpreted TEM resistivity increases to values of 70 and 26 ohm-meters respectively at a depth of 20 meters. These high resistivity zones have thicknesses of at least 50 meters and may be related to the deep (>40 meter), high resistivity zone seen along the axis of Taylor Slough in the HEM data. The cause of these thick, resistive zones in not clear, but could be caused either by fresh-water saturated or low porosity rock.
Our geophysical measurements provide no conclusive evidence of fresh-water saturated zones extending to Florida Bay. In general, thin, high-resistivity layers, which are not detectable geophysically, could be embedded in models satisfying the data. How sign)ficant such freshwater saturated zones, if they actually exist, are hydrologically is an open question. Of greater interest are the two thick, high resistivity zones to the south east of Taylor Slough. The cause of these anomalies is not clear and is being investigated.