Florida Geological Survey I


of atmospheric pressure, rainfall, river and lake stages, and man-induced causes (Domenico and Schwartz, 1990). External loading stresses can cause similar hydraulic behavior (iLe., water level fluctuations in wells) in separate hydrogeologic systems.

 Because Brevard County is located adjacent to the Atlantic Ocean, oceanic tidal loading has a noticeable impact on hydraulic head lluctualions in wells that penetrate confined aquifer systems. The response of water levels in wells due to oceanic tidal loading occurs as a result at three processes:
 1. Mechanical loading of he aquifer at its oceanic extension;
 2. Propagation and attenuation of the pressure wave inland through the aquifer;
 3. Flow of ground water between the aquifer and the borehole.

 Aquiler Loading

 Mechanical loading ot the aquifer at ils oceanic extension causes the water oevel in a well to increase at high tide and decrease at low lide. As wells are located away from the ocean the inland transfer of the pressure wave through the aquifer occurs with a diminishing amplitude and increasing time lag (Enright. 1990) (Figure 27).

 Responses to earth tides in wells occur, by definition, at the same frequencies as ocean tides, but are orders of magnitude smaller (the largest are 0.5-1 inches). Because of a diflerence in phase and amplitude, any earth tidal fluctualions near the coast typically is masked by Ihe oceanic tidal fluctuations, The net effect of earth tides is Io decrease by a small amount the arplitude of oceanic tidal flucluations (Parker and Springfield- 1950: Gregg, 1966: Bredehoett, 1967: Enright. 1990).

 An inverse relationship exists between baro. metric pressure and water levels in wells. An increase in barometric pressure is transmitled to the confined aquifer system through the overly-


ing conining layer and the aquifer responds with an increase in pressure head. This causes water to flow into the well resuLting in an increase in water level, However, the well has a direct connection with the atmosphere, and because the atmospheric load is partially supported by the aquifer skeleton," the net effect of atmospheric loading is a decrease in water level during increased barometric pressures and an increase in water level during decreased barometric pressure (Domenico and Schwartz, 1990: Enright, 1990)- This relationship is demonstrated in Figure 26 where the D_ B. Lee monitor wells are inversely responding to the increases and decreases of barometnc pressure.

 If a well syslern is monitored continuously (such as the D_ B. Lee injection and monitor wells, Figure 28), the response to these external loading stresses can be observed as corresponding fluctuations of water levels. These stresses may conceal ihe true behavior of the aquifer syslem(s) in response in injection testsFor example, the simultaneous increase in well pressure wilhin Iha monilor wells during the injection test could be a result of a hydraulic connection between the injection well and the monitor wells, oceanic tidal loading, a decrease in barometric pressure, or a combination of these phenomena.

 When analyzing the aquifer(s) reactions to injeclion well tesis, the monitor well hydraulic head responses to atmospheric and ocean tidal loading may mask 1he efiect of the injeclicn test on the monilor wells. Therefore, in order tio isolate the hydraulic head response ot the monitor wells to the injection test, the ocean tidal and atmospheric loading influences need to be removed by determining the tidal efficiency and barometric efficiency of the aquifer(s) in which the welJ(s) are localed. Methods bor determining tidal and barometric efficiency are described by Jacobs (1 940) and Domenico and Schwarlz (1990).