HYDROGEOLOGY: THE DEMAND FOR WATER
Figure 6 - Pine plantation clearing and topographic restoration areas.
plantation reduced overland flow to the wetlands by greatly increasing the minimum storm size needed to create overland flow. Planted pines also increased leaf area and leaf litter so recharge decreased because of greater interception and upland storage on the NE wellfield property.
Taken together, it appeared that the altered system stored less water than the unaltered system and the wetlands received less overland stormflow, but that a nearly natural hydrology could be restored.
Restoration of wetland hydrology
The first stage of the rehydration program at the NE wellfield property was to harvest approximately 200 acres of planted pine (Figure 6) and regrade the land to approximate the topography that existed circa 1940. Because detailed land surface elevation information from that period did not exist for the property, a digital topographic model (DTM) was created based on existing topography, the shapes and gradients of natural edges of wetlands, historical wetland hydrobiological indicators, and historical soil types and indicators. The goal was to construct a topography that could recreate a hydrologic flow system consistent with that of the 1940s.
The restored hydrology had to improve on-site wetland hydroperiods but could not cause unacceptable off-site impacts. Blocking of ditches was selected as the best means to hold water in the wetlands and to restore the hydroperiod using water from surface water inflow and the overland flow that would occur after recontouring the land. In addition, blocking ditches could not result in flooding upstream property, nor could ditch blocks prevent downstream off-site discharge. Surface water monitoring stations were installed to record the movement
of surface water to and from the NE wellfield property and between different parts of the wellfield. The surface water flow data, in conjunction with the monitoring well data, were used to construct an interconnected pond routing model to determine where ditch blocks should be constructed (Figure 6) and the overflow elevation that would achieve hydrologic restoration objectives without creating unacceptable up- or down-stream.
The wetland rehydration program at the NE wellfield property achieved the desired results as demonstrated by the monitoring data and analyses conducted before and after completion of construction. Wetlands on the NE wellfield property now generally remain inundated for most of the year, and fill bank-full during the wet season. Wetland ecology has been partially restored, and upland species invasion has decreased significantly in the monitored wetlands.
Figure 7 illustrates the surficial aquifer water levels recorded at monitoring well P5AW, prior to and after pump- ing began and after completion of the wetland rehydration project. Well P5AW was installed in 1995 and is located in the large wetland northwest of the center of the NE wellfield property. Not all fluctuations in water level at well P5AW prior to October 2005 are connected to wellfield operation. Prior to the start of pumping, major storm events (for example, tropi- cal storms and hurricanes) filled most wetlands bank full, but water in most wetlands rapidly drained out and water levels returned to ground surface. The wetland rehydration project was completed in 2012 and the wetland monitored by well P5AW refilled with the wet season rains of that year and has remained wet since.
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