HYDROGEOLOGY: THE DEMAND FOR WATER
of Florida’s potable water (Fernald and Purdum, 1998) and the source of many freshwater springs (Rosenau, et al., 1977).
In northern central Polk County, Florida, transmissivity of the UFA can range from 10,000 ft2-1 to more than 100,000 ft2-1 and leakance1 can range from less than 10-3 ft day-1 ft-1 to 10-5 -1-1 (DRMP, 1990, HAI, 2002). Greater potential for harm from drawdown can occur at the lower range of transmissivity and higher range of leakance; hence, it is critical that these hydrogeologic parameters are reliably known during wellfield design so that excessive groundwater pumping at permitted rates will not cause unacceptable envi- ronmental impacts and groundwater quality remains stable.
Collectively, City administration and City water utility decision makers (hereafter, “City leaders”) recognized that minimizing on-site and off-site impacts was essential, so selection of a new UFA wellfield site included a requirement to purchase enough land to provide a buffer from development and any potential off-site impacts due to withdrawals. To ensure that enough land was purchased, an extensive hydro- geologic testing program was designed to collect sufficient data to calculate transmissivity, storage and leakance for use in predicting scale and extent of impacts. This program was submitted to the Southwest Florida Water Management District (SWFWMD) for review, comment and approval prior to implementation of the program. The SWFWMD is the regu- latory agency responsible for permitting water use in west- Central Florida, so it was important to reach agreement on the aquifer testing and analysis protocol to simplify review of the water use permit (WUP) application. Through an extensive literature and property search the City leaders identified an 800-acre parcel (Figure 1) and entered into a lease-to-purchase agreement with the land owner. Purchase was contingent on
satisfactory reliable yield determined by hydrogeologic testing and on obtaining a WUP from the SWFWMD.
Northeast wellfield area
The top of the unweathered UFA at the wellfield site is roughly 90 feet below land surface and is overlain succes- sively by 50 or more feet of lower yield weathered limestone and calcareous clay (“sandy clay to clayey sand” on Figure 2), by a thin sand or limestone interval ( “sand”), and by clay beds 10 or more feet thick. The uppermost three to five feet comprise clean well-sorted fine sand (Figure 2), (DRMP, 1990; Peterson, 2007). We refer to the semiconfined sand layer as an “intermediate aquifer” to distinguish it from the unconfined surficial sands, rather than to suggest it is connected to or an extension of the Intermediate Artesian aquifer recognized throughout southern and southwestern central Florida.
The northeast wellfield property (henceforth NE wellfield) is roughly equally divided between low relief uplands and shal- low wooded wetlands. Few other than depressional soils are listed as frequently inundated, and upland soils are generally well drained (NRCS on-line soil survey, Polk County, Florida). Historically, as shown in the aerial photograph from 1941 (Figure 3), uplands consisted of South Florida Flatwoods (pine flatwoods) with only a few feet of relief and very gently sloping convex topography. Before alteration, areas most distant from wetlands were nearly flat-lying, and the low relief resulted in shallow groundwater gradients within natural wetlands catchments. The darker areas on the crests of divides between areas of wooded wetlands indicate that these flat crests retain stormwater in ephemeral wetlands or microtopography. This storm water apparently then drains away slowly to the wet- lands proper by overland and shallow groundwater flow.
Figure 2. NW-SE Hydrogeologic Cross-Section of NE Wellfield. Section trace is shown on Fig.5. Modified from HAI(2002).
1. Vertical leakance is defined as the average vertical hydraulic conductivity of the confining unit sediment divided by the thickness of semi-confining beds between the aquifer and source beds.
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