Leakage of groundwater from the shallow surficial sands to underlying aquifers is limited by the low permeability (Grubb, 1977) and moderate thickness of semiconfining beds (DRMP, 1990; HAI, 2001).
HYDROGEOLOGY: THE DEMAND FOR WATER surface water outflow including streamflow and overbank
storm flow (So), and groundwater outflow (Go) (Mitsch and Gosselink, 1986). The NE wellfield wetlands water budget
comprises limited groundwater inflow (Gi), direct precipitation (Pn), and overland stormflow as sources(Si); and evapotranspi- ration (ET), surface drainage (for connected wetlands) (So), and negligible leakage to the Upper Floridan aquifer (Go) because of low permeability bedded clays as sinks. On the NE wellfield
property the generally low relief allowed isolated wetlands to exchange surface water during extreme storm events. NE wellfield wetlands also have naturally relatively low ratios of catchment to wetland area (Figure 3).
Figure 3 is a 1941 aerial photograph illustrating historical land cover and drainage patterns on most of the NE wellfield property, and Figure 4 is a 1990 aerial photograph showing the same area after conversion of uplands areas to improved pasture and with additional ditching to drain the land to pro- mote growth of forage grasses (City of Lakeland, FL contracted aerial photograph).
Site Suitability
Figure 3 - 1941 USDA aerial photograph showing the area of the Lakeland NE wellfield. Darker vegetation was cypress and hardwood wetlands and lighter areas were pine flatwood uplands. Standing water shows as very dark patches and tilled land shows as very light regular features.
In 1990, the first of three aquifer performance tests (APT) to estimate transmissivity, storage coefficient, and leakance factor was performed. The first APT included construction of a test well to production well standards near the center of the property (Fig. 4) and two clusters of observation wells. The test well (NE-1) is 16-inches in diameter, cased to 123 feet below land surface (bls), and open borehole to total depth of 780 feet bls. The potentiometric head in the surficial aquifer, in an intermediate artesian aquifer, and in the UFA aquifer was logged at each of the two observation well clusters. The clusters are located approximately 100 feet and 1000 feet from well NE-1. The observation well clusters comprised a 2-inch diameter surficial aquifer well screened from five to 10 feet bls, a 4-inch diameter intermediate artesian aquifer well screened from 12.5 feet bls to 27.5 feet bls, and a 4-inch diameter UFA production zone observation well open to approximately the same interval as NE-1 (120-780 ft). The potentiometric head response to the APT was also monitored and logged at one on-site existing out-of-service UFA well open from 42 to 376 feet and three off-site UFA wells. The first APT was run for 72 hours with well NE-1 at a discharge of 2,040 gallons per minute (GPM). Drawdown data were corrected for regional interference using SWFWMD monitor wells that were unaf- fected by the APT and corrected for antecedent trends of on-site observation wells using curves fitted to pre- and post-pumping data (DRMP, 1990).
Figure 4 - 1990 aerial photograph of the same area (white outline) as Figure 3 after alteration for enhanced offsite drainage and con- version of pine flatwoods to improved pasture. (Contract Aerial Photograph, City of Lakeland).
Wetlands on the NE wellfield property comprise both con- nected and isolated wetlands. In general terms the water budget for a wetland is:
Delta V = Pn + Si + Gi – ET – So – Go. (eq. 1) Where Delta V is volume of water in storage. Inflow com-
ponents are net direct precipitation (Pn), surface water inflow including streamflow and overland flow (Si), and groundwater inflow (Gi). Outflow components are evapotranspiration (ET),
The aquifer hydraulic properties derived from the APT were used to simulate wellfield drawdown impacts of five UFA supply wells (Figure 4) using the Prickett-Lonnquist Aquifer Simulation Model (PLASM) (Prickett and Lonnquist, 1971) in 1990 (DRMP, 1990). The values used in PLASM were 1.26 x 106 gallons per day per foot (gpd/ft) and storativity of 9.3 x 10-5 (dimensionless). The PLASM simulations showed that proposed operation of the NE wellfield would not cause harm to wetlands or other users when pumping at a combined annual average daily flow of 9 million gallons per day (MGD) and at a maximum monthly average daily flow of 16 MGD. (The original PLASM predictions were confirmed using a regional MODFLOW model for later permit applications.) Predicted drawdown was within acceptable limits of the SWFWMD, and a WUP was issued in 1991.
The City completed purchase of the NE wellfield property, and in accordance with the WUP conditions, a hydrobiological monitoring program was prepared and approved. Pumping from the NE wellfield did not begin until October 2005, but
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