Flooded area and plant zonation in isolated wetlands in well fields in the Northern Tampa Bay Region, Florida, following reductions in groundwater-withdrawal rates

Abstract

The extent and duration of the flooded area were compared in two reference wetlands and nine wetlands in well fields in the northern Tampa Bay region, Florida, to determine whether reductions in well-field groundwater-withdrawal rates resulted in increases in wetland flooded area. Flooded area, expressed as a percentage of the total wetland area, was used to provide a quantitative and comparable line of evidence for describing the hydrologic conditions in isolated wetlands of different sizes and locations. Flooded-area frequencies were quantified for periods with different groundwater-withdrawal rates that bracket reductions in well-field groundwater withdrawals. Four-year pre-reduction and post-reduction periods were applied to wetlands in Cypress Creek and Cross Bar Ranch well fields, whereas 3-year periods were applied to wetlands in Starkey well field. The reduced groundwater-withdrawal rates in Cypress Creek and Cross Bar Ranch well fields were 30 and 24 percent less than their pre-reduction rates, respectively. The reduced groundwater-withdrawal rate in the Starkey well field was 64 percent less. Total rainfall amounts were similar (differed by 1 percent or less) in the respective pre- and post-reduction periods, which minimized the effect that rainfall variability had on the analysis. Flooded-area patterns at the reference wetlands, which were unaffected by groundwater withdrawals, were similar during pre- and post-reduction periods, indicating that short-term rainfall variability within those periods did not affect the longer-term patterns of flooded-area extent and duration. One well-field wetland (W-33) experienced an extent and duration of flooded area similar to that observed at the reference wetlands. About 61–100 percent of W-33 was flooded 41 percent of the time during the pre-reduction period and 45 percent of the time in the post-reduction period. The amount of time the wetland was dry decreased from 40 percent in the pre-reduction period to 26 percent in the post-reduction period. The median elevation of the potentiometric surface of the Upper Floridan aquifer increased beneath this wetland by about 4 feet after reductions in groundwater-withdrawal rates. Four well-field wetlands (W-17, W-56, Starkey N, and Starkey 108) had substantial increases in the extent and duration of the flooded area after reductions in groundwater-withdrawal rates. These four wetlands were dry for 25–45 percent less time during the post-reduction period, when the pre- and post-reduction periods were compared. Up to 20 percent of W-56 was flooded more than three times as long after reductions in groundwater-withdrawal rates. All parts of W-17 were flooded for as much as 10 percent of the time in the post-reduction period. Parts of Starkey N and Starkey 108 were flooded for more than twice as much time after reductions in groundwater-withdrawal rates. The median elevation of the potentiometric surface of the Upper Floridan aquifer was about 4–8 feet higher beneath W-17 and W-56 after reductions in groundwater-withdrawal rates, whereas the median elevation increased beneath Starkey N and Starkey 108 by about 4 feet after reductions in groundwater-withdrawal rates. Four other well-field wetlands (W-41, Q-1, Starkey D, and Starkey E) were mostly dry before reductions in groundwater-withdrawal rates and remained mostly dry after the reductions. W-41 was dry 23 percent less time in the post-reduction period, but most of the increase in flooded area was confined to less than 20 percent of the total wetland area. Q-1 was dry for only 12 percent less time in the post-reduction period. The median elevation of the potentiometric surface of the Upper Floridan aquifer increased beneath W-41 by about 5 feet and beneath Q-1 by about 2 feet after reductions in groundwater-withdrawal rates. The extent and duration of the flooded area was unchanged at Starkey D when the post-reduction period was compared to the pre-reduction period. At Starkey E the extent of the flooded area decreased slightly during the post-reduction period. Even though groundwater-withdrawal rates at Starkey well field decreased in the post-reduction period, the median elevation of the potentiometric surface of the Upper Floridan aquifer did not increase beneath Starkey D and Starkey E after reductions in groundwater-withdrawal rates from this well field. Factors such as the high permeability of sediments beneath the wetlands, subsidence, or sinkholes could contribute to continued downward leakage from these four wetlands and the lack of recovery of wetland water levels. Plant zonation in the two reference wetlands and the nine well-field wetlands was described using data collected by the Southwest Florida Water Management District and Tampa Bay Water, a regional utility, in their Wetland Assessment Procedure (WAP). A scoring system was used to describe the distribution of trees, woody shrubs, and groundcover in zones at three depths along a transect line through each wetland. The locations of the three zones were identified on contoured wetland bathymetry maps and were discussed in relation to areas of the wetland bottom that flooded for different periods of time during the study. Higher scores are characteristic of a greater extent and duration of wetland flooded area. WAP scores and weighted average scores for wetland vegetation were generally consistent with the results of the flooded area analysis. The WAP scores and weighted average scores were higher overall and did not decline with time at four wetlands in well fields (W-33, W-56, Starkey N, and Starkey 108) during the years following reductions in groundwater-withdrawal rates. These four wetlands also had increases in the extent and duration of the flooded area during the post-reduction period. Scores for trees were more consistent than scores for shrubs and groundcover. WAP scores remained relatively low or generally declined at five well-field wetlands (Q-1, W-17, W-41, Starkey D, and Starkey E) during the years following reductions in groundwater-withdrawal rates, and weighted average scores either declined over time or remained low. These five wetlands either did not have an increase in the extent and duration of the flooded area, or if there was an increase, it was small.