HYDROGEOLOGY: THE DEMAND FOR WATER
the City implemented the monitoring plan in 1994 and moni- tored without interruption groundwater level and ecological responses to changes in weather for more than 11 years before wellfield pumping commenced. The City continuously records data from monitoring wells, rainfall gauges, evaporation pans, and wetland staff gauges, groundwater head and ecological responses to operation of the NE wellfield. Those data are collected and evaluated each year and are submitted annually to the SWFWMD. The City’s monitoring network currently consists of 52 monitoring wells in wetlands, in the surficial aquifer, in the local “intermediate” aquifer, and the UFA. Figure 5 shows monitoring stations aboard the NE wellfield.
Before the wellfield was put into operation, the City com- pleted a second APT in accordance with conditions of the WUP, and a third APT was performed by the SWFWMD. When the wellfield was put into production the baseline data were crucial for demonstrating that groundwater pumping was having no adverse impact on wetlands. The Governing Board of the SWFWMD approved the City’s WUP renewal in 2008, but at an allocation of 4 MGD, AADF2 – the rate at which the City had theretofore utilized the wellfield based on system demand. In an abundance of caution, the SWFWMD renewed the WUP with a requirement to implement a wellfield (wetland) improvement plan.
Changes to wetland hydrology
Long term changes to wetland health were noted long before pumping began. Prior to commencement of pumping wetland canopy flora were generally healthy, but some wet-
lands experienced prolonged dry periods, frequent absence of soil saturation or inundation, below normal water levels (i.e., stage), lowering of indicators of normal water elevation (i.e., pool), and invasion of upland vegetation. During the pre-pumping period, offsite discharge from the property was rare. Conditions in the wetlands were essentially unchanged after the start of pumping. Also, pumping of the NE wellfield began in October 2005 near the time when the planted pines had almost completely covered the surround- ing uplands. Pine plantation canopy cover at that time was very dense. The combined effects of the pines (almost no overland flow to the wetlands), and in the opinion of the SWFWMD, a consequence of pumping, resulted in a prolonged dry period in the wetlands. Data from the multiple APTs, soil borings, and labora- tory permeability tests demonstrated that the leakage from the surficial aquifer to underlying aquifers was very low; so low that the head difference between the two aquifers was approximately 10 feet. Realizing early that the change in tim- ing and duration of wetland inundation (hydroperiod) wasn’t due to pumpage and induced leakage, other parts of the water budget that could impact the surficial aquifer and wetlands were considered. The differences evident between Figures 3, 4, and 6 show that significant changes to the property had occurred.
2. Annual Average Daily Flow.
Two significant historical changes that could have caused net negative effects on hydrology of the NE wellfield wetlands were (1) drainage caused by ditching to connect and drain wetlands, and (2) conversion of natural land cover in the catchments of most wetlands from natural pine flatwoods to improved pasture. Ditching occurred before 1941 and
increased surface water runoff from wetlands (So, eq. 1); it also limited the maximum pool elevation in the wetlands. Conversion of uplands from natural pine flatwoods to improved pasture occurred between 1941 and 1958. This change also increased stormflow runoff to the wetlands but reduced bank storage in the catchments,decreased groundwater baseflow, and speeded up off-site discharge of water from the wetlands. Both occurred many years before acquisition of the property by the City in 1990.
Beginning in the late 1990s, many of the upland areas within the NE wellfield were converted from improved pasture to silviculture. The former pasture was plowed into a ridge and furrow system that retained water and limited surface
drainage (Si) to the wetlands. Slash pine was planted in the furrows and within a few years leaf litter had accumulated to thicknesses of 15 cm (~6 in) or more. As the planted trees matured the combined effect of interception of precipitation and increased ET further reduced flow from the catchments to the wetlands. Hydrologic and hydrobiologic monitoring showed that the surficial aquifer and some wetlands recharged quickly after storms but drained quickly as well. Whereas the improved pasture had increased runoff to wetlands, the pine
Figure 5 - Environmental Monitoring and Management Plan Network and other testing locations.
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