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Hydrologic changes in well field areas of northern Tampa Bay

Project Chief: Terrie M. Lee
Cooperator: Southwest Florida Water Management District
Period of Project: October 2009 – September 2013

Problem Statement

Figure 1. The potentiometric surface in June 2005 was used in this mapping product to show (in gold) areas of upward groundwater flow near a well field. Groundwater in the Upper Floridan aquifer flows upward toward the surficial aquifer beneath the channels of two major streams, along some smaller tributary streams, and below wetlands that form stream headwaters.

Presently no consistently applied approach is used to quantify and assess the temporal or spatial trends of the Upper Floridan aquifer (UFA) potentiometric surface within well fields in the northern Tampa Bay (NTB) area. As a consequence, it is difficult to characterize the status of groundwater resources in the well fields, to assess the effect of pumping regimes on the potentiometric surface, or to assess changes in the groundwater status over time. Trends in water levels have been analyzed and described for numerous individual monitor wells. However, the behavior at point locations is not sufficient to assess the temporal response of the UFA water levels in a spatial context over an entire well field. Because wetlands and streams are landscape-scale features, an analogous, landscape view of the potentiometric surface is required to describe and quantify the changing groundwater conditions experienced by these features over time in well-field areas.

The potentiometric surface can exhibit large seasonal variations beneath well fields in the NTB area. Depending upon pumping and climate conditions, its elevation may change over several months by 20 feet in some locations and remain similar in others. Day-to-day variations due to pumping can be 5 feet or greater depending upon the distance from a production well. Tampa Bay Water manages 11 well fields in conjunction with surface water and desalinization facilities to optimize production while minimizing environmental impacts. As a result, groundwater withdrawals within the well fields and the Upper Floridan aquifer potentiometric surface can vary substantially.

Objectives

The objectives of this study were to: (1) create a time series of digital map products of the potentiometric surface of the Upper Floridan aquifer in and around the 11 Tampa Bay Water well fields; and (2) use the spatial time series as hydrologic evidence to directly assess the status of the groundwater and surface-water resources in the well fields.

Approach

This project used a standardized, statistically valid, and repeatable approach for quantifying the average monthly elevation of the potentiometric surface within the well fields, and for describing statistical characteristics of this monthly average surface. The approach was conducted in two phases. The first phase, a correlation analysis of the time-series data acquired from all relevant wells, was used to evaluate techniques needed to gap-fill missing water-level data. The completed time series was then used to generate monthly average values at each well. The second phase involved the spatial interpolation of the head values from the time- series data, which relied on raster-based spatial analysis techniques within a GIS framework.

Water levels measured in existing Upper Floridan aquifer monitor wells were used to generate digital mapping images showing the estimated monthly average potentiometric surface elevation of the UFA in a 400 square mile region that includes 11 municipal well fields. Potentiometric surfaces were interpolated based on a total of about 200 wells that included from 10 to 30 monitor wells per well field. The surface maps were generated monthly, along with estimates of standard deviation around the monthly average values (fig. 1). Statistical properties of the monthly average values, such as the standard deviation of the water level, also were mapped. Data layers describing the potentiometric surfaces were analyzed in conjunction with LIDAR-based topography, hydrography data layers, land use, and field observations of wetland and groundwater levels to interpret hydrologic conditions both regionally and at individual well fields.

Information Product

Monthly maps were produced and interpreted for a five-year period for each well field, generating over 660 data layers for the entire regional well fields in NTB.