Depth-dependent groundwater sampling for City of Tallahassee
Project Chief: Scott McBride
Collaboration: City of Tallahassee
Period of Project: FY 2012-2014
Tallahassee Test Well
Water-quality problems have arisen recently with the groundwater supply for the City of Tallahassee and surrounding areas. The City withdraws water from the Upper Floridan aquifer (UFA) using 27 deep, open-borehole water-supply wells. Although this groundwater is generally of high quality, changes in water quality in the permeable zones of the UFA have led to increased costs associated with treatment of groundwater used for public supply. Problems are related to elevated concentrations of iron, manganese, arsenic, nitrate, perchloroethylene, and bacteria.
The U.S. Geological Survey (USGS) has developed depth-dependent sampling techniques that allow collection of water-quality samples and flow-rate data from discrete depth intervals within public supply wells and boreholes during pumping and non-pumping conditions. Depth-dependent flow and water-chemistry profiles in pumping supply wells are useful for understanding sources and pathways for contaminants to supply wells. Previous studies by the USGS indicate that the majority of contaminants found in wells are often contributed by a single flow zone. Results from depth-dependent sampling will help to optimize the design of new public supply wells and contribute to developing effective strategies for economically sustainable withdrawals of substantial volumes of clean drinking water from the aquifer.
The main objectives of this study are to (1) delineate high-permeability zones in the Upper Floridan aquifer that have substantial rates of groundwater flow, and (2) characterize changes in water quality with depth in the test borehole for a City of Tallahassee public supply well.
Groundwater flow zones will be delineated using geophysical, chemical, and flow-meter data; lithologic logs, core samples, and other available site-specific information will be used to identify candidate intervals. The depth of the test well borehole is approximately 400 feet with a casing depth of approximately 250 ft. and a borehole diameter of 10 inches that will allow geophysical logging tools to acquire data on aquifer properties. Standard geophysical logs such as natural gamma, electromagnetic induction, and caliper will be run in the test borehole. In addition the USGS will use new borehole geophysical logging technologies and procedures, such as borehole image tools using optical and acoustic sensors, to image the borehole wall. From the images, the type and distribution of large-scale porosity can be interpreted. A full waveform sonic tool can quantify bulk porosity and relative permeability.
Vertical flow within the borehole will be measured over a wide range of flow rates during ambient and pumping conditions using high-resolution heat pulse, electromagnetic, and spinner flow meters. A water-quality probe will provide information on the water quality of each of the flow zones encountered. The water-quality probe provides field measurements of fluid temperature, conductivity, pH, dissolved oxygen concentration, and redox potential within the borehole under both ambient and pumping conditions.
Discrete water-quality samples will be collected from groundwater flow zones delineated by the methods described above. Inflatable packers will be used to provide positive isolation of specified flow zones in the borehole and water samples will be collected during low and high pumping rates. Field measurements of pH, specific conductance, temperature, and dissolved oxygen will be made at these different zones.
Water-quality samples will be analyzed for major ions, trace elements, nutrients, volatile organic compounds, bacteria, and dissolved organic carbon at the City of Tallahassee Water Quality Laboratory. In addition, water samples will be collected and analyzed for selected stable isotopes, dissolved gases, and transient age-dating tracers at USGS laboratories for the determination of the origin and age of water from various zones in the aquifer. It is anticipated that samples will be collected from five zones during low and high pumping rates. Isotope samples will include oxygen-18, deuterium, and strontium-87/strontium-86. Transient tracers for estimating groundwater age will include sulfur hexafluoride and tritium.
[Results are not yet available].
A summary data report will be prepared that includes a description of the geophysical and geochemical profiles with depth.