A Geochemical Mass-Balance Method for Base-Flow Separation in the Upper Hillsborough River Watershed, West-Central Florida
Project Chief: George R. Kish
Cooperator: Southwest Florida Water Management District
Period of Project: October 2003 - September 2009
Problem Statement
 Limestone outcrop of the Tampa Member of the Arcadia Formation in the Hillsborough River at Hillsborough River State Park. Photograph by Dan Duerr, U.S. Geological Survey (retired).
In 1997, the Florida Legislature mandated that all of the Water Management Districts in Florida establish minimum flows and levels for surface waters and aquifers to maintain and protect natural systems while providing water for human use. Determining minimum flows for surface waters is difficult in cases where the base-flow contribution to streamflow is not accurately known, as is the case in the upper Hillsborough River. Most methods for separating base flow from streamflow rely on recession-curve methods or recursive-digital filter methods. Geochemical mass-balance methods take advantage of the chemical differences between surface water and groundwater chemistry to estimate base flow.
Understanding the interaction between groundwater and surface water is necessary in west-central Florida because of the proximity of the karstified Upper Floridan aquifer to land surface. Knowledge about groundwater/surface-water interaction also provides information that can be used to evaluate the susceptibility and sustainability of the aquifer and surface-water resources in this part of the Hillsborough River watershed. Because demands for water are projected to continue to increase in the future, an understanding of how groundwater sustains streamflow during dry periods will be necessary for water-resource managers to make informed decisions about future groundwater development.
Objectives
The objective of the project was to use a geochemical mass balance method to estimate the contribution of base flow to total discharge in the upper Hillsborough River.
Approach
Water-quality characteristics of streamflow and precipitation were measured during selected low- and high-intensity local and frontal storm events, and compared to the water-quality characteristics of pre-event surface water and groundwater. A mixing model was developed to estimate the percentage of pre-event (base flow) and event (runoff) water composing the hydrograph. The mixing model, which produces a direct separation of base flow and runoff, was used to evaluate and calibrate graphical hydrograph separation techniques using a chemical mass balance method. The mixing model was applied to point samples of specific conductance, major ions (such as calcium and magnesium), stable isotopes (hydrogen and oxygen) and continuous specific conductance for selected storm events.
Results
Calcium, magnesium, and silica are consistent markers of Upper Floridan aquifer geochemistry, but their use to determine base flow by the geochemical mass balance method was limited because the frequency of the point data collected in this study was not sufficient to capture the complete hydrograph from pre-event base flow to post-event base-flow concentrations.
Streamflow conductivity integrates the concentrations of the major ions, and the logistics of acquiring specific conductance at frequent time intervals are less complicated than collection, sample processing, shipment, and analysis of water samples. The acquisition of continuous specific conductance data reduced the uncertainty associated with less frequently collected geochemical point data.
Total discharge at the upper Hillsborough River sites during low-intensity storm events consisted primarily of base flow. During high-intensity storms, total discharge was substantially diluted with runoff water so that less than 10 percent of the total discharge originated from groundwater.
Information Product
Kish, G.R., Stringer, C.E., Stewart, M.T., Rains, M.C., and Torres, A.E., 2010, A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed, West-Central Florida, 2003-2005 and 2009: U.S. Geological Survey Scientific Investigations Report 2010–5092, 32 p.
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U.S. Department of the Interior |
U.S. Geological Survey
