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Scientific Investigations Report 2010–5092

Prepared in cooperation with
Southwest Florida Water Management District

A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed, West-Central Florida, 2003–2005 and 2009

By G.R. Kish, C.E. Stringer, M.T. Stewart, M.C. Rains, and A.E. Torres

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ABSTRACT

Geochemical mass-balance (GMB) and conductivity mass-balance (CMB) methods for hydrograph separation were used to determine the contribution of base flow to total stormflow at two sites in the upper Hillsborough River watershed in west-central Florida from 2003–2005 and at one site in 2009. The chemical and isotopic composition of streamflow and precipitation was measured during selected local and frontal low- and high-intensity storm events and compared to the geochemical and isotopic composition of groundwater. Input for the GMB method included cation, anion, and stable isotope concentrations of surface water and groundwater, whereas input for the CMB method included continuous or point-sample measurement of specific conductance.

The surface water is a calcium-bicarbonate type water, which closely resembles groundwater geochemically, indicating that much of the surface water in the upper Hillsborough River basin is derived from local groundwater discharge. This discharge into the Hillsborough River at State Road 39 and at Hillsborough River State Park becomes diluted by precipitation and runoff during the wet season, but retains the calcium-bicarbonate characteristics of Upper Floridan aquifer water.

Field conditions limited the application of the GMB method to low-intensity storms but the CMB method was applied to both low-intensity and high-intensity storms. The average contribution of base flow to total discharge for all storms ranged from 31 to 100 percent, whereas the contribution of base flow to total discharge during peak discharge periods ranged from less than 10 percent to 100 percent.

Although calcium, magnesium, and silica were consistent markers of Upper Floridan aquifer chemistry, their use in calculating base flow by the GMB method was limited because the frequency of 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. In this study, pre-event water represented somewhat diluted groundwater.

Streamflow conductivity integrates the concentrations of the major ions, and the logistics of acquiring specific conductance at frequent time intervals are less complicated than data collection, sample processing, shipment, and analysis of water samples in a laboratory. The acquisition of continuous specific conductance data reduces uncertainty associated with less-frequently collected geochemical point data.

First posted July 2, 2010

For additional information contact:
George Kish
U.S Geological Survey
Suite 215
10500 University Center Dr.
Tampa, FL 33612-6427
813-975-8620 http://fl.water.usgs.gov/

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Suggested citation:

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, 33 p.



Contents

Abstract

Introduction

Purpose and Scope

Description of Study Area

Hydrogeologic Framework

Methods of Investigation

Data Collection Sites

Hydrologic Data Collection

Water-Quality Data Collection

Laboratory Analysis

Mass-Balance Mixing Methods

Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation Techniques

Precipitation, Surface-Water Discharge, and Groundwater Flow Gradients

Water Quality

Geochemical Mass-Balance Methods for Base-Flow Separation

Summary

References Cited


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