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AcknowledgmentsThe authors gratefully acknowledge the cooperation and assistance of Michael Hancock from the Southwest Florida Water Management District for suggestions and comments on early drafts of the report and Patrick Potts, Park Services Specialist, Hillsborough River State Park, who provided logistical support during data-collection activities. Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).Altitude, as used in this report, refers to distance above the vertical datum.Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 °C).
AbstractGeochemical 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][2004][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 pointsample 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 calciumbicarbonate 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 ...
