Box 9 Mixing Model Used to Separate Stream Base Flow

Kish and others (2010) examined the use of a geochemical mass balance approach to determine the portion of groundwater contributing to the Hillsborough River in west-central Florida, USA. They used both a geochemical mass balance (GMB) approach that accounted for flows, and anions, cations, and stable isotopes, and a conductivity mass balance (CMB) (Stewart et al., 2007) that relied on continuous and spot measurements of specific conductance. Both streamflow and groundwater water were dominated by similar major ions, a calcium-bicarbonate water type. Stream water became diluted by precipitation and runoff during high flow events. The lack of continuous ion sampling during runoff events and high discharges limited the GMB application to “low-intensity” runoff events. The continuous data sets collected as part of the CMB approach proved more useful in deciphering base flow. Results showed that the average proportion of groundwater (base flow) in stream runoff ranged from 31 to 100 percent (Table Box 9-1).

Table Box 9-1 – Base Flow Analyses for Sites SR39 and HRSP Showing Balance Derived Estimates of Base Flow Using GMB and CMB Approaches, Hillsborough River, Florida, USA (Kish et al., 2010).

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