Investigating the spatio-temporal variability in groundwater and surface water interactions: a multi-technique approach

The interaction between groundwater and surface water along the Tambo and Nicholson rivers, southeast Australia, was investigated using Rn-222, Cl, differential flow gauging, head gradients, electrical conductivity (EC) and temperature profiles. Head gradients, temperature profiles, Cl concentrations and Rn-222 activities all indicate higher groundwater fluxes to the Tambo River in areas of increased topographic variation where the potential to form large groundwater-surface water gradients is greater. Groundwater discharge to the Tambo River calculated by Cl mass balance was significantly lower (1.48x104 to 1.41 x 10(3) m(3) day(-1)) than discharge estimated by Rn-222 mass balance (5.35 x 10(5) to 9.56 x 10(3) m(3) day(-1)) and differential flow gauging (5.41 x 10(5) to 6.30 x 10(3) m(3) day(-1)) due to bank return waters. While groundwater sampling from the bank of the Tambo River was intended to account for changes in groundwater chemistry associated with bank infiltration, variations in bank infiltration between sample sites remain unaccounted for, limiting the use of Cl as an effective tracer. Groundwater discharge to both the Tambo and Nicholson rivers was the highest under high-flow conditions in the days to weeks following significant rainfall, indicating that the rivers are well connected to a groundwater system that is responsive to rainfall. Groundwater constituted the lowest proportion of river discharge during times of increased rainfall that followed dry periods, while groundwater constituted the highest proportion of river discharge under baseflow conditions (21.4% of the Tambo in April 2010 and 18.9% of the Nicholson in September 2010).