The spatial and temporal dynamics of groundwater – river interactions

Abstract

This thesis evaluates the connectivity and geochemical implications of groundwater-surface water connectivity in the Gippsland Basin. Head gradients, temperature profiles, Cl concentrations and 222Rn 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 (1.48×104 to 1.41×103 m3 day-1) than discharge estimated by 222Rn mass balance (5.35×105 to 9.56×103 m3 day-1) and differential flow gauging (5.41×105 to 6.30×103 m3 day-1) due to Cl variability in bank waters. 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). Groundwater residence times increase towards the Tambo River which implies a gaining river system and not increasing bank storage with proximity to the Tambo River. Major ion concentrations and δ2H and δ18O values of bank water also indicate that bank infiltration does not significantly impact groundwater chemistry under baseflow and post-flood conditions, suggesting that the gaining nature of the river may be driving the return of bank storage water back into the Tambo River within days of peak flood conditions. The covariance between 3H and 14C indicates the leakage and mixing between saline (~3,000 µS/cm) old (~17,200 years) groundwater from a semi-confined aquifer and fresh (