The spatial and temporal dynamics of groundwater – river interactions
thesisposted on 22.02.2017 by Unland, Nicolaas Peter
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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 (<500 µS/cm) younger groundwater (<100 years) near the river where confining layers are less prevalent. The presence of this semi-confined aquifer has also been used to help explain the absence of bank storage, as rapid pressure propagation into the semi-confined aquifer during flooding will minimise bank infiltration. Continuous elevation and EC monitoring along the Tambo River bank, indicates that the degree of mixing between the two aquifers and the river varies significantly in response to changing hydrological conditions. Numerical modelling using MODFLOW indicates that saline water moves into the river bank during flooding as hydraulic gradients reverse. This water then returns during flood recession as baseflow hydraulic gradients are re-established. Modelling also indicates that this process will increase groundwater concentrations for up to ~34 days between 20 and 40 meters of the river for flood events as large as 9.7 m in height. For the same flood event, groundwater concentrations within 10 m of the river will only increase for ~15 days as the infiltrating low salinity river water drives groundwater dilution. Continuous high temporal resolution monitoring of 222Rn activities in the stratified tidal estuary of the Tambo River indicates significant variation in groundwater-surface water interactions over a ~5 day period. In contrast to the limited number of studies in similar areas, 222Rn mass balance indicates that the groundwater fraction varies tidally in the subsurface section of the Tambo River estuary but not in the surface section (the upper 50cm of the water column). The maximum groundwater fraction estimated in the subsurface typically ranged from ~4% to ~8% of total river discharge over individual tidal cycles. While this is partially attributed to varying degrees of mixing between subsurface water and surface water over tidal cycles, the input of groundwater during flow reversal is also likely to affect the groundwater fraction. It is proposed that in a tidal estuary, river water may receive groundwater inputs during both downstream movement at tidal minimum and upstream movement at tidal maximum, resulting in an increased groundwater fraction during tidal maximums. A reduction in the maximum groundwater fraction in the surface section (from ~11 to ~2%) coincides with increased rainfall in the catchment and a reduction in δ2H values (from -42 to -62‰), suggesting the dilution of groundwater via inputs from rainfall and runoff. During drought conditions in 2009, low-lying areas of the Heart Morass (0–2 m elevation) were the most affected by acid sulphate soils, with a median soil pH (pHF) of 3.56 to approximately 50 cm depth. Soils below ~100 cm depth in these areas contain pyrite and have reduced inorganic S concentrations of up to 0.85 wt%. Higher areas of the floodplain (2–6 m) do not contain acid sulphate soils, with a median pH of 4.74 to approximately 50 cm depth, an average neutralising capacity of 3.87 kg H2SO4/t, and no appreciable unoxidised pyrite. In low-lying areas concentrations of Co, Ni, Zn, Mn and Fe in soil increased from <2.0, 4.0, 10, 20 and 2000 mg/kg, respectively, at 56 cm depth to 10, 20, 45, 152 and 15,000 mg/kg at 221 cm depth. In areas of higher elevation, concentrations of Co, Ni, Zn and Fe increased from 6, 11, 21 and 12,500 mg/kg at 44 cm depth to 10, 19, 47 and 19,400 mg/kg at 239 cm depth. These data indicate acidic leaching of metals from the upper soil profile in both low-lying and more elevated areas.