Science, Technology and Engineering

Environmental Geoscience

Honours Projects

Groundwater - surface water interactions in the Tullaroop catchment, central Victoria

Sarah Hagerty

Supervisor: Dr John Webb

In the Tullaroop catchment rising salinity in the Tullaroop Reservoir and increased pressure on groundwater resources, which are used for irrigation and urban water supplies, requires an integrated knowledge of groundwater and surface water; this was achieved through a detailed hydraulic and chemical study of water resources.

Hydrograph data from the basalt and deep lead aquifers shows that the deep lead is recharged through downwards flow in the south of the catchment, but the hydraulic gradient reverses upflow of ENE trending faults, where deep lead groundwater is forced to flow up into the basalt. Basalt and deep lead groundwater are fresher under volcanoes, and both aquifers generally become more saline towards the north. All groundwaters have a similar origin, but basalt groundwater has higher Ca²⁺, Na⁺, K⁺ and SiO₂ relative to Cl^-, due to silicate weathering.

Low ⁸⁷Sr/⁸⁶Sr ratios and young radiocarbon ages in the southern part of the deep lead confirm that it is recharged via preferential flow paths through volcanoes in the basalt. Some basalt groundwater samples have abnormally high ⁸⁷Sr/⁸⁶Sr ratios, and this is due to upflowing deep lead groundwater immediately upflow of faults.

Basalt groundwater input into Creswick Creek, Tullaroop Creek and the northern end of Birch Creek is identified through groundwater d¹⁸O and d²H signatures and low ⁸⁷Sr/⁸⁶Sr. These reaches also have a large seasonal salinity increase from ~500 µS/cm in late winter and spring to over 2000 µS/cm in late summer and autumn. Influx of saline basement groundwater is also identified in Creswick Creek, at the margin of basement outcrop, because it has a high ⁸⁷Sr/⁸⁶Sr and low Ca²⁺ and Mg²⁺ relative to Cl^-. The upper reaches of Birch Creek are fresh year-round and d¹⁸O, d²H and ⁸⁷Sr/⁸⁶Sr isotopic compositions show that stream composition is not significantly affected by groundwater.

Lake modelling has showed that salinity in Tullaroop Reservoir is controlled by the salinity of incoming surface flows from Tullaroop Creek; evaporation has little effect on the salt budget for the lake. The salinity of Tullaroop Creek is largely a result of the relative volume of baseflow versus rainfall, and this balance has been upset by recent drought conditions.

This study shows that deep lead groundwater should be allocated separately from each discontinuous section of the deep lead, and that groundwater and surface water in the Tullaroop catchment should be managed as a single resource.

This project was carried out in collaboration with Primary Industries Research Victoria (PIRVic), and was co-supervised by Dr Jon Fawcett (PIRVic).