Science, Technology and Engineering

Environmental Geoscience

Honours Projects

Locating preferential flowpaths within the Willaura catchment using isotopic geochemistry

Darren Bennetts

Supervisor: Dr John Webb

The Willaura catchment in Western Victoria has suffered a substantial increase in salinisation since the 1930's, threatening its highly productive agricultural land. In order to quantify the extent and duration of salinisation, groundwater flow mechanisms have been studied using a multi-faceted isotopic approach.

Stable isotope analyses of groundwaters reveal light (meteoric) waters (d¹⁸O -4 permil to -6 permil) in recharge zones (Grampians Range and large lakes) and heavier enriched waters in discharge areas (-4 permil to 0 permil), particularly the two major springs at Gellerts Swamp and Boggy Creek. The enrichment along the flow path is probably due to the progressive addition of diffuse recharge from the overlying ground surface; this water has a heavy oxygen isotope composition due to evaporation within the soil profile, which also results in very high salinities in the soil water.

Tritium analysis reveals young groundwaters (10-15 years old) close to Grampians recharge areas, and older groundwaters (50 years and older) along flow paths. Apparently young waters (2-3 years) at the two major springs probably reflect the mixing of two different water bodies of separate ages. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) were used to source these water bodies; groundwater in Cainozoic basalt is less radiogenic (0.70882) than that in the underlying Cambrian-Ordovician basement (0.7123). Mass balance calculations show that the spring at Gellerts Swamp (2ML/day) has 85% of its water derived from underlying Cambrian-Ordovician basement and 15% from the overlying basalt, whereas for Boggy Creek (1.5ML/day) the situation is reversed (70% from basalt, 30% from Palaeozoic basement), owing to the much larger basalt recharge area for the latter spring. The basalt groundwater is more dilute than that in Palaeozoic basement, with substantially lower levels of the inert tracers Br and Cl.

The groundwater in the basalt is likely to be relatively young, due to its lower salinity and shorter flow paths, and predominantly diffusely recharged through the soil overlying the basalt. The basement groundwater is older, and recharged relatively rapidly through the thin soils of the Grampians slopes and the permeable sediments underlying the large lakes in the area. Along its flow path it receives slow, diffuse recharge through overlying low permeability soils, progressively increasing the salinity of the water and enriching its stable isotope composition.

The sub-surface relatively impermeable greenstone belts within the basement directly influence the groundwater flow paths, by deflecting groundwaters recharged from lacustrine recharge areas through conductive fault zones to discharge springs, as shown by geophysical mapping using shallow high-resolution electromagnetics (nanoTEM).

Salinity profiles from soils and sediments across the area show that groundwater recharge represents ~2% of rainfall, and lake sediments represent the major salt storage and most important area of salt export. Salt budget calculations indicate that salt export through the springs is at least 6 times greater than input from rainfall; this disequilibrium was probably caused largely by draining and diversion of inflow into the large lakes, reinforced by clearing of native vegetation. At current rates it will take 10-20 years to remove the salt in the lake sediments (along with the high salinity groundwaters), and re-establish equilibrium between salt input and output within the Willaura catchment, thereby substantially reducing groundwater salinity through much of the area.

This project was carried out in collaboration with the Centre for Land Protection Research.