IDENTIFYING SOURCES OF VERTICAL RECHARGE TO A CONFINED AQUIFER, SOUTH EASTERN SOUTH AUSTRALIA

Low permeability aquitards generally control the degree of interconnectivity, and therefore the recharge flux, between aquifers. However, in addition to diffuse recharge through an aquitard, a study of a confined aquifer in south east South Australia identified another mechanism for vertical recharge that of preferential flow of near surface water along fault zones.

A field site consisting of ten piezometers were installed at discrete intervals through the unconfined and confined aquifers and the intervening aquitard. Hydraulic measurements, groundwater sampling and aquitard pore water chemistry were analysed and a 2-D conceptual model of the groundwater profile at the site was constructed. A very low vertical hydraulic conductivity of 5.77 x 10-11 m/sec in the aquitard creates a downward head gradient of ~17m between the unconfined and confined aquifers. There was no significant difference in groundwater chloride taken from both aquifers and aquitard pore water. It is therefore likely that the aquitard is in hydraulic equilibrium with the unconfined aquifer and a discharge flux from the aquitard of ~18 mm/yr is occurring.

Radiocarbon (14C) concentrations in the groundwater decline from the equivalent of modern water at the watertable to 14 pmC (percent modern carbon) at the base of the unconfined aquifer. Below the thin two meter thick aquitard the groundwater in the confined aquifer has a pmC of 55, considerably higher than that of the overlying aquifer. Therefore while the chloride profile supports vertical flow, the relatively younger groundwater in the confined aquifer below the aquitard suggests the aquifer is receiving additional recharge from another source.

A significant lowering of the water table near a major fault coupled with fractures observed from a well drilled on it, which are parallel to the regional maximum horizontal stress direction and are therefore inferred to be likely open and permeable, are used as evidence to support a conceptual model that suggests the faults are conduits facilitating preferential flow of younger groundwater to the confined aquifer. The radiocarbon concentration of groundwater taken from the confined aquifer is therefore a mixture of older water sourced from the aquitard, relatively younger water sourced from preferential flow and lateral inflow.