Objectives
This study used multiple lines of evidence, gathered during and after the commissioning of a newly constructed wetland in an urbanising catchment (Western Port Basin) to examine its effect on recharge and interaction with the underlying aquifer system. Understanding how such wetlands interact with groundwater is critical to effective water-cycle management in areas experiencing rapid urbanisation.
Design and methodology
Nested monitoring bores were installed adjacent to the wetland during early construction, and time series water level, electrical conductivity (EC) and soil moisture data collected. Surface and groundwater major ions, stable and radiogenic isotopes (δ2H, δ18O, 3H and 14C) were analysed before and after the wetland began regularly filling with stormwater runoff from new surrounding suburbs.
Original data and results
Prior to the wetland regularly filling with water, the shallow aquifer contained groundwater with EC >3,000 µS/cm, with fresher groundwater (EC ~1,000 µS/cm) occurring in the underlying deep aquifer. Groundwater in both aquifers contained no detectable tritium, and radiocarbon activities between 72.8 and 75.5 percent Modern Carbon. Following regular filling of the wetland with stormwater runoff, shallow groundwater EC began to fluctuate, periodically dropping below 1,000 µS/cm following rainfall events, before returning to pre-event values. The data from this early period (including hydraulic gradients) indicate dynamic ground- surface water interaction, with two-way exchange of water between the wetland and shallow groundwater. Following more than two years of runoff regularly filling the wetland, shallow groundwater EC stabilised at below 800 µS/cm, with tritium values of 2.29 TU and radiocarbon activity of 98.3 pMC. These data indicate that the original groundwater had been displaced by recharge from the wetland, creating an expanding plume of fresh recharge within the more saline regional groundwater. This plume has yet to affect the deeper groundwater, which experienced water level fluctuations in response to filling of the wetland (likely due to pressure loading), but showed no change in hydrochemical or isotopic composition.
Conclusions
The findings represent the first study to use environmental isotopes to demonstrate leakage from a constructed wetland as a significant new recharge mechanism in a catchment experiencing rapid urbanisation. Overall our data indicate that recent land-use change has modified recharge processes from low rates of diffuse rainfall recharge under pre-urban conditions to focussed recharge via leakage from constructed wetlands. This has significant implications for locations and rates of groundwater recharge, and for water quality protection (particularly if stormwater contains pollutants).