Influence of urban expansion on groundwater recharge in southeast Melbourne, Australia.
Creators: name: Hall, Benjamin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 330404
subject: Land use and environmental planning
classificationCode: 370303
subject: Isotope geochemistry
classificationCode: 370799
subject: Hydrology not elsewhere classified

description: This study undertook a detailed investigation of recharge processes of a Cainozoic aquifer system near the city of Melbourne, Australia that is experiencing rapid urbanisation. The aquifer system supports large-scale irrigated horticulture and groundwater dependent ecosystems, and supplies municipal drinking water. Changes to planning zones in 2012 and demand for housing has led to widespread urbanisation and the area now has the fastest growing population in Australia. There is extensive research on the impacts of conversion of land for agricultural production on groundwater recharge; however, to date there are few if any field-based studies examining impacts of urban expansion on recharge. The aims of this study were to (1) characterise recharge processes and rates and explore their relationships with geology and topography (2) investigate the interaction between new urban water management infrastructure (constructed wetlands) and groundwater recharge, and (3) asses the implications of the current and changing recharge regime for water management and land use planning both in the study area and more broadly in regions experiencing rapid urbanisation.<br><br>Through a combination of techniques, including drilling, hydraulic testing, water level, soil moisture and electrical conductivity time-series analysis, and determination of major ions, stable (δ18O &amp; δ2H) and radioactive (3H &amp; 14C) isotopes in groundwater, a new conceptualisation of recharge was developed, including estimation of rates in different areas and description of the major controls on recharge locations and mechanism(s) in the basin. This model challenges the previous conceptualisation of recharge, indicating that opportunities for recharge are limited in much of the urban development area. Sub-cropping basalt was found to be highly weathered to clay with low permeability, resulting in low recharge rates as indicated by chloride mass balance (5-50 mm/yr) and water table fluctuation-based estimates (&lt;5 to 20 mm/yr). Instead, key recharge areas (estimated rates &gt;100 mm/yr) are spatially restricted to a zone along the edge of the basin, where the basalt is absent and Quaternary sand deposits directly overlie the Tertiary sand aquifer. This conceptualisation is further supported by the spatial distribution of tritium in groundwater (ranging from below detection up to 2.40 TU, consistent with modern recharge) as well as vertical hydraulic gradients and soil moisture profiles.<br><br>The study further demonstrates that constructed wetlands used to manage urban stormwater act as an unintended source of new, focussed recharge in the development area. Dynamic relationships between filling/emptying of a constructed wetland and water level and EC in shallow adjacent groundwater were observed, indicating an oscillating fresh water plume migrating into the aquifer away from the wetland. Shallow groundwater quality changed significantly over 2.5 years of monitoring from high salinity (EC = 3,800 µS/cm), low 3H (&lt;0.03 TU) and pre-modern radiocarbon (75.5 pMC), to fresh (EC = 600 µS/cm), high 3H (2.29 TU) and modern radiocarbon (98.3 pMC), resembling surface water in the adjacent constructed wetland. At least 18 constructed wetlands have been built or are planned in the study area to manage stormwater flows. It is estimated that these wetlands may induce an additional 440 to 880 ML/yr of focussed recharge into the basin as urbanisation progresses.<br><br>The findings have significant management implications including (1) the importance of protecting the relatively small area where active recharge occurs, (2) the potential for constructed wetlands to provide additional recharge to the regional groundwater resource, if designs are modified to optimise this effect, and (3) with increasing recharge comes a risk of water logging, salinization, engineering failure and water quality impacts that need to be carefully evaluated and managed. The study highlights the value of collecting a variety of robust field data to substantiate and refine groundwater recharge models. The findings also provide valuable insights into hydrological processes occurring in areas experiencing rapid urbanisation above zones of potential or actual groundwater recharge, particularly where constructed wetlands are used as a stormwater management technique.<br>