Managed Artificial Recharge in Small Urbanising Catchment

Abstract

Aquifers are natural water reservoirs and one of the most import sources of potable water in urban areas. In recent years, the world-wide urban population has doubled, and it is estimated that by mid-21st century more than half of humankind will live in urban areas. A consequence of population growth in urban areas is increase in water demand and thus an increase in groundwater abstraction rates. Furthermore, urbanisation reduces natural recharge due to the increase in impervious surfaces. These two factors result in aquifer over-exploitation and therefore depletion of groundwater availability. In addition, urbanisation increases stormwater runoff and thus the risk of stormwater flooding. Previous studies on artificial groundwater recharge have mainly focused on artificial groundwater recharge in rural areas. This PhD study explores sustainable stormwater management and its relationship with urban growth, using the principles of low impact development (LID). This study, for the first time, explores possibility of using treated excess stormwater to enhance natural recharge, using deep-well managed artificial recharge (MAR), as a method of mitigating the negative effects of urbanization. The aim is to maintain catchment's pre-development hydrology (hydrologic neutrality) during urbanisation. Investigation of the effects of urbanisation on the catchment and proposed methods to mitigate these effects is accomplished with the use of numerical models. The study explores the level of complexity of the numerical model required to adequately assess the natural state of the groundwater system, regional impacts of the reduced natural recharge and the effects of AR. It is concluded that a simple model is adequate for investigating regional effects. Furthermore, the performance was assessed of two different numerical methods applied to a small complex urbanising catchment. The results of the study confirmed that software (FEFLOW) that uses the finite element method (FEM) is better for a representing complex catchment than the software (MODFLOW) that uses the finite difference method (FDM). The first step in investigating the effects of MAR is to assess the pre-developed state of the groundwater system. Secondly, the effects of reduced recharge due to urbanisation are assessed, providing a baseline for determining the effects of MAR. Lastly, the effects of different MAR scenarios are investigated. The results showed significant increase in groundwater table rise in areas of high abstraction rates, confirming that AR can be used as a method for mitigating the adverse effects of urbanisation. Finally, a framework for artificial recharge in urban areas using treated stormwater is proposed.