Evolution of nutrient export under urban development in areas affected by shallow watertable

Abstract

Surface water quality in catchments undergoing urbanisation may be affected by the release of pre-existing (or legacy) solutes, such as nutrients, as well as new sources associated with urban land use. This paper examines both for a number of urbanisation scenarios and adopting the modelling capability developed for the analysis of urbanisation effects on catchment water balance. The flat relief of the study catchment and its sandy soils, in combination with a Mediterranean-type climate, lead to large rates of diffuse gross recharge and diffuse (evaporative) discharge with low overall runoff from the catchment (<1mm per unit area). Under these conditions solutes stored in shallow groundwater have long residence times (longer than 100 years). Urbanisation of such a catchment leads to significant changes in water regime, leading to a reduction in groundwater residence time and 'flushing' of legacy solutes towards the surface water network. Concurrently, urban development introduces new sources of solutes. It was found that the modelled concentrations of legacy solutes in the urban drains are greater than the water quality standards in the region; though, legacy solute concentrations reduce by 50% within the first 2-3 years and become less than 5% within 10 years for all urban scenarios. The full effect of new urban landscape on water quality was estimated to be longer than 5 years. Urban density and groundwater abstraction for irrigation of public open space and domestic garden have an effect on the surface water quality, as they influence the rate of legacy solute replacement and accumulation of the solute associated with the new urban forms. It was shown that water quality control measures in new urban developments should be directed to legacy nutrients during the first 2-3 years but measures reducing nutrient leachate from soil, such as soil amendments, should be considered for long-term solutions.