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

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.

Identification of phosphorus export from low-runoff yielding areas using combined application of high frequency water quality data and MODHMS modelling.

In basins combining flat-sandy valleys and hilly-bedrock sub-catchments, the assessment of nutrient (phosphorus) exports from low-runoff yielding environments is difficult. To overcome this issue hydrological modelling and high frequency phosphorus measurements were simultaneously employed. A coupled surface water-groundwater interaction model (MODHMS) was used to determine runoff from the low-runoff yielding part of the catchment. The modelling results indicated that the lower catchment contributed less than 10% of annual catchment discharge over a number of weeks during mid-winter. High frequency phosphorus (P) measurements showed a threefold increase in P concentration during this period in 2008, which lasted for 3 weeks. Concentration-discharge analysis suggested that the increase in P concentration was associated with runoff generation processes in the low-runoff yielding sub-catchment. It was estimated that this area contributed 32% of the annual P load though only 2% of total annual discharge in 2008. Both runoff and P contributions occurred during the period when the water table rose to the surface causing inundation. It was shown that the P concentrations in discharge from the low-runoff yielding sub-catchment were similar to those observed in the shallow groundwater layers.