Effect of urban development on solute transport in groundwater

[摘要] The Southern River catchment is one of the fastest developing areas of Perth. Local urban development is challenged by shallow groundwater and multiple wetlands, some of which are conservation category. It is expected that the future land use alteration may significantly affect the current hydrological cycle. As inundation and shallow watertables are incompatible with much urban infrastructure, it is expected that the urban development will include implementation of watertable control measures. A project funded by the Department of Water and CSIRO National Flagship “Water for a Healthy Country” aims to evaluate the likely impact of the changes in the land use on the water fluxes and the water quality within the Southern River catchment. This report examines the effects of urbanisation on solute transfer with groundwater to urban drainage. The adopted modelling is based on previously reported the Southern River catchment MODHMS model (Barr and Barron, 2009), which was also applied to the Wungong Urban Water (WUW) sub-catchment: (Barron and Barr, 2009). This previous work concentrated on the water levels and flows in the region under predevelopment and post development conditions. The current work extends the analysis to examine the fate of pre-existing (or legacy) solutes in the upper superficial aquifer and the addition of alternative solute sources associated with urban development.The main objectives of the undertaken research were:•to define the likely fate of the pre-existing or legacy solute pool within the upper superficial aquifer after urbanisation•to examine the accumulation and discharge of alternative solute sources associated with urban development such as roof runoff and irrigation throughflowThe model, based on the MODFLOW package, was used to examine the effect of land and water management variables and their combinations under three climate scenarios over a 10 years period including:•shallow groundwater table control measures using only subsurface drains or a combination of surface fill and subsurface drains•density of urban development, including high (R35), medium (R25) and low (R17.5) urban density (The R code refers to the number of residential blocks per hectare (ha), thus R25 has 25 residential blocks per ha)•effect of groundwater abstraction from the lower superficial aquifer for non-potable water use in the new development for irrigation of Public Open Space (POS) and domestic gardens and for indoor use for toilet flushing and hot water use in the laundryThe results of undertaken analysis indicate that the legacy solute stored in the shallow groundwater may influence water quality in subsurface drains for up to 9 years, depending on climate variability and adopted water and land management options. Within the constraints of the undertaken modelling it was indentified that flushing rates of the legacy solute from shallow groundwater are greater for the scenarios that are associated with a greater rate of the groundwater recharge. These are wetter climate and higher urban density. Groundwater abstraction has the most significant effect on the legacy solute fluxes to drains, which has a smaller rate for scenarios under higher groundwater abstraction. An increase in the downwards flux from shallow to deep superficial aquifer as the vertical hydraulic gradient increases during abstraction. These fluxes carry the legacy solutes resulting in depletion of the solutes mass in the top layer. Subsequently the solute concentration in drains is smaller when groundwater abstraction rate increases.The different watertable control options of “subsurface drains only” and “fill and subsurface drains” have a negligible long term impact on the solute balances. However within the first 1-2 years of urbanisation, the “subsurface drains only” option showed a greater rate of the legacy solute removal from shallow groundwater.The analysis also indicated that the effect of solutes introduced in new urban forms increases over 5-7 years before reaching equilibrium and the maximum concentrations. Two solute sources were considered: the solutes associated with groundwater recharge within irrigated/fertilised areas (named garden recharge) and with groundwater recharge from roof runoff. A constant concentration in these recharge sources was assumed. Accordingly the mass flux of solutes entering the shallow groundwater increases with the rate of the groundwater recharge and the equilibrium concentrations in drains is higher for scenarios with wetter climate and higher urban density. Deep groundwater abstraction reduces the accumulation of these solutes in shallow groundwater and leads to a lower concentration in the drainage water. The different watertable control options have a negligible impact on these solute balances.Combining the observed and previous reported ranges of the concentrations of the Chloride, Total Phosphorus and Total Nitrogen from different sources with the volumetric fluxes of each source, the evolution of the solute concentration in drainage discharge after urbanisation was evaluated.It was demonstrated that there is a transition in a prevailing source of solutes to the drainage discharge from the predominately legacy solute to the combined garden recharge and roof runoff solutes. The legacy solutes deplete rapidly within a few years and are displaced by the roof runoff and garden recharge solutes. The transition between the pre-existing and new sources can be well defined over time. The length of time to displace the legacy solutes depends on the relative concentrations in groundwater prior to urbanisation and the new urban sources. There are also seasonal changes in the solute concentrations of the drainage discharge, as the concentration increases during the summer dry period (and the volume of discharge decrease), and decreases during the winter period. This is in agreement with the observed data in the existing urban drains, such as Bayswater Main Drain or Mills Street Main Drain. The limitation of the undertaken analysis is related to limited opportunities for validation in terms of both water fluxes and solute transport. The model was a subset of the large urban area model. Limitations are also related to the adopted assumption of the future land cover within the model domain. Solutes were considered to be conservative tracers, so no reactive processes were included in solute transport analysis. As a result the model was used to explore the relative effects of water and land management options for new urban developments. The model outcomes are indicative of the maximum concentrations of the solute fluxes from the subsurface to the drainage network.Future work may include monitoring and therefore validation on these results in newly developing areas; consideration of temporal changes in concentrations of the solute sources; and an extension of the solute model to a reactive nutrient transport model, which allows simulation of nutrient attenuation, enrichment and speciation within the unsaturated, saturated and phreatic zones.

[发布机构] CSIRO

[学科分类] 地球科学(综合)