Nutrient contribution to hyper-eutrophic wetlands in Perth, Western Australia

This thesis investigates nutrient contribution to six hyper-eutrophic lakes located within close proximity of each other on the Swan Coastal Plain and 20 kilometres south of the Perth Central Business District, Western Australia. The lakes are located within a mixed land use setting and are under the management of a number of state and local government departments and organisations. These are a number of other lakes on the Swan Coastal Plain for which the majority are less than 3 metres in depth and considered as an expression of the groundwater as their base is below the regional groundwater table throughout most of the year. The limited amount of water quality data available for these six lakes and the surface water and groundwater flowing into them has restricted a thorough understanding of the processes influencing the water quality of the lakes. Various private and public companies and organisations have undertaken studies on some of the individual wetlands and there is a wide difference in scientific opinion as to the major source of the nutrients to those wetlands. These previous studies failed to consider regional surface water and groundwater effects on the nutrient fluxes and they predominantly only investigated single wetland systems. This study attempts for the first time to investigate the regional contribution of nutrients to this system of wetlands existing on the Swan Coastal plain. As such, it also includes new research on the nutrient contribution to some of the remaining wetlands. The research findings indicate that the lake sediments represent a considerable store of nutrients (nitrogen and phosphorus). These sediments in turn control the nutrient status of the lake's water column. Surface water is found to contribute on an event-basis load of nutrients to the lakes whilst the groundwater surprisingly appears to contribute a comparatively low input of nutrients but governs the water depth. Analysis of the regional groundwater shows efficient denitrifying abilities as a result of denitrifying bacteria and the transport is localised. Management recommendations for the remediation of the social and environmental value of the lakes include treatment of the lake’s sediments via chemical bonding or atmospheric oxidation; utilising the regional groundwater’s denitrifying abilities to ‘treat’ the surface water via infiltration basins; and investigating the merits of managed or artificial aquifer recharge (MAR).

A new modelling approach to simulate preferential flow and transport in water repellent media: parameter sensitivity, and effects on crop growth and solute leaching

A modified version of the popular agrohydrological model SWAP has been used to evaluate modelling of soil water flow and crop growth at field situations in which water repellency causes preferential flow. The parameter sensitivity in such situations has been studied. Three options to model soil water flow within SWAP are described and compared: uniform flow, the classical mobile-immobile concept, and a recent concept accounting for the dynamics of finger development resulting from unstable infiltration. Data collected from a severely water-repellent affected soil located in Australia were used to compare and evaluate the usefulness of the modelling options for the agricultural management of such soils.

The study shows that an assumption of uniform flow in a water-repellent soil profile leads to an underestimation of groundwater recharge and an overestimation of plant transpiration and crop production. The new concept of modelling taking finger dynamics into account provides greater flexibility and can more accurately model the observed effects of preferential flow compared with the classical mobile–immobile concept. The parameter analysis indicates that the most important factor defining the presence and extremity of preferential flow is the critical soil water content.

Comparison of the modelling results with the Australian field data showed that without the use of a preferential flow module, the effects of the clay amendments to the soil were insufficiently reproduced in the dry matter production results. This means that the physical characteristics of the soil alone are not sufficient to explain the measured increase in production on clay amended soils. However, modelling with the module accounting for finger dynamics indicated that the preferential flow in water repellent soils that had not been treated with clay caused water stress for the crops, which would explain the decrease in production.

Appraisal of random and systematic land cover transitions for regional water balance and revegetation strategies

This study describes the use of landscape transition analysis as a means for effective basin management. Land cover transitions from 1995 to 2002 were analyzed using a cross-tabulation matrix for an important economic zone in south-west Victoria, Australia. Specifically, the matrix was used to determine whether the transitions were random or systematic. Random landscape transitions occur when a land cover replaces other land covers in proportion to their availability. Systematic landscape transitions occur when there are deviations from random patterns, and land use types ‘target’ other land use types for replacement. The analysis was conducted with 11 land cover categories and showed that dryland pastures have been systematically losing area to dryland crops and blue gum (Eucalyptus globulus) plantations. Dryland crops have systematically expanded in the north-east of the catchment, an area where increasing in-stream salinization has occurred concurrently with this transition. The systematic expansion of the blue gum plantations has been predominantly in the south-west of the catchment and has the potential to reduce stream flows and groundwater recharge in an already water-stressed region, as blue gums use more water than the dryland pastures they are replacing. All other transitions were largely random. These findings have implications for land use planning in the study area for regional water balance and revegetation strategies. Landscape transition analysis is a cost-effective means of contributing to the management of water resources at a regional scale, and is highly recommended for future basin planning.

Geochemical changes during recharge with tertiary-treated wastewater at a coastal sandfill

Results of experiments investigating geochemical changes during artificial recharge of treated wastewater at a coastal sandfill, reclaimed with sand dredged from the seabed, are reported in this paper. Laboratory batch experiments were conducted using secondary effluent (SE) and SE treated with an additional ultrafiltration process (UF), and wastewater treated by reverse osmosis (RO) process, mixed with surface sand obtained from the sandfill. Experiments with RO showed a net increase of 0.41 meq/L, 0.12 meq/L and 0.31 meq/L for Ca(2 + ), Mg(2 + ) and HCO(3) (-), respectively. UF and SE also exhibited net increase in Ca(2 + ), Mg(2 + ) and HCO(3) (-) indicating carbonate mineral dissolution. All three waters were found to be over-saturated with respect to calcite. Carbonate dissolution reactions were observed in the field experiments. However, the presence of imported clays from the borrow source gave rise to ion exchange reactions where Na(+) attached to the clay particles were exchanged for Ca(2 + ) and Mg(2 + ) inducing mineral dissolution, driven by sub-saturation conditions. This resulted in an increase in pH with maximum values in excess of 9.0. It was also found that the sodium adsorption ratio remained high (>10) even after the groundwater had been diluted sufficiently to freshwater levels (ionic strength, I <0.015) indicating a potential for the dispersion of clay particles. This could have a deleterious consequence on porosity and hydraulic conductivity.

DOC and UVA attenuation with soil aquifer treatment in the saturated zone of an artificial coastal sandfill

Results from a direct recharge experiment conducted in the field to investigate DOC and UVA(254) attenuation rates during the direct injection of UF treated wastewater into a artificial coastal sandfill are presented in this paper. Approximately 500 m(3) of ultra-filtered wastewater was injected into the saturated zone, over a period of 9 days. The movement of the plume was tracked over 80 days, during which time samples were obtained from multilevel samplers installed in transects across the drift axis of the plume. An analysis of fluorescein in the samples obtained during the drift of the UF plume showed that DOC and UVA were attenuated beyond rates predicted by conservative mixing, by up to a maximum of 45%. A degradation coefficient of 0.0175 day(-1) was found to be applicable for DOC degradation. After a drift period of 80 days, DOC and UVA reduced to approximately 4.5 mg/l and 0.100 cm(-1), respectively, from initial values of 8.06 mg/l and 0.199 cm(-1).