William Glamore

The effects of tidal buffering on acid sulphate soil environments in coastal areas of New South Wales

One-way floodgates installed on flood mitigation drains in regions affected by acid sulphate soils restrict carbonate/bicarbonate buffering, thereby creating reservoirs of acid water (pH < 4.5) that discharge during the ebb tide. The drain water quality and hydrodynamic conditions prior to and following floodgate modifications that allowed for controlled saline intrusion are described with reference to data collected from intensively drained and floodgated coastal lowlands located in southeastern New South Wales, Australia. Cl:SO4 ratios taken from groundwater samples depicted an acidic environment with little soil buffering capacity. Prior to modification, water quality upstream of the one-way floodgate was consistently acidic (average pH 4.6) with high concentrations of aluminum and iron that fluctuated with precipitation. Over a two-week period before modifications, floodgate leakage permitted alkaline water to intrude upstream of the floodgate and react with H+ ions. This period showed the strongest supporting field evidence for tidal buffering via modified floodgates. After installing vertical lifting, two-way floodgates average drain water pH increased to 5.89 and aluminum and iron concentrations decreased by more than 30%. A large rainfall (131.8 mm) during the post-modification period caused acidic groundwater flushing, however, in comparison to the pre-modification period, recovery time and average pH were markedly improved. Preliminary investigations of groundwater salinity in response to tidal intrusion has shown that electrical conductivity fluctuates with rainfall and it is predominately limited to 10 m perpendicular to the drain.

Acid sulphate soil remediation techniques on the Shoalhaven River Floodplain, Australia

A commonly used flood mitigation technique in coastal areas of Australia during the late 1960s was the installation of one-way floodgates on flood mitigation drains. In regions affected by acid sulphate soils (the oxidation of pyrite in the soil forms sulphuric acid), the floodgates prevent tidal carbonate/bicarbonate buffering of the drains and thereby create reservoirs of acidic water (pH < 4.5) that discharge during low tide. Several acid sulphate soil remediation techniques have been used in coastal lowland in southeastern NSW, Australia. Following extensive monitoring and finite element modelling of groundwater conditions and quality, fixed level V-notch weirs were installed at three elevations to maintain elevated groundwater levels. The weirs successfully maintained the groundwater level above the acid sulphate soils, preventing additional pyrite oxidation, and reduced the rate of discharge of acid to the drain. Following further monitoring, investigation into anaerobic acid sources, and finite element-based geochemical modelling, modified two-way floodgates that allow tidal ingress were installed. The modified floodgates were successful in buffering the drain water pH before discharging the drain water into adjacent waterways. Numerical analysis based on finite element modelling was extended to illustrate that saline intrusion into the surrounding soil (as a result of tidal ingress and acid buffering in the drains) was not a major concern for the pastureland or other agricultural activities.

A two-stage decision support tool for restoring tidal flows to flood mitigation drains affected by acid sulfate soil: case study of Broughton Creek floodplain, New South Wales, Australia

A 2-stage flood estimation and water quality decision support tool (DST) was developed, calibrated, and applied to a field site in south-eastern New South Wales (NSW) to simulate tidal restoration in a flood mitigation drain affected by acid sulfate soils leachate. The first stage of the DST employs a digital terrain map, geographic information tools, and measured water levels to calculate drain water overtopping due to tidal variations. Simulations using the GIS technique at the study site indicated that the primary drainage network can safely contain full tidal flushing (0.91 m AHD or a 58% increase), whereas at the same level the secondary drainage network overtops along relict drainage channels. The second stage of the DST simulates the change in drain water quality using an ion-specific program code written within the open interface PHREEQC program. The results from the water quality model were calibrated against laboratory titration tests. Drain water pH was shown to increase above 6.0, and soluble aluminium and iron concentrations decreased by 73% and 56%, respectively. The extent of water quality change is directly related to the ionic strength of the intruding water and the ion-specific reaction kinetics of aluminium, iron, and sulfate. Based on the DST simulations, floodgate modifications to restore tidal flushing were successfully undertaken at a study site near Berry, in south-eastern NSW. DST predictions accurately simulated field results (±10%) and slight variations between data were attributed to the prolonged drought at the field site, which increased the ionic strength of intruding waters, and the high concentration of sulfate in seawater. The DST can be easily adapted to other sites throughout Australia.

Optimising subsurface well design for coastal desalination water harvesting.

Subsurface techniques which make use of a natural sand filter are often a preferred means of extracting saline water in coastal environments, and beach-well systems (both vertical wells and radial lateral wells) have been successfully used with minimum pre-treatment for large desalination plants in Israel, Malta, Saudi Arabia and numerous islands. Through a case study, this paper details the limitations of standard vertical wells in coastal aquifers and provides the general design considerations for radial lateral wells with reference to international best-practice. A feasibility assessment for a proposed desalination source site on the Central Coast of New South Wales in eastern Australia is described. Further work, including intensive monitoring and 3D numerical flow modelling, is in progress for the purpose of detailed design.

Tidal forcing groundwater dynamics in a restored coastal wetland: implications of saline intrusion

Tidal restoration projects are currently being undertaken throughout coastal Australia without a full understanding of the influence of tidal forcing on groundwater salinity. To determine the impact of restoring tidal flows on groundwater salinity levels, field investigations were undertaken at a study site near Berry, New South Wales. Fluctuations in groundwater and surface water chemistry (soluble chloride, pH, electrical conductivity) and hydrodynamics were measured over a 12 week period using multilevel piezometers and submersible data loggers spaced at discrete distances from a flood mitigation drain. Additional parameters, including saturated hydraulic conductivity (Ksat), were undertaken to determine baseline conditions and to provide initialisation data for a three-dimensional finite-element model. The finite-element model was calibrated within ±5% of field data and developed to simulate saline intrusion during a ‘worse-case’ scenario. Results from the model simulations indicate that saline intrusion at the site is limited even under extreme conditions. To quantify the influence of Ksat(h) levels on saline intrusion, material properties in the numerical model were altered to represent 1, 10 and 20 m/d. Model simulations showed that Ksat(h) values >10 m/d permitted saline intrusion in excess of ANZECC criteria. Based on the model findings a series of management criteria are proposed that detail acid sulfate soil remediation techniques, including tidal restoration, dependent on Ksat(h) values. The proposed management criteria suggest that tidal restoration projects are most suited in sites where the Ksat(h) values are <10 m/d.

APPLICATION OF TMDL AND RISK ASSESSMENT PRINCIPLES FOR PATHOGEN MANAGEMENT AT AN URBAN RECREATIONAL LAKE

Lake Parramatta was a disused water storage located at the bottom of an 8 km2 urbanised watershed in Sydney, Australia in which primary contact recreation had been actively discouraged, because of the perceived risk of infection from waterborne pathogens. However, a recent survey of the Lake showed that safe swimming might be possible during summer dry weather periods, providing high contamination periods (during rain events) could be identified and avoided. To this end we characterized inflow/lake water quality (bacterial indicators, fecal sterols, Cryptosporidium, viruses & turbidity) and hydrology (rainfall, streamflow & Lake depth measurements) to quantify the cycle of lake contamination and water quality recovery. Fecal contamination became noticeable for rainfall inputs > 10 mm in 25 MJ.m-2.d-1, with 10% visible radiation reaching 1 m). None of a range of enteric viruses was detected in the Lake immediately after high stormwater inputs. Fecal sterol analysis indicated that sewage only represented some 0.1% of run-off into the Lake. This data was used to develop a water safety management plan for efficient and timely recreator protection from pathogen-related events, and the Lake was reopened in December, 2006.

Incorporating Innovative Engineering Solutions into Tidal Restoration Studies

Restoring tidal flows to salt marsh wetlands is typically undertaken for environmental, social, or compensatory (i.e., compensating for loss elsewhere) purposes. In most circumstances the objective of the project is to create or restore an environmental setting that historically existed on-site or nearby. During the process of restoring the tidal environment, significant attention is given to the desired environmental outcomes with specific interest on the notable flora and fauna. To achieve these end goals attention has become increasingly focused on the importance of reestablishing a desired hydrologic and/or hydrodynamic regime. The process of restoring the site’s hydrology, however, is often complicated by a range of factors, and, in many cases, it is not possible to restore the site back to some preexisting “natural” state. In these circumstances, engineering solutions are required to achieve the desired hydrologic and/or hydrodynamic outcome.

Salinity intrusion in coastal flood mitigation drains and creeks

This paper reports on mathematical model developed to simulate salinity intrusion in coastal waterways in the south eastern coastlands of the Gold Coast (Australia) to neutralize acidity caused by pyrite oxidation. The model was used to predict the effect of salinity intrusion upstream of existing floodgates. Predictions of the model compared favourably with the field results. Instrusion of saline water can increase the pH of the upland water from 2.4 to 6.5, while the salt levels remain low enough so that it is not of concern to the landholders. Based on the computer predictions and experimental observations, it was concluded that the Nerang river flood mitigation system ought to be redesigned to allow for controlled tidal water intrusion upland of the floodgates to neutralize acid leached from groundwater. Such a structure would have to be constructed and installed in the creeks replacing existing one way floodgates thereby allowing the flow of saline water upland during inland tidal periods. In this manner, the saline creek water can neutralize much of the acidic water stored upland of the floodgates thereby reducing the ill-effect on fish and other estuarine communities and their habitats downstream following heavy rainfalls. Dear Colleague, All papers published in the Wessex Institute of Technology conference proceedings are subject to peer review. This review process is carried out by the conference chairmen and members of the International Scientific Advisory Committee (ISAC). The names of the ISAC are published in the front of each conference book and the names and affiliations of the committee for each conference are published on our website, www.wessex.ac.uk. The review process takes place at the abstract stage and again at the final paper stage. At either stage the authors may be required to alter his/her paper in accordance with the reviewers comments and any alterations are also subject to review. Coverage of WIT conference proceedings appears regularly in a number of publications, including Applied Mechanics Reviews (AMR); Engineering Index and related publications from Elsevier Engineering Information Inc; INSPEC (IEE); Mathematical Reviews: Scitech Book News; ISIs Index to Scientific and Technical Proceedings, Index of Scientific Book Contents, Index to Social Science and Humanities Proceedings and Current Contents; and Various Cambridge Scientific Abstracts publications. Proceedings are also listed in the Directory of Published Proceedings. Kind regards Rachel Rachel Green Senior Conference Co-ordinator Email: rgreen@wessex.ac.uk Tel: +44 (0) 238 029 3223 Fax: +44 (0) 238 029 2853 ---------------------------- WESSEX INSTITUTE on the Web - http://www.wessex.ac.uk See our conference programme at: http://www.wessex.ac.uk/conferences/2005/index.html