How does groundwater affect civil engineering construction

Underpinning the feasibility assessment, design and construction phases for a range of engineering infrastructure works requires a sound knowledge and understanding of how hydrogeology (groundwater) can be impacted and managed. With prolonged drought, emerging climate change impacts and population growth, increased demand for groundwater to augment water supply and greater recognition of the groundwater value presents new challenges and requirements for the engineering sector.

Built engineering works will temporarily or permanently change in-situ or surrounding groundwater conditions during construction and post commissioning. Groundwater level decline due to open or underground excavation works can cause land subsidence impacts to adjacent or further afield built environments. Falls in groundwater levels affect other stakeholders dependent on this water source by altering natural processes (reduced stream base flows) or engineered methods of abstraction (yield decline for stock or domestic water supply bores).

Rising groundwater level impact on built infrastructure needs to be understood in terms of active pressures or chemical deterioration processes (salt attack) and managed accordingly. Similarly, groundwater inflow estimates for site water management arrangement and post construction structure stability should be determined.

Both horizontal and vertical movement of groundwater during constructing civil works can mobilise polluted groundwater sources or change groundwater (aquifer) geochemistry with resultant water quality impacts for beneficial users over variable temporal and/or spatial scales.

From a groundwater water supply perspective, to design and construct the extractive and distribution engineering works, knowledge about the sustainable aquifer yield of the target groundwater resource to be exploited is necessary. In addition, groundwater chemistry should be ascertained for any necessary treatment as part of a sole or augmented supply source for a water treatment plant. Identifying and appraising the effectiveness of engineering options to manage groundwater disposal during construction activities or within the operation of commissioned works to meet statutory environment discharge requirements is dependent on knowledge of the chemical composition of groundwater.

There are many relevant examples of areas where groundwater management requires particular attention in terms of implementing engineering works for urban development infrastructure, augmenting potable water supply (demand management and drought response purposes) and for the health and wellbeing of the industry. These projects demonstrate the increasing importance of how engineering works need to be designed and operated, conscious of the interdependency groundwater has in the biophysical and hydrological cycles of our environment.

Urban development infrastructure - groundwater implications for engineering works

The suburb of Pakenham in south-east Melbourne is located in a region historically affected by water logging and land salinisation processes that have restricted land use and development. Significant population growth over the past decade, which is projected to continue for Melbourne, has subsequently increased pressure to fully utilise remaining land parcels in urban growth corridors for residential and associated development. The Pakenham area, located in the Cardinia Shire Council, is one of these key growth areas where significant investment is occurring; in particular, on land prone to the effects of shallow ‘brackish’ groundwater levels.

Land developers in these areas must fully appreciate both direct and indirect groundwater impacts before identifying and investing in engineering options that can adequately protect residential developments. The first step involves identifying, characterising and distinguishing the relative contribution that local and/or regional groundwater systems have on existing or potential salinity and water logging processes. We must then gauge the effectiveness of a range of engineering options that may be adopted to control groundwater levels and suppress salinity impacts on subsurface infrastructure. The sustainability implications in implementing various options should then be considered, in terms of construction cost, post construction operation and maintenance costs, estimated annual capital replacement costs and wider environmental issues.

Ranking each engineering option in terms of overall effectiveness in groundwater level control and associated ongoing cost then occurs, including consideration of establishment costs, monitoring and management requirements for the council/water authority, and capital works infrastructure refurbishment/replacement frequency. From this process, the overall preferred option is selected - the most effective at imparting salinity control, resulting in the lowest economic cost to the community, and that is environmentally sustainable. Representation of salinity impacts and options for management in this manner can streamline statuary planning approval requirements and residential development investment business decision making.

Anglesea Borefield Project: groundwater resource development for urban supply

The Anglesea Borefield Project (ABP) is expected to provide an average of 7000 ML/year of groundwater to the greater Geelong region, Victoria’s second largest city, with a current population of 275,000. The ABP was required to meet future demand and address the challenges of climate change over the next 50 years. The project also supported Barwon Water’s strategy of a diversified supply system that utilised surface water, groundwater, recycled water and conservation initiatives.

The Anglesea borefield system generally consists of seven bores, a pipeline to collect the extracted groundwater and transfer it to a pre-treatment plant, a pump station to transfer the pre-treated water via a pipeline to Geelong’s main water storage and a significant amount of monitoring infrastructure.

A Bulk Entitlement (BE), granted by the Minister for Water in June 2009, authorises Barwon Water to extract 35,000 megalitres of groundwater from the Lower Eastern View Formation aquifer over any five-year period. Extraction limits of 40 megalitres in any one day and 10,000 megalitres in any one year must not be exceeded in the borefield’s operation.

The BE requires it to have an approved Monitoring and Assessment Program (MAP). The purpose of the program is to provide data and information about the long-term sustainability of groundwater resources in the area and to protect the environmental values and health of groundwater dependent ecosystems. The aquifer system is located in an area that contains ecosystems of environmental significance and includes the Anglesea estuary, swamplands in the lower parts of the Anglesea River and Salt Creek catchments, the upper Anglesea River and Breakfast Creek.

A numerical model was constructed and is utilised to assess potential effects of groundwater drawdown with extracting 7000 ML/year. The extent of potential groundwater drawdown processes on: neighbouring users; base flow to swamplands, creeks and rivers; inter-aquifer flows; saline intrusion and subsidence is appraised with the model. The modelled groundwater processes provided input into the Environment Effects Statement (EES), the Environment Protection and Biodiversity Conservation (EPBC) referral and the BE, and information to commence feasibility and concept designs for the anticipated water treatment and supply works. Production and observation bore installation and pumping tests were also undertaken to assess the bore yields, provide aquifer parameters and data for calibration of the numerical model.

Barwon Water monitors the groundwater level in 38 monitoring bores, surface water flow and quality at six sites, frog populations at eight sites, fish populations at 11 sites, vegetation composition and abundance at eight sites, hydraulic regime of six swamplands and wetland sites, and subsidence at 30 sites as part of the MAP.

Groundwater extraction commenced on 5 October 2009, from one bore, with the remaining available bores progressively tested and commissioned as they were completed. Water from the project was first pumped to Geelong’s main water storage in November 2009. The commissioning of the project contributed to the decision made in February 2010 by the Barwon Water Board to relax water restrictions from Stage 4 to Stage 3. The Anglesea Borefield Project has provided the Geelong region a new water resource at a time when surface water resources have become less reliable because of climate change. Extensive consultation and investigations proved invaluable to the project’s success. The comprehensive groundwater monitoring program will ensure the environment is protected. The project is one of several projects delivered by Barwon Water to secure the long-term water supply of the Geelong region and complements a diversified system that combines surface water, groundwater, recycling and conservation.

Groundwater augmentation for Maryborough Water Supply

The year-to-year drought conditions in Victoria that prevailed throughout most of the 2000s have had significant impact on the water resources in Victoria’s Central Highlands region. After more than 10 years of below average rainfall, pronounced effects on the security of supply to regional towns, historically reliant on surface water systems, have manifested. This has cumulated in widespread water restrictions, a deterioration in water quality and, in some cases, community distress and pressure (social and political) on the responsible water supply authority.

Central Highlands Water, the authority responsible for providing safe and secure water supply to urban populations in central Victoria, embarked on major groundwater investigations to secure supply for impacted communities. The authority faced significant challenges within Maryborough in terms of selecting appropriate initial water supply options to progress, negotiating commercial contracts for transferable groundwater licences, licensing approvals, and managing the water supply system while water supply and treatment engineering works were investigated and infrastructure construction was required to be implemented in parallel.

A selection of sites was hydrogeologically reviewed for Maryborough (including nearby Moolort, Carisbrook and Evansford regions) to narrow down areas to target more explorative hydrogeological investigations which included extensive drilling and production bore construction, sampling and pumping test programs to confirm the aquifer yield, pumping response and groundwater quality.

An assessment of extraction impacts on neighbouring groundwater users and the environment was undertaken to satisfy the Victoria Guidelines for licensing urban authorities. While understanding the requirement for provision of essential urban supplies, Goulburn-Murray Water also had to ensure that the rights of existing users and environmental values of the system were not compromised. Consistent with ministerial guidelines, and in consultation with Goulburn-Murray Water, the authority developed monitoring and reporting programs to be established to assess the impacts of extraction over time. In addition, through-flow analysis was undertaken to understand the impacts down gradient in the aquifer system, particularly under the influence of prolonged drought conditions and implication for the water authority’s water resource management requirements.

Rye Geothermal Hot Springs - waste stream disposal considerations

Victoria’s first natural hot springs spa centre opened in 2005. Natural hot mineral waters flow into thermal pools and private baths offering both relaxation and rejuvenation to users. The hot mineral spring waters flow from an aquifer 637 m below the surface. The geothermal heated water (which varies from 37 to 43°C) rises under its own pressure through a groundwater extraction bore to within 10 metres of the surface. The water contains a range of naturally occurring minerals including sulfur, calcium, magnesium and potassium, amongst others.

The extracted groundwater salinity is too high for the spa’s waste stream to be used for irrigation or many domestic functions. Disposal of spa water back into the aquifer that provides the geothermal waters is currently being assessed. However, disposal water is of significantly lower temperature than ambient aquifer temperature and concern over cooling of the extracted groundwater via this option is being assessed.

To successfully achieve waste stream re-injection requires succession of several technical uncertainties, principally through innovative injection bore and pre-injection treatment system design. Technical advice on the feasibility of the re-injection scheme, achieved by conducting chemical analysis and geochemical modelling of the extracted water, and geothermal modelling of the aquifer has been recently undertaken. Geothermal modelling indicated that re-injection of disposal water would significantly cool the resource in the medium term. However, adaptive engineering measures have been proposed that will provide a suitable re-injection bore design, using the results of the geochemical and geothermal modelling.

Through adaptive design, engineering and operation, consultants have the opportunity to steer water authorities and stakeholders towards valuable awareness of the interdependency groundwater has in the biophysical and hydrological cycles of our environment.

*Michael Mozina - Water Resources Manager for GHD - discusses the significance of management of groundwater in engineering works, water supply and natural resource management. With prolonged drought and emerging climate change impacts, Michael observes that the need for groundwater resources and their management is ongoing and increasingly pressing.