Water Journal : Water Journal August 2012
refereed paper groundwater management water AUGUST 2012 89 With reservoir inflows at record low levels, and the community under some of the most severe water restrictions (Stage 4) imposed throughout Victoria, Central Highlands Water examined other water supply options for the Maryborough system. Historically, a number of 19th century miners were defeated by groundwater inflows and this provided promising evidence of a resource of sufficient capacity to supply a relatively large city. GHD's hydrogeologists identified groundwater aquifer systems in the Maryborough district that had the potential to alleviate water supply stress in terms of volume, but at the same time maintaining a desired water quality. Accessing Groundwater The Victorian Department of Sustainability and Environment (DSE) has recognised areas of intensive groundwater use with the potential to be developed throughout Victoria. These areas have been defined as Groundwater Management Areas (GMAs) for resource management purposes. In the Maryborough region, the Upper Loddon and Mid Loddon Water Supply Protection Areas (WSPAs) had been proclaimed with water resources (surface and groundwater) managed by Goulburn-Murray Water. Licensed allocations had been capped over most of the aquifer systems deemed suitable for supply to the Maryborough system and, as such, before embarking on a groundwater development program, Central Highlands Water had to secure access to a supply by obtaining licence entitlements. Based upon the review of their existing water supplies, current levels of usage and predicted demands, Central Highlands Water was able to identify spare allocations within these management areas that could be transferred to sites closer to Maryborough. In addition, Central Highlands Water advertised in the local press and successfully negotiated transferable water entitlements from existing licensed groundwater users in the Upper Loddon WSPA. Investigation Program Owing to the urgency to drought-proof Maryborough, GHD undertook groundwater investigations at both Evansford (Stony Creek) and Moolort. These investigations included a number of phases of exploratory drilling, water sampling, production bore construction and pumping test programs. Key aspects of the investigations included: • Undertaking background hydrogeological reviews to identify sites with development potential. This included reviews of historical mining reports and boring records, and geological mapping. • Prioritising sites based on perceived water quality (and likely treatment requirements), but also delivery infrastructure requirements such as pipelines, power and pumping stations. GHD and Central Highlands Water worked closely to ensure that infrastructure approvals, contracts and construction were timed closely with groundwater development to minimise any delay in establishing the connection of the new supply to the delivery system. • Obtaining drilling, land access and environmental approvals. GHD's hydrogeologists and ecologists worked closely with the regulatory authorities of Goulburn-Murray Water and the Department of Sustainability and Environment to ensure that bore construction licenses were obtained promptly and sites approved in terms of native vegetation clearances. The priority targets for groundwater development were 'Deep Lead' aquifer systems. These are geologically young (lower to mid-Tertiary in age), unconsolidated sediments of the Calivil Formation. These sediments were laid down under fluviatile conditions as river channel deposits and consist of interbedded sand, gravel and minor clay. The leads do not form a laterally continuous sheet across the region and consist of a few tens of metres of sand and clay 'drift' overlying several metres of gravel material. They are restricted in extent, being deposited along channels typically incised into the Palaeozoic bedrock surface. The physical extent of these systems is extremely difficult to determine based on the surface topographic expression. The geometry and lithology of the Deep Leads have been primarily identified through drilling programs principally undertaken by government in the late 1800s to the early 1900s in an attempt to locate the floors of the lead valleys where alluvial gold accumulated in the gravel wash. The Deep Leads outcrop or sub-crop buried at shallow depth beneath thin Quaternary-age alluvium along the upper reaches of the highland valleys; however, elsewhere along much of their course they are buried beneath basalt flows of the Newer Volcanics. The interpreted alignment of the Moolort and Greenock Deep Leads, targeted at the Moolort and Stony Creek Borefields respectively, has been shown in Figure 1. At Moolort, the Deep Lead was targeted at the nearest location to Tullaroop Reservoir to facilitate rapid connection to the Maryborough potable supply. Water transmission pipeline (and power to operate the borefield) approvals were initiated early during the groundwater investigation program to avoid construction delays. GHD drilled a number of pilot exploratory bores to identify the thickness and lithology of the Deep Lead, in order to locate the optimum site for production bore/s installation. These bores were tested to confirm water quality before the larger-diameter production bores were constructed. The exploratory bores were subsequently converted into monitoring bores to support both the licensing of the supply (and pumping test investigations), but also to provide information during the borefield's operation. A simplified geological cross-section through the Moolort Borefield has been shown in Figure 2, which shows the Deep Lead sediments overlain by two main flows of Newer Volcanic basalt. At Stony Creek the Greenlock Deep Lead was targeted (see Figure 1) at a location close to the water pipeline between Talbot Reservoir and Maryborough. The geological profile at Stony Creek is similar to that at Moolort, with approximately 40m of Newer Volcanics overlying a thin section of the Greenock Deep Lead. Observation bore at Middle Creek. Installation of production bore casing.
Water Journal September 2012-1
Water Journal July 2012