Preliminary hydrological, environmental and economic assessments of the ACT Salt Management and Environmental Improvement Project

[摘要] EXECUTIVE SUMMARY: This report details the results and recommendations from a study to inform of the benefits and impacts of a proposal by the ACT Government to divert water within the upper Murrumbidgee catchment for environmental purposes. The study includes hydrological modelling, environmental risk assessment and economic assessment as the primary methods. The proposed diversion involves transferring water from Tantangara Dam, situated at the headwaters of the Murrumbidgee River in NSW, to Googong Dam, one of Canberra’s main water supply storages located in the adjacent Queanbeyan-Molonglo River catchments. The proposed flow path uses the stream network along with a new pumping station and pipeline from Angle Crossing on the Murrumbidgee River, to Burra Creek. The primary objective of the proposed transfer is to increase flows in the lower Molonglo River to dilute salt concentrations to within licensed limits at/below the Lower Molonglo Water Quality Control Centre (LMWQCC). The secondary objective is to improve the environmental health of the sections of river affected, through increased low flows, improved seasonality of flows and provision of flushing flows where possible. The main focus of this report is an analysis of four flow scenarios defined by annual volumetric releases from Tantangara Dam of: 10 GL, 18 GL, 25 GL and 13.5 GL. For the latter scenario, 3.5 GL a-1 is included to replace poorer quality outflows from the Queanbeyan Sewage Treatment Plant (QSTP) which discharges into the Molonglo River. The analysis considers the hydrological and ecological benefits and impacts and, economic costs of each scenario with respect to the objectives of the proposed diversion. Additionally, reverse osmosis of treated effluent from the QSTP and LMWQCC, along other alternatives to address the salinity problem are also considered. The preliminary hydrological, ecological and economic assessments presented here showed clear differences in recommendations between the scenarios examined. This was largely the result of the differences in focus and perception of desirable outcomes. The differences in recommendations suggest that trade-offs are an inevitable component of decision making on this issue. In spite of this, the assessments all provide support in favour of further consideration of the proposal to transfer water from Tantangara Dam to Googong Dam for the purposes of salt dilution and improvements in environmental health. Based on the preliminary assessments, we recommend that the 18 GL a-1 and 13.5 GL a-1 scenarios be given further consideration. These scenarios appear to provide the best possible combination of outcomes relating to the achievement of salt dilution targets in the lower Molonglo River, improvement in ecological and environmental health in streams affected by the transfer, improvements in flow regimes towards natural (without development conditions), and cost effectiveness taking into consideration purchase, construction and operational costs associated with the water transfer. Estimated costs per annum are upwards from $5.7 million. The 10 GL a-1 and 25 GL a-1 scenarios, as well as the reverse osmosis options, provide lesser hydrological or ecological benefits or are less cost effective but may warrant further consideration if a greater level of risk or trade-off is accepted. More detailed investigation is warranted on the effects of the proposed transfer on Burra Creek, specifically relating to substantial increases in flow and concerns for channel stability and ecosystem decline. Additionally, alternatives that directly address the problems of high salinity outputs from STPs and declining river health should also be explored.

[发布机构] CSIRO

[学科分类] 地球科学(综合)