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Aquaculture as a Relinquishment Option for Final Voids

Open Cut » Environment

Published: May 03Project Number: C9027

Get ReportAuthor: Glen Whisson, Louis Evans | Curtin University of Technology

Relinquishment of final mining voids is a significant issue for coal mining companies. The cost of rehabilitating mine voids to land fill and re-establishment of pre-existing terrestrial habitats is prohibitive and not considered to be a viable commercial option by most coal mining companies.

Following closure, mine voids gradually fill through seepage from ground water sources and surface runoff to create mine lakes. Mine lakes represent a potential water resource for a range of community and commercial activities. This project aimed to establish a demonstration system to evaluate aquaculture using treated mine lake water as a beneficial end use for final mining voids.

Six aquaculture ponds, a water remediation system (settling pond, macrophyte pond, and compost pond) and an integrated water treatment system incorporating an anoxic limestone drain and an algal tank were constructed. Water was pumped from a submersible pump mounted on a pontoon anchored in the void through the algal tank, the anoxic limestone drain, the macrophyte pond and the compost pond before being stored in the settling pond and then passed through separate water intakes into each of the aquaculture ponds. Each pond had an outlet that led into a drain and delivered effluent water back into the void.

Two different treatment systems were evaluated - an anoxic limestone drain with as associated algal treatment tank (ALD system) and a fluidised limestone bed system (FLB system). Successful amelioration of pH from the mine lake water (pH 2.8 to 4.2) to pH>6.0 was initially achieved with the ALD system but the outlet pH levels subsequently declined. An alternative method of limestone treatment utilising a fluidised bed technology, was therefore explored. The results were encouraging and further prototypes were constructed and evaluated as part of a new research project funded through the Centre for Sustainable Mine Lakes.

A demonstration aquaculture project was conducted in the culture ponds to examine density effects on polyculture production. Marron and silver perch were stocked in three treatment groups: marron monoculture, low density polyculture (marron plus 100 caged perch), and medium density polyculture (marron plus 200 caged perch). Each treatment was allocated to two ponds and survival, growth and polyculture production were evaluated over 200 days. Pond yields were significantly higher in polyculture than monoculture, with net biomass increase highest in polyculture ponds with 200 perch/cage. While the need for research into higher, commercial densities is evident, the trial successfully demonstrated the potential of crayfish polyculture as a beneficial end use for coal mining lakes in Southern Western Australia.

A bio-economic model was developed containing three major components - the biological factors, technical components and economic factors. The results of the polyculture trial were used in developing the combined biological/technical component of the model. The base model showed that for the biological indices used in the polyculture trial, gross margin for a typical year in a 20 year period is negative (-$71,784). However, the second model constructed using commercial stocking rates gave negative gross margins for years 1 and 2 and a positive gross margin of $98,275 in year 3. A cash flow analysis conducted over a 20-year period at a discount rate of 7% showed an internal rate of return of 16.9% and a benefit cost ratio of 1.27. All these financial indices indicated a favourable outcome. However, further studies are required to validate the findings of this study in specific relationship to mine lake aquaculture.

Desktop studies of the feasibility of mine lake aquaculture at Moura and another site in the Hunter Valley were undertaken. The study was based on site visits and examination of existing water quality data provided by mining companies. Studies of reports on fish fauna in existing mine lakes indicated that some lakes would be suitable for commercial aquaculture and/or recreational fish stocking. The need for monitoring of seasonal changes in environmental conditions and for examination of water samples for the presence of heavy metals or other pollutants was highlighted.

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