Jones, D.R., and Townley, L.R. (1994), Hydrological and water quality consequences of closure of the tailings dam at the Lucky Draw Mine, Burraga, NSW, Report to Renison Goldfields Consolidated, CSIRO Minesite Rehabilitation Research Program, 35pp., January 1994. [Confidential]

Concerns have been expressed about the possible future impacts of the decommissioned Lucky Draw tailings dam on the quality of nearby surface water and groundwater. In particular it has been suggested that a water table containing elevated levels of cyanide could be expressed at the surface of the tailings, and that strategic planting of trees might be essential to prevent this occurrence. The issue of constructing a stilling pond with an overflow structure on the surface of the final landform has also been raised.

After reviewing the historical records of the construction, operation, and water quality monitoring of the dam and its surrounds, inspecting the site, and leach testing core samples of surficial tailings, we have reached the conclusion that water quality is unlikely to be a problem. Rainfall on the surface of the tailings will be removed primarily by evapotranspiration and surface runoff. The volume of seepage through the tailings is very small relative to other components of the water balance, and seepage from the base of the dam wall is estimated to be of the order of 2 m 3 d-l. It is likely that such a small volume will be readily evaporated and/or transpired by vegetation at the base of the dam wall. Any cyanide in the seepage will therefore be contained and degraded at the point of seepage and will not be transported into nearby streams.

The concentration of cyanide likely to be present in surface runoff resulting from a 1 in 100-year storm event is calculated to be only twice the Schedule 2 Clean Water Act limit of 0.05 mg/L assuming that all of the tailings surface are uncovered and susceptible to scouring. In reality, the surface of the tailings will be covered with soil and vegetation. Thus the quality of storm runoff should not be of concern.

There is no physical mechanism whereby contaminated pore water from deep within the tailings mass could be induced to migrate to the surface of the tailings, either during or after significant rainfall events. Consequently, it is considered that there is no especially preferred location for the planting of trees to maximise lowering of the water table. However, trees should not be planted too close to the dam wall in order to reduce the risk of roots penetrating the clay lining.

The major role of a stilling pond on the surface of the tailings mass would be to act as a sediment trap. Its secondary role would be to allow time for the already low levels of iron cyanide to be further degraded by sunlight and biological activity before discharge into the channel leading down to the tailings embankment runoff dam. In order to maximise the rate of photodegradation the pond should be less than 1 m deep. The cyanide decay model developed in Appendix A1 is based on published data for the rate of photolysis of iron cyanides in pure solution, and assumes that volatilisation is the sole mechanism for loss of cyanide from the water column. The extent of enhancement (over that predicted by the model) of cyanide degradation by site¡ªspecific biotic and abiotic processes could be determined by field or laboratory¡ªbased studies with water and soil from the tailings dam.

The horizontal dimensions of the stilling pond are not critical and are almost an issue of choice. The elevation of the lowest point between the pond and the overflow channel will determine the maximum area of the pond. A maximum area between 0.5 and 1 ha would be reasonable given the rainfall regime in the area. However, it should be borne in mind that a water body of this size could still dry up at times during the year when evaporation exceeds precipitation.

The planting of suitable trees around the periphery of the stilling pond could greatly accelerate the loss of water from the ponded area via evapotranspiration. This would facilitate drawdown of the pond between rainfall events and maximise the availability of storage capacity.

There is a possibility that the causeway on the dam provides a preferred hydraulic connection between the pond currently present on the surface of the dam, and the base of the decant tower. The only way to unambiguously establish the extent of the hydraulic connection, and the time of travel of water from the surface, would be to use a chemical tracer. A visible dye would probably be the most practical substance to use for this purpose. The test would start following the injection of dye into the surface pond. Water samples would then be collected on a daily basis until tracer was detected in the decant tower. However, such a test might be of academic interest only given that pumping from the tower will cease, and it will be backfilled, during final decommissioning of the site.

It is currently proposed to install standpipes, followed by backfilling, in the decant towers to provide piezometric head and water quality data. However, as discussed in section 7 of this report, the standpipes might not provide the most reliable samples for water quality measurements. It is suggested that a combined multilevel bore and piezometer array located towards the centre of the tailings surface might be more satisfactory for this purpose.

The sporadically higher than background levels of cyanide being measured in observation bore P3 are somewhat puzzling since they do not appear to be correlated with other indicators (for example, conductivity and concentrations of major ions) that should signal the escape of pore water from the tailings. It is recommended that high quality measurements of conductivity, using a calibrated conductivity meter, be made whenever water samples are collected from P3. RGC Ltd might consider carrying out a pumping test to measure recharge rates in this bore. If this is done, a series of water samples should be collected for analysis. In view of their importance, the samples should be collected by qualified personnel, and selected subsamples dispatched to a number of laboratories for comparative purposes.