Biological Remediation of Low Sulphate Acidic Pit Lake Waters With Limestone PH Neutralisation and Nutrients

Pit lake water in Collie is typically acidic due to Acid Mine Drainage (AMD) caused by pyrite oxidation and ferrolysis coupled with a low natural buffering capacity in surrounding geologies. These pit waters have typically low sulphate, Fe and heavy metal concentrations. Nevertheless, Collie pit lakes have low biological productivity probably due to Al toxicity and low nutrient concentrations. Lake Kepwari (24 GL, max depth 65 m) is an acidic former coal open cut mine in Collie (Western Australia) that has been extensively revegetated and contoured for a water-ski park to facilitate its relinquishment back to the State. The main water treatment approach was rapid fill by river diversion but this has failed to prevent ingress of Fe2+ rich groundwater which has reduced pH to 4.8. In a manipulative mesocosm experiment using 1,200 L enclosures containing Lake Kepwari sediment and water, a control and three treatments were replicated three times. Treatment one, acidity was neutralised to pH 7 with powdered limestone, two had P concentrations maintained at eutrophic levels of approximately 20 μg L-1 (using K2HPO4), and three combined these same liming and P additions. Mesocosms were monitored for water quality (physico-chemical, nutrient and metal parameters) at 0, 77, 140, 191 and 255 days after establishment. Rainfall dilution resulted in the pH of the control increasing slowly from 4.8 to 6.5. Neutralised treatments remained circum-neutral, whilst P addition alone produced a higher final pH of \u3e7. Sulphate concentrations fell slightly in all mesocosms by about 10 mg L-1, presumably due to SO4 2- reduction or secondary mineralisation. In the treatments with P additions, stimulation of NO3 2- reduction appeared to be the principal mechanism for increasing pH above those of the control. Addition of P had the additional benefit of reducing toxic mono-valent Al concentrations; probably by coprecipitation. This study demonstrated that P additions alone might be a useful remediation approach for the Collie pit lakes, although remediation effectiveness is enhanced with concurrent limestone neutralisation. This combined approach, in addition to ongoing alkalinity production by nitrate reduction and primary production, may facilitate these lakes remaining circumneutral over longer periods of time than single acidity neutralisation treatments alone