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• SSR151 - The chemistry of Magela Creek: A baseline for assessing change downstream of Ranger (PDF - 628 KB)

Executive summary

The compositions of waters in Magela Creek upstream and downstream of Ranger uranium mine were reviewed. The water quality parameters examined were pH, electrical conductivity (EC) and turbidity, and dissolved calcium, magnesium, sodium, potassium, chloride, sulphate, ammonium, nitrate, copper, lead, manganese, zinc, uranium and radium-226. The frequency distributions of each of these parameters in waters upstream of the mine were characterised and statistically described to provide a baseline which allows a change in water chemistry downstream of the mine to be assessed.

With the exception of pH, EC, turbidity, magnesium, calcium, sodium and manganese, data that comprise the baseline are not normally distributed. The frequency distributions of copper, lead, zinc, uranium and radium-226 forming the baseline are characterised by a large proportion of values at or near analytical detection limits and contamination in a relatively large proportion of the remainder.

A comparison of upstream and downstream data shows that there is good conformity in pH, EC, turbidity, sodium, potassium and chloride. For calcium, nitrate, ammonium, lead, uranium, radium and zinc less than 40% of the downstream data fall outside the 20^th and 80^th baseline percentiles but in the case of U, data are biased towards relatively high values. More than 40% of downstream magnesium and sulphate data are outside these percentile boundaries and are skewed towards relatively high concentrations.

Copper, lead and zinc in mine waters (characterised by the composition of waters contained in the former RP4) do not appear to pose a risk as contaminants based upon the results of toxicity testing and water quality guideline trigger levels with risk minimised for greater than 1 in 20 dilution.