THE EFFECT OF DISCHARGE ON WATER QUALITY AND CU FLUX FROM THE BEATSON MINE SITE, PRINCE WILLIAM SOUND, AK

Several abandoned Cu mines are located along the shore of Prince William Sound, AK, and the effect of mining-related discharge upon shoreline ecosystems is unknown. To determine the magnitude of the effect at the Beatson mine, the largest Cu producer in the region, our first step was to measure Cu flux from remnant, mineralized workings and waste. Pyrite-pyrrhotite-chalcopyrite ore from the Beatson mine averages 12.4% Fe and 2.2% Cu. Dissolved metals in mine surface waters have been observed as high as 650 ppb Fe, 380 ppb Zn, and 350 ppb Cu.

The stream discharge from Beatson emanates from an adit, and flows through mine waste before crossing a beach and emptying into the Sound. We instrumented the stream with a flume, automatic water sampler, and continuous dataloggers to record water level, pH, and conductivity, for the period 6/4/05-8/15/05. We also installed a datalogger at a site upstream from the mine, to record baseline pH and conductivity.

The pH values averaged 6.8 at the adit and 7.2 at baseline. Rainfall caused a pH decrease of several tenths of a unit before rebounding over a period of days. Baseline specific conductivity was ~40 µS/cm, dropping to ~10 µS/cm after rain events. At the adit, conductivity was 120 µS/cm, dropping to 50 µS/cm after rain events. The decrease in conductivity after rainfall suggests a dilution control on major element concentrations.

The concentration of trace metals such as Cu and Zn show an opposite behavior; they increase with increasing discharge. They increased from 5 to 80 ppb, and 100 to 300 ppb, respectively, as discharge increased from 0.06 to0.85 cms. Our preliminary interpretation is that of a net weathering effect: increased flow volume results in a greater wetting of reactive surface area of the sulfide ore minerals and secondary precipitates, resulting in a greater overall reaction rate. Cu flux from the Beatson mine ranges from 1-50 ng/s, with the greater flux occurring during times of peak discharge.