STAMP SAND ALONG THE KEWEENAW SHORELINE: SOLID AND DISSOLVED COPPER & EFFECTS ON BIOTA

Abstract

Stamps sand refers to the regional colloquialism for the mine-tailing byproducts generated from copper (Cu) ore processing mills located in Michigan’s Keweenaw Peninsula. In the Keweenaw region, copper extracted from basalt ores resulted in a legacy of 100’s of millions of metric tonnes of stamp sand wastes, including 22.7 million metric tonnes at Gay, MI. Trace amounts of Cu persist in stamp sands and when leached have toxic enough concentrations to influence aquatic biota. To better understand stamp sands’ properties, we ran experiments and compiled data about the solid and dissolved phases of Cu. Solid phase concentration of Cu was assumed to be 2863 mg/kg from MDEQ’s previous studies of Gay’s tailings pile and was compared to Cu concentration data from AEM and other published works. Dissolved Cu concentrations were determined through leaching experiments, measured in stamp sand ponds, and compared to other literature/published results. The physical properties of mean density (2.88g/cm3), grain sizes, and mean percentage stamp sand were also determined. Both field and laboratory studies on the chronic effects of stamp sands were done using native Daphnia species. Acute toxicity for Daphnia was determined through an LD50 test for Cu (8.89 µg/L). Laboratory chronic toxicity studies tested local waters with low dissolved organic carbon (DOC) and neutral pH against those with high DOC and lower pH. These experiments demonstrate that lower pH and higher DOC may lead to higher rates of Cu leaching from stamp sands, compromising complexation of Cu. The chemistry of overlying water plays a significant role in the leaching of Cu from stamp sand deposits, and the particle size on dispersal and Cu concentration.