DEVELOPMENT AND APPLICATION OF A SIMPLE SCHEME TO DETERMINE THE AQUATIC TOXICITY OF MINE WASTES

Using leachate tests developed by the US Geological Survey (USGS), the Colorado Division of Minerals and Geology (CDMG), and modifying the 1311 TCLP test of the EPA, a simple scheme was developed to determine the aquatic toxicity potential of solid mine wastes. The multi-element power of modern inductively coupled plasma, atomic-emission spectroscopy (ICP-AES) is also a necessary component of the scheme. At two sites in Colorado, approximately 25 sediment samples and the water flowing over the sediments were collected and the water and extracts from the three leachate tests were analyzed for 31 elements by ICP-AES. When the pHs of the water and the leachate from the CDMG were below 5.0, the element concentration patterns of all four solutions were quite similar. When the pHs of the water and the leachate from the CDMG test were above 5.5, the element concentration patterns from the four solutions were often different. The results suggested that when the paste pH of a mine waste is below 5.0, the element concentration patterns from the three tests will give an excellent indication of the metals being released from a mine waste and thus a good estimate of the aquatic and human toxicity of water flowing over that mine waste.

In 2002, such tests were used to assess the toxicity of mine wastes at two sites in the Lake City area of Colorado. The Chicago Tunnel site is an actively draining adit whose water has a pH of 6.5. The Golden Wonder Complex consists of two adits which had only a trickle of water flowing from them whose pH was between 4.5 and 5.5. At the Chicago Tunnel, all waters sampled from the site and extracted from the leachate tests were found to have a pH greater than 5. Zn was found to slightly exceed the aquatic toxicity limits for water that has a hardness of 100 mg CaCO3 / L. It was designated as a low priority reclamation site. On the other hand for the Golden Wonder Complex, even though the adit waters were close to a pH of 5, the waters leached from the representative samples taken from the surface of the waste piles had pHs below 3. The leachate waters were above aquatic toxicity levels for up to 11 different metals and were above human toxicity levels for arsenic. In the drought year of 2002, no water was flowing in the gulch below the waste piles. However, in a normal year the water washing from the piles could prove to be toxic to the Gunnison River.