Southeastern Natural Sciences Academy, Center for Ecological Restoration
American Ecological Engineering Society 2005
Oscar Flite, SNSA, flite@naturalsciencesacademy.org
Gene Eidson, Ph.D., SNSA, eidson@naturalsciencesacademy.org
James E. Schindler, Clemson University , jesch@clemson.edu
Open pit mining usually results in a void that, over time, fills with water and becomes a pit lake. The goal for pit lakes is to create sustainable systems that positively contribute to local and regional watersheds. However, development of these manmande systems is driven by maximization of safe resource extraction, physical reclamation, and cost minimization, not future pit lake sustainability. One long standing hypothesis for attaining the goal of sustainability has been to create and maintain meromictic systems. It is believed that meromixis minimizes atmospheric oxygen exposure to pit walls and concomitant acid generation and minimizes reentrainment of metals to the upper waters during seasonal mixes. This seems to be a reasonable goal but few lakes achieve and maintain meromixis so it is not clear whether this should be the goal for all pit lakes.
A study of two filling pit lakes at the Kennecott Ridgeway Gold mine in South Carolina was conducted from April 2000 through April 2004. While monitoring the physical, chemical, and biological parameters within both lakes we observed the development of a persistent meromictic state within one of the two lakes. Differences between the lakes and the stochastic events that occurred in only one of the lakes during the study helped to elucidate the importance of the biology in developing and maintaining the meromixis. We found that photosynthetic activity resulting from high phytoplankton productivity (784 mgC/m 2/d) induced formation of precipitates in the upper water (manganese, iron, and calcite). Upon dieoff the associated plankton biomass stimulated benthic bacteria, lowered the redox in the bottom water, and allowed resolubilization of the upper water precipitates . Both of these ancillary events of the large plankton biomass induced a strong enough density difference between upper and lower waters to overcome the destabilizing force of wind mixing and a meromixis was established in the winter of 2000-2001. Physical characteristics of this particular lake (e.x. lake shape, groundwater influent, and surface water influent) aided in counteracting the destabilizing force of wind and most likely helped to maintain the meromixis.