Date of Award

12-2006

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Biological Sciences

Advisor

Schindler, James E.

Abstract

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. One long-standing hypothesis for attaining the goal of sustainability has been to create and maintain meromictic lakes (lakes that do not completely mix on a yearly cycle). 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 sustainable goal for pit lakes. The goal of this study was to determine the key limnological parameters (physical, chemical, and biological) that may facilitate meromixis within pit lakes with the assumption that meromixis should be the desired goal.
This study was conducted at the Kennecott Ridgeway Gold mine in Ridgeway, South Carolina from April 2000 through April 2004. During the course of the study, a persistent meromictic state developed. Observations of the system's response to stochastic perturbations were crucial in determining the mechanisms leading to the persistent meromixis. The most significant event was the introduction of a nutrient laden input (~25% of the pit lake volume) in the summer of 2000. Key factors regarding onset of meromixis resulting from this input included the biogeochemical cycling of calcium, sulfate, magnesium, sodium, carbonate, iron, and manganese. These elements were controlled by biological processes throughout the water column, which lead to a water column density discontinuity. Key factors regarding the persistence of the density discontinuity resulted from the interplay between the physical, chemical, and biological domains where biogeochemical cycles, meteorological forcing (wind field and precipitation), dilute watershed influents, and lake bathymetry were found to be significant. This study emphasized the importance of lake bathymetry and that this often disregarded factor may be the key to sustainability within meromictic systems and manmade pit lakes.

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