The neutralization of acidic coal mine lakes by additions of natural organic matter: a mesocosm test

Cylindrical polyethylene enclosures 3 m in length and 1 m in diameter reaching from the surface to the bottom were constructed in an acid (pH=3.1) lake on a coal surface mine in southern Illinois. Wheat straw was added to the enclosures to test the effects of dissimilatory sulfate reduction on water chemistry. Added straw increased sulfide concentrations, raised pH to 6.5, reduced O₂ and increased acid neutralizing capacity of the enclosed water columns when compared with a control enclosure and with the open lake. Generation of acid neutralizing capacity exceeded the standing stock of sulfide indicating that sulfide was removed either by precipitation of FeS or outgassing of H₂S. The pH and acid neutralizing capacity within the enclosures eventually returned to the level of the surrounding lake because of water exchange around the enclosure walls. Our results show that additions of organic matter to acid surface mine lakes result in the generation of acid neutralizing capacity.