EXSOLUTION OF CARBON DIOXIDE FROM THE SILVER CREEK AMD TREATMENT SYSTEM, NEW PHILADELPHIA, PA

Abandoned mine drainage (AMD) has resulted in degradation of streams, their surrounding landscapes, and the biodiversity of the local ecosystem. AMD treatment systems provide the necessary conditions to remove iron and other metals from AMD discharges to decrease their impact on the local streams. The design and installation of these treatment systems is highly dependent upon available space and the chemistry of the raw outfall. Silver Creek Treatment System, located in the Anthracite Coal Field of PA, is an entirely passive and properly functioning system. Of the many processes required to support effective treatment systems, the exchange of CO₂ and O₂ between the water and the atmosphere is of critical importance. As CO₂ decreases and O₂ increases in the water, pH rises and iron oxidation and precipitation reactions accelerate. The CO₂ and O₂ exchange can occur across the pond surface, at inter-pond riffles, and through the aerenchyma of wetland plants.

In this study, we report on the measurements of CO₂ gas exchange at twelve locations along the treatment system at Silver Creek. Gas exchange was determined using a dynamic flux chamber (DFC) coupled to an Infrared Gas Analyzer (IRGA) to measure the CO₂ flux directly from the surfaces of the ponds. Measurements were made at each pond, riffle, and wetland in the system. Dissolved CO₂ concentrations were measured in the water samples using headspace equilibration and gas chromatography. Unfiltered, diluted samples were used to determine alkalinity. We find that CO₂ flux is greatest at entry and decreases through the system, with peak flux occurring at the riffles between ponds. This research will help us understand which pathways are most important for gas exchange in natural and engineered systems, with important implications for the design of smaller, more efficient AMD treatment systems.