GEOCHEMICAL LESSONS LEARNED FROM LABORATORY TESTING THROUGH OPERATION OF A WATERSHED-SCALE ACID ROCK DRAINAGE WATER TREATMENT SYSTEM

A large ARD watershed-scale neutralization treatment system near Ducktown, Tennessee has been operating in North Potato Creek (NPC) for nearly two years. Investigations started with laboratory experiments followed by a pilot scale plant, and ultimately the construction of a cost-effective full scale hydrated lime neutralization plant at the base of the 9,000-plus acre watershed. The in-pit lime treatment facility uses a 20 acre pit (South Mine Pit, SMP) as a settling basin and “clarifier” and is designed to treat mine water and variable NPC flows ranging from dry weather flows of 9 cfs to a 10-yr, 24-hr storm of 972 cfs for approximately 15% of the cost of a conventional lime treatment plant. In 2005 average daily removal rates were 131 lbs of aluminum, 13 lbs of copper, 419 lbs of iron, 123 lbs of manganese and 68 lbs of zinc.

Water quality monitoring data that continues to be collected is quite extensive and includes daily field iron and pH measurements in North Potato Creek above the plant, just below the plant outfall, and at the discharge from the pit. The system to date has met or exceeded expectations in terms of downstream water quality improvements. Dissolved iron concentrations above the plant have been found to generally range between 6 mg/L and 10 mg/L and dissolved iron concentrations leaving the watershed are below 0.1 mg/L with associated pH values ranging from 7 to 9 and alkalinities generally ranging from 12 to 25 mg/L. Observations from the operating data that have a broader interest to other ARD-impacted watersheds are that iron nucleates in the water treatment facility much more readily than observed in the laboratory testing and manganese removal efficiencies from the operating system show a large variability ranging from 20 to 90%.