George R. Watzlaf

Long-term Performance of Anoxic Limestone Drains

Ten anoxic limestone drains (ALDs), buried beds of limestone gravel that add alkalinity through dissolution of calcite, have been monitored for a decade. Influent and effluent water quality data have been analyzed to determine the long-term performance of each ALD for treating coal mine drainage. Alkalinity concentrations in the effluent of the ten ALDs ranged from 80–320 mg/L as CaCO3 with near maximum levels of alkalinity being reached after approximately 15 hours of detention. ALDs that received mine water containing less than 1 mg/L of both ferric iron and aluminum have continued to produce consistent concentrations of alkalinity since their construction. However, an ALD that received 21 mg/L of aluminum experienced a rapid reduction in permeability and failed within eight months.

LONG-TERM PERFORMANCE OF ALKALINITY-PRODUCING PASSIVE SYSTEMS FOR THE TREATMENT OF MINE DRAINAGE

Ten passive treatment systems, located in Pennsylvania and Maryland, have been intensively monitored for up to ten years. Influent and effluent water quality data from ten anoxic limestone drains (ALDs) and six reducing and alkalinity-producing systems (RAPS) have been analyzed to determine long-term performance for each of these specific unit operations. ALDs and RAPS are used principally to generate alkalinity. ALDs are buried beds of limestone that add alkalinity through dissolution of calcite. RAPS add alkalinity through both limestone dissolution and bacterial sulfate reduction. ALDs that received mine water containing less than 1 mg!L of both ferric iron and aluminum have continued to produce consistent concentrations of alkalinity since their construction. However, an ALD that received 20 mg!L of aluminum experienced a rapid reduction in permeability and failed within five months. Maximum levels of alkalinity (between 150 and 300 mg!L) appear to be reached after 15 hours of retention. All but one RAPS in this study have been constructed and put into operation only within the past 2.5 to 5 years. One system has been in operation and monitored for more than nine years. Alkalinity due to sulfate reduction was highest during the first two summers of operation. Alkalinity due to a limestone dissolution has been consistent throughout the life of the system. For the six RAPS in this study, sulfate reduction contributed an average of28% of the total alkalinity. Rate of total alkalinity generation range from 15.6 gd·m· to 62.4 gd·m· and were dependent on influent water quality and contact time. Additional

Design and Performance of Limestone Drains to Increase pH and Remove Metals from Acidic Mine Drainage

Publisher Summary This chapter presents data on the construction characteristics and the composition of influent and effluent at 13 underground, limestone-filled drains in Pennsylvania and Maryland to evaluate the design and performance of limestone drains for the attenuation of acidity and dissolved metals in acidic mine drainage. On the basis of the initial mass of limestone, dimensions of the drains, and average flow rates, the initial porosity and average detention time for each drain are computed. Calculated porosity ranged from 0.12 to 0.50 with corresponding detention times at average flow from 1.3 to 33 h. The effectiveness of treatment was dependent on influent chemistry, detention time, and limestone purity. At two sites where influent contained elevated dissolved Al (>5 mg/liter), drain performance declined rapidly. Elsewhere the drains consistently produced near-neutral effluent, even when influent contained small concentrations of dissolved Fe3+ (<5 mg/liter). Rates of limestone dissolution computed on the basis of average long-term Ca ion flux normalized by initial mass and purity of limestone at each of the drains ranged from 0.008 to 0.079 year–1. Data for alkalinity concentration and flux during 11-day closed-container tests using an initial mass of 4 kg crushed limestone and a solution volume of 2.3 liter yielded dissolution rate constants that were comparable to these long-term field rates. An analytical method is proposed using closed-container test data to evaluate long-term performance (longevity) or to estimate the mass of limestone required for a limestone treatment. This method considers flow rate, influent alkalinity, steady-state maximum alkalinity of eflluent, and desired eflluent alkalinity or detention time, and applies first-order rate laws for limestone dissolution (continuous) and production of alkalinity (bounded).

MODELING OF IRON OXIDATION IN A PASSIVE TREATMENT SYSTEM

Iron oxidation rates were modeled in a passive system using pH, temperature, dissolved oxygen and iron concentrations. The system consists of a 224 meter ditch receiving the effluent from an anoxic limestone drain (ALD), which treats drainage from a reclaimed surface coal mine in Clarion County, Pennsylvania. The ditch was divided into ten sections. Depth and width were measured for each section. Three water samples (raw, unfiltered and acidified, filtered and acidified) were collected at the beginning and end of each section. Water analyses included field-measured pH, dissolved oxygen, and temperature, and laboratory-measured net acidity and major and trace elemental concentrations (including Fe2+, Feror, Ca, Al, Na, Mn, S04-, K, As, Ba, Be, Cd, Co, Cr, Cu, Ni, Pb, Sb, Se and Zn). Field pH, dissolved oxygen and temperature varied between 5.89 6.37 s.u., 0.52 7.75 mg/L, and 12.1 22.0°C, respectively. The average flow rate for the system was 92.8 L/min. Iron concentration decreased to approximately 70% of the original level by the end of the ditch. A kinetic model for loss of ferrous iron from solution was compared to the traditional sizing criteria for iron removal of 10 20 gd·m·. Because of the geometry of the ditch, a plug flow model was used. The majority of the sections had removal rates near the 20 gd·m· traditional value, and modeling provided insight as to why certain sections performed better than others. All significant changes occurred soon after aeration, indicating that net alkaline water should be aerated immediately in order to optimize iron removal. Additional

CHARACTERIZATION AND RECOURCE RECOVERY POTENTIAL OF PRECIPITATES ASSOCIATED WITH ABANDONED COAL MINE DRAINAGE

Sludge samples from untreated and passively treated mine drainage discharges were characterized using INAA, ICP-AES, XRD and SEM. Iron content ranges from 25 to 68 dry wt%, and goethite is the dominant mineral (40 90 dry wt%). The majority of particles have a spiky spherical morphology (0.5 2.0 μm diameter). Within several passive treatment systems, iron content remains relatively constant, and concentrations of Mn, Co, Ni, and Zn increase, while As concentration decrease. Additional