| dc.description.abstract | Abstract
Mine drainage is threat to water systems in legacy mining districts as elevated concentrations of dissolved iron, sulfate, and trace metals have an unmitigated impact on water quality. Changes in pH due to acidity loading as well as the mobilization of trace metals poses an unacceptable risk to environmental and human health. A variety of active remediation strategies exist, but differ in their initial capital investment, operational requirements, and maintenance making them less attractive options for remote or abandoned locations due to cost. Passive treatment systems (PTS) have become an increasingly more popular technology for the treatment of acid mine drainage (AMD) with the goal of improving water quality through (1) acid neutralization, (2) metals removal and retention and (3) alkalinity generation. Passive treatment systems are composed of a series of treatment cells, in which each unit is designed to meet one or more of the afore mentioned goals through the control of physical, chemical, and biological aspects of the treatment cells. The preliminary oxidation cells of a passive treatment system focus on the removal and retention of iron specifically due to its roll in physical (solids accumulation and retention to maintain hydraulic conductivity through the system), chemical (latent acidity produced via oxidation and hydrolysis; trace metals sorption to FeOOH(s)), and biological (use of emergent hydrophytes to facilitate solids sedimentation) system functions.
The premise of this dissertation is that passive treatment system performance is dependent on the dynamic removal, fate, and transport of iron oxides over time. The following chapters each contribute to a detailed assessment of the design and performance of the oxidative unit of a full scale passive treatment system under expected (design driven) operational conditions and under periods of disturbance due to frequent storm activity. The performance of the oxidative unit, and the performance of the system overall for the first seven years of operation are addressed through intracellular transport, removal, and accumulation profiling.
Chapter One, “Full Scale Passive Treatment of Net-Alkaline Ferruginous Acid Mine Drainage at the Tar Creek Superfund Site” describes the need for site specific passive treatment, and the critical decisions involved in treatment system design. This chapter represents data as a collaborative work of monitoring by the Center for the Restoration of Ecosystems and Watersheds over a period of nearly 10 years (2004-2015) leading up to the installation and application of full scale treatment technologies in fall of 2008, and their performance evaluation over the next seven years of operation to assess effectiveness in achieving the goal of water quality improvement. The Mayer Ranch Passive Treatment system meets water quality improvement expectations as seep concentrations of iron (192 mg/L), zinc (9.78 mg/L), nickel (0.933 mg/L), cadmium (15.1 µg/L), lead (60 µg/L) and arsenic (66 µg/L) are attenuated prior to discharge into a tributary of Tar Creek [99% (Fe), 95%(Zn), 83% (Ni), 93%(Cd), with Pb and As being removed to levels below detection limits]. The system also generates alkalinity in multiple steps (Cells 3N/S; 5N/S) to mitigate what has been lost due to metals latent acidity yielding a net alkalinity of nearly 200 mg/L as CaCO3 equivalence. The MRPTS has successfully removed iron within the oxidative unit specifically (iron oxidation pond+ two surface flow wetland cells) over the lifetime of the system, yet the variability in the efficiency of the preliminary oxidation cell (Cell 1) demands additional investigation.
Chapter Two, “Spatial Profiling of Seasonally Influenced Iron Removal in an Oxidation Treatment Cell”, provides a detailed evaluation of seasonal iron removal within Cell 1 thorough a series of samples collected between the influent and effluent flows typically used for cell performance evaluation. This detailed survey of iron removal corresponds well with the solids accumulation profiling detailed within Chapter 5, “Characterization of the Spatial Iron Accumulation in the Preliminary Oxidative Cells of a Passive Treatment System”, as the accumulation of precipitated iron oxides follows spatial orientation consistent with average removal dynamics. Periods of colder temperatures (winter: ~6oC) decrease the rate of iron removal within Cell 1 with the majority of material transported into Cells 2N/2S being in the dissolved state (Fe2+). The overall function of removal for the oxidative unit is not compromised during the winter months of operation as the surface flow wetlands provide additional hydraulic residence time for the removal of iron prior to discharge on to the vertical flow bioreactors (VFBR). Although iron removal has not been impacted by the accumulation of iron oxides thus far, the hydraulic retention time of Cell 1 has been reduced from a design time of 7.7 days to 5.5 days based on the results of a rhodamine dye tracer study. Cells 2N/2S were assessed to have shorter retention times (2.5 days) versus design (3.5 days) in 2009, yet demonstrate extended retention times approach 9 days due to successive rain events and flow restriction due to the vertical flow bioreactors (VFBRs) indicating that short term storm events play a role in iron transport and removal dynamics.
Chapters Three and Four focus on the role of acute storm disturbance on iron transport between the cells of the oxidative unit, exported from the oxidative unit to the VFBRs, and exported out the system into the receiving stream. Storm frequency, intensity, yield, and duration were evaluated from archived data from the Oklahoma Mesonet to determine a storm classification criteria based on intensity (Low: 0.25-0.99 cm/hr; Moderate: 1.00-1.99 cm/hr; High: 2.00-2.99 cm/hr; Extreme: >3.00 cm/hr). Iron transport out of cells 1, 2N, 2S, and 6 was determined for a select group of individual storms between 2009-2013 and mass transport of iron was determined on a storm by storm basis. The amount of iron transported during a 30-hour sampling window following the storm event did not correlate to rainfall intensity, and thus the mechanism of transport is not believed to be due to resuspension of accumulated materials. Rather, disruption of sedimentation of iron oxide flocs is suspected due to the frequency between rain events. Low intensity rainfall events dominate the precipitation profile for the MRPTS, and significance in transport is not only observed for individual rain events, but also for seasonal and annual transport within the oxidative unit. Iron transport out of the passive treatment system due to storm events was minimal as iron removal and storage occurs multiple cells before the final polishing wetland (spatially isolated from oxidative unit transport). | en_US |
| dc.contributor.committeeMember | Strevett, Keith, David, Elizabeth, Andrew, Robert Sabatini, Butler, Madden, Nairn | |
