Environmental issues related to coal ash disposal with emphasis on boron and arsenic behavior

Jennifer L Kolbe, Purdue University

Abstract

The use of high pH fly ash to mitigate acid mine drainage conditions was evaluated at the first and currently only field site where this strategy has been implemented. Three concerns addressed in this study were: (1) the ability of the ash to neutralize the pH to an acceptable value, (2) the potential for leaching of contaminants from the ash to groundwater, and (3) the attenuation of transport of leached contaminants by downgradient soil. Fresh and weathered ash samples from the site were used to determine the acid neutralizing capacity and leaching potential of the ash and attenuation of transport of leached constituents was evaluated using well water and downgradient soil samples in batch and column systems. Results were compared to site water quality data. The ash exhibited a high acid neutralizing capacity that increased with retention time, and neutralization of ground and surface water was observed at the site. However, pore-water within the ash fill contained potentially hazardous constituents, most of which were depleted slowly from ash and reached higher aqueous concentrations at extended retention times. The two main constituents of concern were arsenic and boron. Arsenic transport was highly attenuated by soil with As(III) being more mobile than As(V). Arsenic has not been detected in downgradient monitoring wells and its transport is highly dependent on preferential flow conditions. Boron exhibited low sorption to soil and its transport lagged slightly behind groundwater flow. Thus, release of boron from ash and groundwater flow rate control its mobility. A fraction of the boron in ash was solubilized immediately upon water contact and other fractions were released more slowly, consistent with boron partitioning during combustion. Over 50% of the total boron was released slowly indicating that ash may be a long-term source. For both elements, groundwater flow rates, preferential flow, and dilution are critical in evaluating environmental impacts. Assuming no preferential flow, boron and arsenic require over 100 and 30,000 years to reach a downgradient creek which is utilized by wildlife. A creek flow rate of at least 1.6 L/s, which is reasonable, would dilute both elements to acceptable levels.

Degree

Ph.D.

Advisors

Lee, Purdue University.

Subject Area

Environmental engineering

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