The staff at SCS Engineers (SCS) has talked at length about how changing the parameters of a coal ash remediation project impacts the eventual outcome of that project. That involves not only the factors present at a particular site but also the regulatory environment in which that site operates, certainly as rules evolve regarding the disposal of coal combustion residuals (CCRs).
Two primary means of coal ash remediation are closure-in-place, or cap-in-place, of an existing coal ash storage site, and closure-by-removal. Closure-in-place involves dewatering the storage site, or impoundment, in effect converting from wet storage to dry storage of ash. A cover system is then used to prevent more water from entering the site.
Closure-by-removal involves dewatering of the coal ash, and then excavating it, and transporting it to a lined landfill or a recycling center.
“There are lots of technical reasons and site-specific factors that can influence a project’s outcome,” said Eric Nelson, vice president of SCS and an experienced engineer and hydrogeologist. “These might include the type and volume of CCR, the geologic setting [e.g., groundwater separation], presence and proximity of receptors [e.g., drinking water supply], and physical setting [e.g., constraints such as access, available space onsite for re-disposal, proximity/availability of offsite re-disposal airspace, etc.].”
Sherren Clark, an SCS team member with experience in civil engineering and environmental science, said “risk evaluation is a key component of remedy selection. A CCR unit undergoing an assessment of corrective measures [ACM] could be a 100-acre ash impoundment containing 30 feet of fly ash, but it also could be a 2-acre bottom ash pond. It could have numerous groundwater constituents exceeding drinking water standards by a significant margin, or it could have a single parameter slightly above the limit at a single well. And there could be water supply wells nearby in the same aquifer, or none for miles around. All of these factors play into the selection of a remedy that addresses the existing risks, without creating other negative impacts such as site disturbance, dust, or truck traffic.”
Tom Karwoski, a hydrogeologist and project manager for SCS who has designed and managed investigations and remediations at landfills as well as industrial, Superfund, and other waste storage sites, noted the challenges inherent to individual sites and stressed careful planning is needed to achieve the desired result. At some sites, “given the size and the nature of the impoundments, transport of CCR off-site may not be the best option.” When moving from the ACM to the remedy [selection], it’s extremely important to have multiple meetings with the client to set the schedule. Based on the way the [CCR] rule is written, things have to progress logically. There’s time available for careful planning. The last thing we want to do is start making assumptions without input from the client and other interested parties. Regulatory compliance and concern for the surrounding community and the environment are important to us and our clients.
“If the nature of the site in its current condition allows it, capping of the site will reduce surface water moving through the waste and significantly cut down on the risk of groundwater contamination,” Karwoski said. “At sites where you have CCRs that may be distributed across a site, to consolidate that onsite and then the cap will address CCRs impacting groundwater.”
Jennifer Robb, vice president and project director with SCS’s Solid Waste Services Division, and the company’s Groundwater Technical Advisor for the Mid-Atlantic region said her group has “done corrective measures for cobalt, arsenic, and thallium,” all contaminants found in coal ash. “There are some in situ bio-remediation that can be done, where basically you’re trying to alter the chemistry to immobilize the metal.” Jennifer noted that there are also more physical remedies where contaminated groundwater is extracted from the subsurface by pumping or the groundwater plume is contained or treated in-situ with the construction of “cut off trenches.”
Karwoski said, “we have no preconceived notions about what is best for all sites, but if you consolidate [waste] onsite and then cap, it will certainly take care of a lot of situations where you have CCRs impacting downgradient groundwater.” This approach may not be appropriate in every situation, but, if arrived at after thoughtfully navigating the remedy selection process defined in the current Federal CCR rules (40 CFR 257 Subpart D—Standards for the Disposal of Coal Combustion Residuals in Landfills and Surface Impoundments), should result in an approach that is effective based on the site-specific factors present.
Read last month’s blog “Many Factors Influence Remedies for CCR Control and Disposal.”