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15 March 2017

Landfill mining – what are the impacts on groundwater quality?

William Soukup, PGThere’s gold in them thar landfills! Well, you might think so, given the increase in mining of municipal solid waste (MSW) from landfills in recent years. From less than ten landfill mining operations in the early 1990s we now have upwards of 100 MSW mining operations in progress in the U.S.

MSW is mined for a variety of reasons, including recovering resources, such as precious metals and even just plain old granular material (soil) for reuse as daily cover. More common reasons however include increasing airspace through facility re-design, and the remediation of leaky landfill caps and/or leachate collection systems.

So what’s the problem?

Mining MSW requires the cap (or cover soils) to be removed from the landfill. This significantly increases surface permeability and alters surface grades, which, at least in the short term, increases the amount of recharge from precipitation that enters the waste. In addition, the mining operation itself may unearth and expose different wastes to this recharge, increasing leachate volume and concentrations of leachate constituents. So before mining landfills without bottom liner systems, it is critical to evaluate the potential for short-term impacts of these changes on groundwater quality.

I have worked on several MSW landfills considering or undergoing mining projects. From this experience, I have found that one key tool to help predict the impact of mining on groundwater flow and quality is numerical groundwater transport modeling. We typically begin by calibrating the model to groundwater elevations and contaminant concentrations, and then use it to predict the impact of increased recharge due to mining operations. We also use modeling to evaluate various operational modifications that can help to reduce these impacts, such as changing the mining sequence and using temporary covers. Finally, we use the model to simulate groundwater remedial alternatives (should they be necessary), including pump and treat, barrier walls, and in-situ treatment.

Is landfill mining worth it?

soukup-miningIn most cases, I have found that the biggest impact is a relatively short-term increase in contaminant concentrations in downgradient groundwater. Depending on the site, this spike in concentration will not cause surface water quality in the downgradient receptor to exceed applicable standards. Surface water sampling is conducted to confirm that contaminant concentrations are below drinking water standards in downgradient surface water receptors. In cases I’ve worked on, I found that the groundwater was moving slowly, which allows plenty of early-warning time to assess model predictions and to implement operational modifications if warranted. Thus, with proactive sampling, groundwater modeling, and the ability to implement minor adjustments to mining operations, the benefits of landfill mining often seem to far outweigh the risks.

Have you considered landfill mining and if so, what has been your experience with groundwater quality issues?

Mr. William Soukup, PG, has 38 years of professional experience in groundwater hydrogeology and related areas of contaminant transport evaluation.

Categories: Environmental Planning & Compliance, Environmental Remediation, Landfill Engineering and Design, Solid Waste
Posted By Bill Soukup at 11:30 AM  |  No Comments on Landfill mining – what are the impacts on groundwater quality?

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