EFFECTS OF COAL FLY ASH AND SHALE DRILL CUTTING AMENDMENTS ON GREEN ROOF PLANT GROWTH

Green roofs are layers of planting material and plants on building roofs that retain, filter, treat, use, and reduce storm water runoff. The most commonly used primary components of growth substrates are expanded shales and clays, which are produced by heating in a high-temperature rotary kiln, requiring significant energy input. In this study, we examined the potential supplementation of conventional green roof growth substrates using available coal combustion by-products (CCBs) such as fly ash, and waste Marcellus gas well drill cuttings. These materials may represent satisfactory replacements for a fraction of the less energy efficient growth substrate materials. They also have the potential to introduce unacceptable levels of inorganic and organic contaminants to roof runoff, adversely modify soil and runoff pH, and limit plant growth.

Over 50 fly and bottom ashes from coal combustion power plants were leached with natural rainwater - these produced a wide range in pH and metal concentrations and varied in their ability to retain water. Soluble components of CCBs were generally released in distinctive and explainable patterns based on the initial pH of the starting material and could be grouped by their relative abundance in solution. Soluble Ca, S, Na, K, Sr, and Ba were found in greatest abundance, with Ca and Na release being primarily associated with high pH CCB materials. A wide variety of trace elements, including Mn, Cr, Mo, Co, As, Li, Cu, Sn, U, Cd and Fe, were detected. Heavy metal release was usually associated with low pH CCB materials, whereas As and Cr were released from high pH materials. Lettuce seed germination was optimal in circumneutral pH soil mixtures produced from neutral pH CCBs or from appropriate mixtures of low and high pH CCBs. Marcellus shale drill cuttings were the only soil amendments to completely inhibit plant germination, possibly due to incorporated organic drilling mud components. Plant pot studies on a building green roof demonstrated the ability of sedum cuttings to become established in selected CCB-amended substrate mixtures adjusted to produce a circumneutral pH.