COAL COMBUSTION BY-PRODUCTS AND SHALE GAS WELL DRILL CUTTINGS AS PLANT GROWTH MEDIA FOR GREEN ROOFS - RESULTS OF ROOF EXPERIMENTS

Green roofs are layers of planting material and plants on building roofs that retain, filter, treat, and reduce storm water runoff, as well as insulate buildings. Green roof growth substrates must be relatively light-weight, retain moisture, be composed of non-toxic materials that do not pollute the associated rainfall and runoff, and allow plant growth. Expanded shales and clays are the most commonly used commercial substrates for this purpose, but they are produced by heating in a high-temperature rotary kiln that requires significant energy input. We are examining the short- and long-term impacts of supplementing conventional green roof growth substrates with available waste products from coal-fired power plants and gas well operations, such as fly and bottom ashes and drill cuttings. These materials may represent satisfactory substitutes for some fraction of less energy efficient growth substrate materials. However, 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. The screening of over 50 fly and bottom ashes from coal combustion power plants leached with natural rainwater showed a wide range in metal concentrations, with specific heavy metals (Co, Cu, Mn) associated with low pH materials, and As and Cr associated with high pH material. Drill cuttings from a Marcellus gas well in southwestern PA generated high salinity water that was detrimental to seed germination but was rapidly rinsed by rainfall from the solid material. Sedum plants were grown for 16 months on a green roof in pots containing different combinations of coal combustion materials, drill cuttings, conventional green roof soils, and slow-release fertilizer. Plant growth was maximal in the presence of added fertilizer regardless of the primary growth substrate used. Cumulative observations on the growth, germination, and the bioaccumulation of metals and metalloids in the plants are discussed.