USE OF COAL COMBUSTION BY-PRODUCTS AND SHALE DRILL CUTTINGS AS SUPPLEMENTS FOR GREEN ROOF GROWTH SUBSTRATES

Green roofs are layers of planting material and plants on roofs that retain, filter, treat, use, and reduce storm water runoff. The benefits of green roofs can include: 1) The capture and evaporation of precipitation, reducing the volume and speed of storm water runoff leaving the site; 2) The lowering of the temperature of storm water runoff, which helps maintain the cool stream temperatures needed by fish; 3) The improvement of air quality by reducing smog; 4) The reduction of “heat island” effects by cooling of the ambient air; 5) Increasing the amount of vegetation and wildlife habitat; 6) Thermal insulation of existing roofs, with lower cooling and heating costs for buildings; and 7) Longer lifespans of existing roofs due to reduced exposure to the weather.

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. The most commonly used substrates are produced by heating in a high-temperature rotary kiln, which requires significant energy input. We are examining the 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 a 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 resulting pHs, metal concentrations, and ability to retain water. Drill cuttings from a Marcellus gas well in southwestern PA likewise indicated the potential mobility of several heavy metals and oxyanions, including As, Cd, Cr, Pb, Se and U. Preliminary results of plant toxicity and growth studies on mixtures of these materials and growth substrate are presented, and scenarios for the geochemical immobilization of potentially toxic compounds via the control of pH and other variables are discussed.