Trace Elements Leachates
Coal ash contains various elements, some of which, such as heavy metals are toxic above some specific level. Utilization of coal ash in infrastructure and agriculture has a potential for causing pollution of soil and groundwater as a result of the release of those elements from the ash to the environment and their eventual reaching groundwater or uptake by edible plants. The elements’ concentration in the leachate depends on the amount of water that comes in contact with the ash and the leaching period, as well as the physical and chemical properties of the coal ash itself and its surrounding (i.e. soil and soil solution) and the form in which the elements are found in the ash.
Since the year 2004 the National Coal Ash Board (NCAB) monitors annually fly ash leachate from the main coal sources imported to Israel, using TCLP procedure of the USEPA as an extraction method. This method involves an acidic solution (pH=4.9) as the leaching medium, and the elements’ concentrations are compared to EPA Toxicity levels which were originally set for the evaluation the hazard potential of acidic wastes. Although the toxicity levels were not defined for fly ash and its intended uses, it was decided in 1997 by the Scientific-Professional Committee of NCAB to adopt the TCLP method since no other toxicity limit values were available at that time for coal ash. Since 2007 the fly ash is also monitored using the EN 12457-2 method of the European Landfill Directive, since the extraction solution employed in this method is based on deionized water (neutral pH) which better reflects the ambient conditions in Israel. Yet, the suitability of the EU method to evaluate the potential hazard emanating from the use of coal ash is still relatively low. It is indeed more suitable than the TCLP method, but coal ash behaves differently following each type of application. Thus, while the EU method is originally designed for granular material, coal ash will develop with time into an impermeable monolith when used in road infrastructure, or it will interact both chemically and physically with soil components when incorporated into an agricultural field.
The environmental aspects of the use of coal ash in Israel are evaluated today by the LEAF (Leaching Environmental Assessment Framework) system, which covers a wide range of hazard scenarios and performs risk assessments of many types of wastes and materials (including coal ash), exposed to the environment under various conditions. The LEAF procedure is a useful tool which was formulated during more than 20 years of cooperation between a research group at Vanderbilt University, USA, led by Prof. David Kosson, and a research group from ECN, The Netherlands, led by Dr. Hand van der Sloot, with the involvement and support of the USEPA.