Effect of Vadose Zone on CDF Leachate Concentration

Contaminated dredged material is often placed in confined disposal facilities (CDFs) designed and operated to control environmental impacts of the disposed sediment. A CDF is a diked enclosure having structures that retain dredged material solids. When contaminated dredged material is placed in a CDF, contaminants may be mobilized to form leachate that may be transported to the site boundaries by seepage. The purpose of this paper is to present the results of sensitivity analyses of the impacts of contaminant and foundation soil characteristics on leachate transport to the water table below. The main factors affecting leachate production and transport out of the facility through the vadose zone are evaluated to develop guidelines to assist in decision making regarding the use of leachate controls. The factors evaluated relate to contaminants in the CDF and the vadose zone properties. The results highlight the significant parameters upon which leachate screening should be based. Estimates of peak contaminant concentrations reaching the water table are developed for use in leachate screening applications. Further, an equation for the time of travel of contaminants through the vadose zone is developed to compute the time it takes peak contaminant concentrations to reach the water table.