Reactive barriers can be formed with a single type of filling, a mixture of media, or down-flow sequences of media. Barriers filled with a single type of media work effectively where all contaminants present are treated in the same manner. However, for some contaminants a mixture of media works more effectively, delivering optimal treatment while balancing hydraulic characteristics and cost. For example, mixtures of quartz sand, crushed limestone and iron and aluminum oxides were trialled to optimise the removal of phosphorus from wastewater (Baker et al. 1998). Variable mixtures within the barrier have also been proposed to reduce precipitation plugging at the entry face of the barrier (Mackenzie et al. 1999). Sequential barrier media can also form a treatment train approach, whereby different contaminants can be treated in turn. For example, it is possible to deliver nutrients in the upstream part of a barrier to encourage petroleum hydrocarbon degradation, a middle compartment to retain hydrocarbons for treatment, with an adsorbant at the back of the barrier to remove surplus nutrients from the treated effluent water. Further examples might be reductive degradation of an organic compound at the front of the barrier, with an adsorbant to remove dissolved metals at the rear of the barrier, or the treatment of a complex assemblage of organic compounds (Devlin et al. 2004; Kalin 2004; Bastiaens et al. 2005; Finkel and Bayer 2005; Ferguson et al. 2007). Conca et al. (2002) used a four-component barrier to remediate groundwater contaminated with radionuclides, other metals and nitrates.
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