Bioclogging of leachate drains is ubiquitous in landfills for municipal solid wastes. Formation of calcium precipitates and biofilms in pore space is the principal reason for clogging. But the calcium speciation in leachte rich in dissolved organic matters (DOM) remains to be uncovered. In spite of its complexity, NICA-Donnan model has been used to compute the speciation of metals and the binding capacities of humic substances. This study applies NICA-Donnan theory into the simulation of calcium speciation during the formation of precipitate-dominant clogging in leachate drainage aggregates for the first time. The consideration of DOC-Ca complexation gives reasonable explanation to the speciation of calcium, which is viewed as oversaturated, in leachate with concentrated DOM. The modeling results for calcium speciation are in good agreement with a large collection of experimental observations, suggesting that NICA-Donnan theory could be used in the modelings of reactive transport and clogging of landfill leachate collection systems. (C) 2014 Elsevier B.V. All rights reserved.