LandSim is a probabilistic quantitative risk assessment model issued by the Environment Agency for use by regulatory staff in landfill performance evaluation. LandSim takes into account factors such as liner and leachate-removal system and design, and the geological and hydrogeological regimes beneath and around the site in question. Among the input values required for the model is the cation exchange capacity (CEC) of geological materials around the landfill and the partition coefficient (Kd) of key solutes in leachates. CEC can be defined as the sum of exchangeable cations that a mineral, rock or soil can adsorb at a specific pH. Kd represents overall sorption of a particular species in leachate, irrespective of process. Both CEC and Kd are critical parameters in modelling the ability over time of the geosphere to retard the migration from landfill sites of leachates in groundwater.