DECAY is an analytical solution for one-dimensional transport of multiple species along a decay chain with branching. The treatment of branching accommodates species potentially having more than one parent and species having more than one daughter product. The DECAY solution considers advection, dispersion, equilibrium sorption, and first-order straight-chain and branching decay reactions. The species may have different dispersion and sorption coefficients. The solution accommodates the specification of different decay rates for the dissolved and sorbed phases for each species. Yield coefficients can be specified so that the calculations can be conducted in terms of concentrations expressed in units of either mass or moles per unit volume of water. The DECAY solution allows Dirichlet and Cauchy inflow boundary conditions with Bateman-type source histories. 


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