Biological activity in a dam containing an organic depository (the Hufeisen See region) in East Germany cleanses organic contaminants, by a factor of about a million, from water seeping through the dam to an outside lake. This article sets up several possible steady-state models to account for the observations, depending on whether the biologically active components are tied to sedimentary particles in the dam material or are free to move with the seeping water. Allowance is made for the death rate of the biological components and also for their variable growth rates. Numerical examples are provided to illustrate the patterns of response in the different cases possible for seepage speeds that are either fast or slow compared to the biological cleansing rate. Comparison with the data available from the Hufeisen See indicates that, while unique determinations of parameters and limitations on model possibilities are not yet possible, nevertheless it is possible to determine dependences of parameters in each model in order to be in accord with the observations. Allowing for the known uncertainty in the measured data of contaminant concentration ratio, from the interior to exterior of the depository, permits an assessment to be made of the range of uncertainty of parameter relations. For the Hufeisen See depository it is shown that the uncertainty is relatively small in comparison to mean values of parameter relations. The overall implication is that the choice of appropriate model cannot yet be better constrained without new measurements of relative organic concentration through and in the dam material itself.