If limited data regarding water flow, heavy-metal sorption, mobility and bio-availability are available, a more advanced assessment is feasible. The ranking of the threat to the different compartments of the system, at steady state, depends on which limit will eventually be exceeded by their greatest margin (but not necessarily first). The degree of threat can be assessed relatively, however, by comparing the steady-state values of the soil content, the crop uptake rate and the leaching rate with the corresponding critical values. The corresponding ratios can be regarded as sustainability factors for ecology (Fe), crop uptake (Fu) and soil solution or leaching (Fs).
The critical sustainability factor (Fc) is given by:
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