Why does SRisk give a warning message for a calculated critical concentration in application I or III?
In applications I and III, SRisk tries to find critical concentrations corresponding to certain threshold risk levels ((pseudo)risk index or concentration index is 1 or excess cancer risk is 1/10^{5}). In mathematical terms, SRisk executes an optimization algorithm that searches the lowest soil/groundwater concentration at which, for instance, the risk index becomes more than 1.
This optimization is run for a concentration range of 0  10^{6} mg/kg dm for soil and 010^{9} µg/L for groundwater. The efficiency of this optimization procedure not only depends on the algorithm itself, but more importantly, on the complexity of the relation between soil/groundwater concentration and risk index. For instance, when in application III, several soil layers or (a) soil layer(s) in combination with an entered groundwater concentration are used, the layer determining the risk can shift while the concentration is modified during optimization, and in certain cases the optimization algorithm can be hampered in finding the correct critical concentration. In other cases, the calculated risk may be lower or higher than the critical threshold for all soil concentrations in the preset concentration range. In these cases, a critical soil concentration cannot be found by SRisk as it does not exist.
The situations above are flagged by SRisk, using one of the following warning messages:

“The index is lower than the critical level for all realistic concentrations.”
This message appears when SRisk could not find a concentration within the considered realistic range that causes a risk. The critical concentration will be set to 10^{6} mg/kg dm for soil layers or to 10^{9} µg/L for groundwater. 
“The index exceeds the critical level for all realistic concentrations.”
This message appears when the layer to be optimized always involves a risk or another layer than the optimized one is determining the risk. The critical concentration will be set to 0 for both soil and groundwater. 
“This substance uses nonstandard calculation models. Double check the result using the Graph tab.”
This message appears always for the chemical substances Cu, Ni, Hg, Zn and Pb as – mathematically speaking – multiple critical concentrations could be found due to discontinuities or a nonmonotonous behavior of the relation between soil/groundwater concentration and threshold risk level and the optimization algorithm does not necessarily finds the lowest concentration. You should use the Graph tab for a concentration range between 0 and the critical concentration reported by the optimization algorithm to check whether there is a lower critical concentration (the lowest should be used in your risk assessment). 
“An unknown error has occurred while searching for the critical concentration. Use the Graph tab to plot the index in function of the concentration in the optimized layer.”
When this message appears, it is recommended to check if all necessary input data were provided correctly. When the Graph tab also does not give any explanation, it is best to contact the helpdesk.
last modified on 09/09/2016  12:17