Why do mercury, copper, nickel, zinc and lead simulations sometimes give counterintuitive results?

For mercury compounds, copper, nickel and zinc, S-Risk adopts non-standard model equations for most plant uptake factors (the so-called BCFs or bioconcentration factors). For mercury compounds, copper and nickel, these BCF models include a discontinuity (jump) at a soil concentration equalling four times the background concentration of the chemical (official background concentrations in 2007; values can be found in the substance data sheets). For zinc, the BCF models include two jumps, namely at a soil concentration of 60 and 360 mg/kg dm. Also for lead, a discontinuity takes place (at a soil concentration of about 1000 mg/kg dm and more) due to the non-standard model used for the determination of the soil-water distribution coefficient Kd.

These jumps can sometimes be seen clearly in the graphs of the risk index (RI) versus soil concentration. For mercury compounds, copper and nickel, this typically occurs in scenarios with the intake via locally produced vegetables, meat and milk and/or eggs as an active pathway. For zinc, the graphs of scenarios with the intake via locally produced vegetables checked as active pathway might include discontinuities. For lead, a jump typically occurs in scenarios with the intake via locally produced meat and milk and/or eggs as an active pathway or where groundwater is used as drinking-water. The graph below presents the function discontinuity for the overall risk index (RI) for systemic effects of mercuric chloride versus soil concentration: the RI crosses the threshold value of 1 at two soil concentrations.

Attention: The optimization algorithm currently used in S-Risk (i.e., from version 1.1.5 onwards) for application I and III will systematically find the higher critical soil concentration of the two concentrations shown in the graph above, i.e., the wrong/less stringent choice for a soil remediation value. The basic principle in this situation is that the lowest soil concentration should be retained in your risk assessment. For these situations, S-Risk will always generate a clear warning message in the reports and advise you to double check your optimization result using the Graph tab.

last modified on 24/03/2017 - 9:54
FAQ Category: