IDMM-ag v3: an improved model for prediction of metal concentrations in soils and receiving waters of agricultural catchments

The previously submitted draft report (November 2015) used version 2 of the IDMM–ag. Since submission of that draft, Parts of the model dealing soil erosion have been rewritten to deal with minor errors discovered. The predictions presented in this report have been done using the updated model (IDMM–ag v3). The updated model produces results which are broadly similar to those previously obtained, both in trends across the scenarios and in the differences in surface water and sediment concentrations within different waterbody types using the same soil scenario. A single scenario (D5 pond) now has predicted PNEC exceedances for surface water copper which were not predicted by the previous model. All other patterns of PNEC exceedance or nonexceedence are the same as were predicted using the previous model. The influence of high soil erosion rate on metal accumulation in scenario R2 was not seen when using IDMM–ag v3. Therefore, discussion of this has been removed from this version of the report. A summary of the differences in predicted concentrations relevant to PNEC exceedance is presented in Appendix 2.