Critical loads of acidifying components for surface waters have been modelled and reflect catchment sensitivity to acid deposition. Calculation of the difference between the critical load and actual load of S deposition results in the critical load exceedance – which is either positive (load of acid deposition is too high compared with the acid-tolerance of the water body) or negative (no exceedance).

Measured lake chemistry in ICP Water sites indicates that the acid neutralizing capacity (ANC) of the lakes are consistent with the calculated critical load exceedances. In lakes with no exceedance, ANC was mostly above the critical lower limit. Conversely, in those lakes where critical loads were exceeded, ANC was mostly below the critical lower limit. The agreement between empirical lake chemistry and modelled critical loads indicates that critical load assessments for surface waters have a solid empirical foundation.