Testing the validity of a critical sulfur and nitrogen load model in southern Ontario, Canada, using soil chemistry data from MARYP.

The validity of a steady-state mass balance model (Arp et al., 1996; referred to as ARP) was tested using physicochemical soil data from the Monitoring Acid Rain Youth Program (MARYP). Four ARP sites were matched with ten MARYP sites according to proximity, bedrock type and subsoil pH to test the validity of the ARP model for critical load exceedances. Soil solution pH, base concentration and A1 concentration from MARYP sites, which were well matched to ARP sites, validated the modelled critical load exceedances. Higher exceedance areas were associated with more acidic pH and lower base and higher A1 concentrations from matched MARYP sites and vice versa. One ARP site was inappropriately matched with MARYP sites and could not be validated using base and A1 concentrations. This study also confirmed the southern limit of the zero critical load exceedance isopleth from the model. However, variability of the other exceedance isopleths was noted due to the limited number of sites used in the model. The validation of these sites in the ARP model and the zero critical load exceedance isopleth nonetheless allows greater confidence in using this model as a management tool for acidic deposition.