Critical loads of acidity for Irish lakes

Abstract.

Critical loads of acidity have been estimated for Irish lakes (277) using survey data from the Republic of Ireland and Northern Ireland, and compatible inputs and methods across both regions. This is the first trans-region application of the Steady-State Water Chemistry model for the island of Ireland. Results indicate that a small percentage of lakes (10.8%, 30 lakes) have low critical loads of acidity (<50.0 mmol_c m^–2 yr^–1). Based on bulk sulphate deposition, exceedance of critical load was observed in 6.9% of the sampled lakes (19 lakes). Inferences relating to the whole lake population cannot be made, as statistical lake surveys were not carried out. However, as the surveys were weighted towards acid-sensitive lakes, the current study suggests that Irish lakes are not under a significant threat from anthropogenic acidification. The acidifying impact of nitrogen was incorporated into the exceedance calcula tion by superimposing the present leaching of nitrogen onto sulphate deposition. This increased exceedance by 40% (i.e., from 11 to 19 lakes). Despite the potential uncertainties associated with using default values and relationships in the Steady-State Water Chemistry model, uncertainty analysis indicated that the critical loads were relatively stable. The critical acid neutralising capacity limit (specified water quality criterion) had the greatest effect on the critical load. In the current study, a catchment dependent limit was employed (ranged: 0–50 μmol_c L^–1; mean: 35 μmol_c L^–1) rather than a single fixed value.