Surface water acidification and critical loads: exploring the F-factor

L Rapp,K Bishop

doi:10.5194/hess-13-2191-2009

Abstract

Abstract. As acid deposition decreases, uncertainties in methods for calculating critical loads become more important when judgements have to be made about whether or not further emission reductions are needed. An important aspect of one type of model that has been used to calculate surface water critical loads is the empirical F-factor which estimates the degree to which acid deposition is neutralised before it reaches a lake at any particular point in time relative to the pre-industrial, steady-state water chemistry conditions. In this paper we will examine how well the empirical F-functions are able to estimate pre-industrial lake chemistry as lake chemistry changes during different phases of acidification and recovery. To accomplish this, we use the dynamic, process-oriented biogeochemical model SAFE to generate a plausible time series of annual runoff chemistry for ca. 140 Swedish catchments between 1800 and 2100. These annual hydrochemistry data are then used to generate empirical F-factors that are compared to the "actual" F-factor seen in the SAFE data for each lake and year in the time series. The dynamics of the F-factor as catchments acidify, and then recover are not widely recognised. Our results suggest that the F-factor approach worked best during the acidification phase when soil processes buffer incoming acidity. However, the empirical functions for estimating F from contemporary lake chemistry are not well suited to the recovery phase when the F-factor turns negative due to recovery processes in the soil. This happens when acid deposition has depleted the soil store of BC, and then acid deposition declines, reducing the leaching of base cations to levels below those in the pre-industrial era. An estimate of critical load from water chemistry during recovery and empirical F functions would therefore result in critical loads that are too low. Therefore, the empirical estimates of the F-factor are a significant source of uncertainty in the estimate of surface water critical loads and related calculations for quantifying lake acidification status, especially now that acid deposition has declined across large areas of Europe and North America.

Highlights

The critical load concept (CL) has become the basis for reducing the acidifying air pollution in Europe, where the CL is defined as “a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified elements of the environment do not occur according to present knowledge” (Nilsson and Grennfelt, 1988)
Our results suggest that while the F-factor worked best within the acidification phase when it was introduced, the empirical F functions used for estimating F will be significantly in error during the recovery phase, when acid deposition has gone down considerably
The reason is that the F-factor should turn negative when the concentrations of base cations decrease below the pre-industrial level during the recovery phase, provided that the predictions of the SAFE model are reasonable

Summary

Introduction

The critical load concept (CL) has become the basis for reducing the acidifying air pollution in Europe, where the CL is defined as “a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified elements of the environment do not occur according to present knowledge” (Nilsson and Grennfelt, 1988). The uncertainties in data and methodologies for calculating critical load become more important as judgements have to be made of the extent to which further emission reductions are needed. Acidification of surface waters is driven to a large extent by soil acidification This is a process that involves a change in a number of catchment parameters, such as soil base saturation and pH, alkalinity and the ANC (Acid Neutralising Capacity) of soil water. If a large change in base saturation of the catchment soil has occurred due to a large input of strong acid, the alkalinity and ANC of the lake may not have changed to nearly the same extent. The buffering capacity of the soil can decline to the point where the input of acids to the soils results in lake acidification

Methods

Results

Discussion

Conclusion