Quantifying the error in EQC introduced by not including aluminium buffering in the conversion of ANC to pH

Abstract

A fundamental strength of the revised Environmental Quality Criteria is the coupling of water chemistry to biological response. Since pH the chemical property that best describes biological response, the classification of human influence is made in terms of changes in pH. Useful as this is in connecting chemistry to biology, the use of pH as the main chemical indicator presents a major operational challenge. Tying the acidification EQC to pH requires the ability to convert between ANC and pH. That conversion is dependent on the buffering systems that are present in water. There are three such systems of interest in Swedish surface waters, the bicarbonate system, organic acids and aluminum hydrolysis. The bicarbonate system is well defined. It was more recently that the organic acid system has been quantified. The ability to reliably and accurately define the buffering of organic acids made it possible to consider an acidification assessment system based on changes in pH. Aluminum buffering, however was not explicitly considered. This report has evaluated the influence this has had on the conversion of changes in ANC to a 0.4 pH unit change as used in the EQC. The most important result is that estimate of the ANC needed to reduce pH by 0.4 units is not significantly affected (an error of 0.1 pH units) in the weakly buffered pH range between pH 5 and 6 when Al concentrations are changed. In this region small changes in ANC will lead to a large pH drop and a high precision in the prediction of pH changes are vital. Here both models predict equal pH changes. What little affect increasing Al between the preindustrial and present time has is to overestimate the pH drop associated with a given ANC decline. This means that the EQC will err on the side of overestimating human influence. In the well buffered region of pH above 6.5, on the other hand, differences do occur. In that pH range, Samples with higher total aluminium concentrations than the average sample population need to be treated individually using the buffering curves in the annex. 1) Differences in buffering response below pH 6.2 are always below 0.05 pH units irrespective of the TOC and Al concentration. This implies that samples with a pre-industrial pH of 6.2 or lower may safely be evaluated using the Swedish 3- pKa model. In the appendix three such calculations are given (C through E ) where it is assumed that the sample has now a pH of 5.7 (5.0, 4.4) and a preindustrial pH of 6.1 (5.4, 4.9). 2) Significant differences occur from pH 6.3 upwards when TOC is larger than 5.0 ppm. The results imply that the buffering of samples with TOC larger than 5 ppm is overestimated when using the Swedish 3-pKa model. As described in the discussion section we suggest accounting for this by increasing the pH difference with 0.1 pH units. This observation applies for all surface waters with a pre-industrial pH of 6.7 or higher and a TOC of 5 ppm or higher. In the appendix one calculation is given where it is assumed that the sample has now a pH 6.5 and a pre-industrial pH of 6.9

Published in

Rapport / Sveriges lantbruksuniversitet, Miljöanalys
2005, number: 2005:22
Publisher: Department of Environmental Assessment, Swedish University of Agricultural Sciences

SLU Authors

• Bishop, Kevin

  • Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Fish and Aquacultural Science
Environmental Sciences

Permanent link to this page (URI)

https://res.slu.se/id/publ/7747