Estimation Of The Acid Sensitivity Of A Soil
Authors: Thimm Harald F. 
Published: March 2010

Current regulations For environmental monitoring in the sour gas industry require annual reporting of soil pH. It is well known that this procedure may produce results with wide scatter, and without a clear trend over time.

An alternative method, which overcomes this problem, is proposed.Rather than relying on soil monitoring to indicate the beginning of an irreversible pH drop, the new method allows the time of this occurrence to be calculated if the mean SO2 or sulphur deposition rate is known or can be estimated. It is also possible, in at least some cases, to identify the minerals that govern initial pH control of the soil.

The method rests on kinetic measurements of soil pH change with time after acidification in the laboratory. It is recommended for monitoring, and especially for environmental impact assessment submissions to regulatory authorities.

Introduction

The acidification of soil, as a potential consequence of sour gas processing operations, has been of concern to industry and government in Alberta for some 20 years. While fears of regional groundwater pollution from trace elements, thought to be released from acidified soils and free to leach into aquifers, appear not to be a serious threat in the part of Alberta overlain by glacial till soils(1, 2) there is residual concern about yield reductions of acid sensitivie crops over the long term.

During this period, there have been a number of strategies to monitor or control potential acidification effects. Prior to 1980, mere were continuing efforts by government and the private sector to ncrease sulphur recovery at sour gas plants, measure stack emissions directly and continuously, and to improve mathematical predictions of sulphur dioxide deposition; sulphur dusting was monitored by normal dustfall monitoring techniques.

With the increase in the price of sulphur around 1980, there was an incentive to rapidly break up sulphur blocks by mechanical means. The attendant problem of sulphur dusting, as well as the analysis of several years of monitoring data by Alberta Environment, showed that the true sulphur dusting near these plants was seriously underestimated by dustfall methods. In some cases the discrepancies were more than an order of magnitude.

In 1980, dustfall methods were abandoned in favour of regular soil pH measurement at predetermined sampling sites. In addition to these measurements, the requirement of the determination of a "buffering curve" was introduced(3). The buffering curves were dropped after a few years, when it became apparent that they underestimated the assimilative capacity of most soils for sulphur and sulphur gases. The reason for this may be the finding of Thimm and Weleschuk(1) that, after an initial acid addition, the equilibration of the soil sample requires some 15 to 30 days, instead of the days specified in the prescribed method. At present, the pH monitoring requirement still stands, but the details of it have been expanded on significantly to include the analysis of soil sulphur, and of carbonate in some cases. Details are given in the Alberta Environment Reference Method (AERM)(4).