Deposition of anthropogenic sulphur dioxide on soils and resulting soil acidification

Abstract

Emission of sulphur dioxide (SO2) takes place in Alberta from tall stacks at natural sour gas or oil sands processing plants. The local quantities of sulphur (S) deposition on soil and any resulting soil acidification are poorly known. The main objective was to quantify S deposited by wet and dry deposition at distances from a large emitter (135 t SO2-S day-1) and determine the effect on soil pH. Crushed soil samples were set in containers under rain shelters without walls at sites varying in distance from the source (4 to 173 km). After 4 months a sandy soil had accumulated 24 kg total S ha-1 when comparing the nearest and farthest sites, but the value was only 4 kg ha-1 for an organic soil. During 5 months in the next year, lichen over a thin layer of sandy soil accumulated 3.5 kg total S ha-1. The three soils had slight but significant pH decreases near the source, and they all had much less sulphate compared to total S increase. Lysimeters were set at 9 sites for a 17-month period and wet deposition of S near the SO2 emitter was < 2 kg S ha-1. A sandy Brunisol had an estimated 19 kg total S ha-1 increase by dry deposition while there was an increase of 56 kg for a sandy loam Luvisol when comparing the site nearest the emitter to the most remote. In all, dry S deposition in the vicinity of the S source was approximately 5 to 70 times greater than the S deposition in rain and snow. Dry deposition S was found mostly as total S instead of sulphate and slight decreases in soil pH occurred. In the laboratory, four ground soils were exposed to an air stream containing 185 micrograms SO2 m-3 of SO2 for 30 days in a 0.15 m3 chamber. The soils sorbed from 23 to 77 micrograms sulphate-S g-1 soil with slight pH depression. In a similar experiment, blocks of intact lichen (5 cm deep) over sand (2.5 cm deep) received an air stream with 265 micrograms SO2 m-3 for 7 days. The soil layers retained 2.6 kg total S ha-1 with little of it as sulphate. Injection of SO2 (200 micrograms S g-1 soil) into closed 1-liter vessels over one 15 minute period or over 40 days (5 micrograms S g-1 day-1), substantially decreased the pH of two soils (range of 0.6 to 1.3) whether the soils were dry or wet and essentially all of the injected SO2 was sorbed by both the short concentrated and long dilute exposures.