Long-term and seasonal variations in CO
نویسندگان
چکیده
As atmospheric emissions of S have declined in the Northern Hemisphere, there has been an expectation of increased pH and alkalinity in streams believed to have been acidified by excess S and N. Many streams and lakes have not recovered. Evidence from East Bear Brook in Maine, USA and modelling with the groundwater acid-base model MAGIC (Cosby et al. 1985a,b) indicate that seasonal and yearly variations in soil PCO 2 are adequate to enhance or even reverse acid-base (alkalinity) changes anticipated from modest decreases of SO 4 in surface waters. Alkalinity is generated in the soil by exchange of H from dissociation of H 2 CO 3 , which in turn is derived from the dissolving of soil CO 2 . The variation in soil PCO 2 produces an alkalinity variation of up to 15 meq L in stream water. Detecting and relating increases in alkalinity to decreases in stream SO 4 are significantly more difficult in the short term because of this effect. For example, modelled alkalinity recovery at Bear Brook due to a decline of 20 meq SO 4 L in soil solution is compensated by a decline from 0.4 to 0.2% for soil air PCO 2 . This compensation ability decays over time as base saturation declines. Variable PCO 2 has less effect in more acidic soils. Short-term decreases of PCO 2 below the long-term average value produce short-term decreases in alkalinity, whereas short-term increases in PCO 2 produce shortterm alkalization. Trend analysis for detecting recovery of streams and lakes from acidification after reduced atmospheric emissions will require a longer monitoring period for statistical significance than previously appreciated.
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