TRENDS IN SURFACE-WATER ACIDIFICATION AT ECOLOGICAL MONITORING SITES IN SOUTHEASTERN CANADA (1981-1993)

Atmospheric deposition and surface water chemistry have been monitored intensively at 5 geologically ''sensitive'' sites in southeastern Can ada. The sites receive differing acid inputs that span the entire rang e found in Canada. Surface water data collected at 9 stations from 198 1 to 1993 for SO42-, NO-3, Alkalinity, DOG, pH, Ca2+ and Mg2+ have bee n analyzed to detect monotonic trends. Similarities between the tempor al patterns and trends for SO42- in deposition and surface water sugge st that they are strongly linked at our sites. Our 13-year datasets sh owed significant negative SO42- trends at the 3 Ontario sites and a po sitive trend in Nova Scotia. A climatically-induced SO42- increase in northwestern Ontario has been reversed. Mobilization and export of ads orbed SO42- and/or reoxidized S from the basins of central Ontario sit es is delaying their recovery. Two of our 9 stations (in Quebec and ce ntral Ontario) are continuing to acidify. The 2 Nova Scotia stations h ave the highest DOC levels and both exhibit a decreasing trend. Ionic compensation for declining SO42- varies from station to station, somet imes involving an Alk increase, sometimes a base cation decrease, and sometimes more complex combinations. Additional factors (e.g. climatic variation) also influence variable trends, and data records longer th an those presently available will be needed to unequivocally verify ac idification recovery.