Sources of sulphate and acidity in wetlands and lakes in Nova Scotia

Abstract

There is a declining gradient of wet SO₄ deposition from south to north in Nova Scotia with the highest values being in the south, along with a localized increase around the Halifax metropolitan area, due to local SO₄ emission. Edaphic conditions such as drainage from soils containing gypsum or drainage on disturbed rocks containing pyrite, provide additional SO₄ to surface waters.Acidity is usually absent in the former (pH > 7.0) and very high in the latter (as low as pH 3.6). By contrast peaty, organic drainages release water low in SO₄ during the growing season but they release high amounts of organic anions (A−), consequently, these waters maintain decreased pH values, usually < 4.5. A study of over 80 wetlands and lakes during the ice free period in Nova Scotia showed that sea salt corrected SO₄ concentrations range from 45 ueq L⁻¹ in the south end of the province, ∼30 ueq L⁻¹ in the Kejimkujik area and < 17 ueq L⁻¹ in the northern areas with values > 85 ueq L⁻¹ in the Halifax area, reflecting the atmospheric deposition pattern of SO₄ The SO₄ concentrations may be > 2000 ueq L⁻¹ in drainages containing gypsum, > 700 ueq L⁻¹ in drainages over pyrite bearing socks but < 20 ueq/L⁻¹ in streams draining bogs. The SO₄ concentrations change considerably during the non-growing season when the ground is saturated with water or frozen, and the runoff is high (snow and rain often alternate in winter). Under such conditions SO₄ concentration drops in the two former cases and increases in bog drainages, accompanied with a considerable drop in (A−) concentrations. Care should be taken when interpreting SO₄ concentrations in surface waters in Nova Scotia with respect to atmospheric SO₄ deposition.