Atmospheric Deposition of Nutrients and Major Ions at the Experimental Lakes Area in Northwestern Ontario, 1970 to 1982

Abstract

Up to 24 chemical parameters have been monitored in individual precipitation events captured in bulk collectors at the Experimental Lakes Area (ELA), from 1970 to 1982. Although sample to sample variation was high, seasonal trends were discernible for several ions. Annual mean concentrations of H⁺, NO₃⁻, and SO₄²⁻ were low and quite constant until 1981, but increased dramatically in 1982. Hydrogen ion concentration increased by an average of 0.63 μEq∙L⁻¹ over this period. The order of annual average cation dominance was Ca²⁺ > NH₄⁺ > H⁺ > Mg²⁺ > K⁺. The dominant anion was SO₄²⁻, followed by NO₃⁻ > Cl⁻. Calcium, Mg²⁺, SO₄²⁻, Si, suspended P and N, total dissolved P and N, and total ionic concentrations were greatest in spring and fall, correlating with periods of maximum agricultural activity on the prairies, over 250 km away. Concentrations of NO₃⁻ and H⁺ were higher in snow than in rain, while SO₄²⁻, Ca²⁺, Mg²⁺, and NH₄⁺ were greater in rain. The total deposition of most substances was higher in summer than in winter, because of the greater proportion of annual precipitation occurring in summer months. A comparison with other sites revealed higher Ca²⁺ and NH₄⁺ at all sites near agricultural areas, and higher H⁺, SO₄²⁻, and NO₃⁻ at sites in eastern North America. A comparison of 1980 bulk versus wet-only chemistry indicated that dryfall contributed 52% of the total annual deposition of Cl⁻, 22% of the SO₄²⁻, 0% of the NO₃⁻, 48% of the Mg²⁺, 31% of the NH₄⁺, 28% of the Ca²⁺, and 8% of the Na⁺. Charge balances were excellent for both wet and bulk samples.