The use of calibrated lakes and watersheds for estimating atmospheric deposition near a large point source

Abstract

We have measured the input and output rates of substances to and from both lakes and watersheds in the Sudbury and Muskoka-Haliburton areas of Ontario. At the former location, we have conducted mass balance studies on 5 lakes and their watersheds for 2½ yrs. At the latter site, we have measured mass balances for 6 lakes and about 30 individual watersheds for the past 5 yrs. Substances studied included SO₄ ²⁻, NO₃ ⁻, NH₄ ⁺, H⁺, major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) and HCO₃ ⁻.

During the course of the investigation at Sudbury we have made several observations that indicate that the inputs of some substances, specifically SO₄ ²⁻ or SO₄ ²⁻-precursors and strong acids, to lakes and watersheds are underestimated when measured as bulk deposition (i.e. by collection in a continuously open container):

(a) The output of SO₄ ²⁻ from the calibrated watersheds was substantially greater than the input measured as bulk deposition.

(b) The SO₄ ²⁻ concentrations of the lakes could not be explained on the basis of the measured inputs. An additional input directly to the lake surface was needed to obtain a mass balance.

(c) The net input of acids measured as bulk deposition to the watersheds was much less than the acid consumed, which was estimated by the net output of Ca²⁺, Mg²⁺, Na⁺, K⁺, Al³⁺, and the net retention of NO₃ ⁻.

(d) The major cation content of the study lakes could be explained on the basis of weathering reactions in the lakes' watersheds only if the input of strong acid had been underestimated.

When these observations were quantified, they indicated a major portion of the total input of SO₄ ²⁻-precursors and of strong acid was not included in our bulk deposition measurements. Deposition of SO₂ is the most likely explanation for these observations.