Nature of natural organic matter (NOM) in acidified and limed surface waters

Abstract

Natural organic matter (NOM) was isolated by reverse osmosis (RO) from the two inlet brooks and the outlet of Lake Terjevann with estimated recoveries of 85–90%. The lake has been receiving liming treatments since 1980 due to the acidic drainage from the catchments. High-performance size exclusion chromatography (HPSEC) analyses of the RO samples and of ultrafiltered fractions indicate that there are five main size fractions of carbon with molecular weights between 410–1170 Da in all three samples. However, the molecular weight distributions differ significantly in that the outlet sample have lower proportions of larger molecular weight NOM than do the inlets. Studies of spectral absorption coefficients at 254 nm support this molecular size distribution. Absorption coefficients of the samples at both 254 and 410 nm decrease with decreasing molecular weight so that the ratio E 2/ E 4 increases with decreasing molecular weight. This trend appears independent of pH changes and especially effects the absorption coefficients of the higher molecular weight NOM. Studies of C:N ratios of the RO samples and of ultrafiltered fractions indicate that the outlet RO sample and that the lower molecular weight NOM are enriched in nitrogen. Supported by studies of spectral absorption coefficients and C:N ratios of the total RO samples and ultrafiltered fractions and effects of pH changes on the ultrafiltered fractions, it is hypothesized that in-lake processes are removing larger molecular weight NOM. This is possibly an effect of the liming which increase pH and calcium(II) concentrations in the lake water coinciding with hydrolysis and precipitation of aluminium(III) and iron(III).