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

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. Highperformance size exclusion ch romatography (HPSEC) analyses of the RO samples and of ultrafiltered fracti ons indicate that there are five main size fractions of carbon dth molecula r weights between 410-1170 Da in all three samples. However. the molecular weight distributions differ significantly in that the outlet sample have lo wer proportions of larger molecular weight NOM than do the inlets. Studies of spectral absorption coefficients at 154 nm support this molecular size d istribution. Absorption coefficients of the samples at both 254 and 410 nm decrease with decreasing molecular weight so that the ratio E-2/E-4 increas es with decreasing molecular weight, This trend appears independent of pH c hanges and especially effects the absorption coefficients of the higher mol ecular weight NOM. Studies of C:N ratios of the RO samples and of ultrafilt ered fractions indicate that the outlet RO sample and that the lower molecu lar weight NOM are enriched in nitrogen. Supported by studies of spectral a bsorption coefficients and C:N ratios of the total RO samples and ultrafilt ered fractions and effects of FH changes on the ultrafiltered fractions, it is hypothesized that in-lake processes are removing larger molecular weigh t NOM. This is possibly an effect of the timing which increase pH and calci um(II) concentrations in the lake water coinciding with hydrolysis and prec ipitation of aluminium(III) and iron(III). (C) 1999 Elsevier Science Ltd. A ll rights reserved.