SULFATE IN COLORED WATERS .2. EVALUATION OF APPROACHES FOR CORRECTING HISTORICAL COLORIMETRIC DATA

Historical colorimetric sulfate data are known to be biased high and cannot be used as reliable SO//4**2** minus concentrations for scientific interpretation. They need to be corrected. The various approaches for correcting these data were discussed and evaluated. A simple and direct approach was found to be satisfactory. To correct historical colorimetric data, data sets are first categorized and grouped together based on geographical locations and seasonal variations of dissolved organic carbon (or color) of a specific site, river, lake (or groups) which have similar amounts and nature of organic matter. For each group, the colorimetric and chromatographic sulfate values are then obtained and related by an equation, which is used to convert the corresponding historical values to 'true' sulfate values.