Terrestrial and In-Lake Contributions to Alkalinity Budgets of Drainage Lakes: An Assessment of Regional Differences

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Abstract
Using two independent procedures, we estimate relative contributions of in-lake alkalinity generation to total basin alkalinity budgest for drainage lakes in selected regions of the Eastern United States. The first method is based on the assumption of approximately equal areal rates of alkalinity production by watersheds and lakes, and uses watershed area to lake area ratio as a direct estimator of relative akalinity contributions. The second approach is based on existing models for in-lake alkalinity generation estimated from sulfate retention. Projections from the two methods are comparable, and show that for most drainage lakes in Northeastern United States, Southern Blue Ridge Province, and Upper Midwest, in-lake alkalinity generation is a minor contributor to net basin alkalinity production, contributing .ltoreq. 7% of total net basin alkalinity production in typical watersheds in any of the regions. The close agreement between the two procedures lends support to the use of watershed area to lake area ratio as a convenient estimator of relative alkalinity sources requiring only map-derived parameters. Regional assessment of current or future effects of acidic deposition on surface water chemistry in the Northeastern United States, the Southern Blue Ridge Province or other areas dominated by drainage lakes with high watershed to lake area ratio would not appear to be seriously comprised by use of models limited to consideration of terrestrial processes. In areas such as the Upper Midwest which are characterized by high proportions of seepage lakes or for lakes with long hydrologic residence times, in-lake processes and hydrologic influences must be considered.