Liming Dosages for Acid Lakes and Watersheds

Abstract

As Walski clearly states, computing liming dosages for acid lakes is not straightforward. Liming as a remedial action for neutralizing acid surface waters requires accurate dose calculations because of the large mass of neutralizing agent required and because of the ecological consequences of overdosing. A majority oflake neutralization projects have been done using limestone as the neutralizing agent. Limestone is a natural choice because of its availability and relatively low cost. One disadvantage of limestone is the relatively slow dissolution kinetics. This low dissolution must be taken into account when calculating the appropriate dosage for limestone, since some of the applied limestone does not have a chance to dissolve before it settles to the bottom of the lake. In some cases the settled limestone will eventually dissolve, and in other cases it will be covered with detritus or become coated with chemical precipitates. In the latter cases little or no acid neutralization is accomplished. Accurate computation of limestone dosages requires a knowledge of the dissolution kinetics. Therefore, considerable research has been aimed at delineating these kinetics for acid lakes. Of equal importance, but unfortunately of less research interest, is the procedure used to calculate final pH. Olem (1991) provides a summary of these dosage calculation procedures. The most sophisticated of these assume that the lake will be in equilibrium with atmospheric carbon dioxide after the neutralizing agent has been added. For example, the dosing models proposed by Scheffe et al. (1986), DePinto et al. (1989), and Driscoll et al. (1982) used the open-system assumption to calculate the resultant pH. Interestingly, many of the procedures for predicting limestone dissolution assume that dissolving limestone particles are in equilibrium not with an open, but rather with a closed system (Olem 1991). The purpose of this forum discussion is to suggest that a closed system is a more appropriate assumption for calculating target pH. The basis for this suggestion is a lO-year neutralization study conducted at Wolf Pond, a 21 ha dimictic lake located in the Adirondack region of New York. In this study Wolf Pond was neutralized with NaHC03 • Since NaHC03 is extremely soluble, dissolution kinetics did not obfuscate the observations. Results of the study showed clearly that most of the lake behaves as a closed system relative to atmospheric carbon dioxide. This is demonstrated in Figs. 1 and 2 which are plots of in-situ measured pH versus pH calculated using openand closed-system equilibrium assumptions. Computation of pH was based on: