IS AL CHEMISTRY THE KEY TO LAKE TROPHIC STATUS IN ACID RAIN-SENSITIVE GEOLOGIC TERRANES

Pearce (2000) studied two oligotrophic lakes in western Maine with pH near 6. Both had anomalously low water-column Total P. Both watersheds are in granitic bedrock. We hypothesize that the low P, and their oligotrophic status, may be controlled by Al chemistry as described below. Al is soluble in acidic conditions of well-drained forest soils. High concentrations of dissolved organic carbon (DOC) commonly produce a soil pH in the forest floor near 4. Descending soil water undergo increases in pH from loss of the acidic DOC, chemical weathering, and ion exchange of H+ for base cations and Al. Exchangeable Al is mobilized if the pH is depressed. Al is then transported downward and either deposited as Al(OH)3,am or transported as ionic Al to surface waters. P is strongly adsorbed by the amorphous Al(OH)3. Degassing of CO2 in surface water increases the pH, precipitating Al(OH)3 (Norton et al., 1983). Roy et al. (1999) demonstrated that acidification of a watershed was accompanied by increased export of P adsorbed on particulate Al(OH)3,am. Al may control P in lakes (Kopacek et al., 2000) through formation of insoluble Al-P phases, or possibly by adsorption of P onto Al(OH)3,am. Such reactions form the basis for tertiary treatment of wastewater. Stripping of Al from lake water would be most effective for lakes with pH 5.5 or higher, with inlet streams with pHs in the low- to mid-5s. Such streams would be derived from groundwater with pH in the high 4s. Ground water with pH>5.5 would not mobilize sufficient Al for this mechanism to be important. In the absence of acidic deposition, groundwater in granitic terrane may have a low enough pH to mobilize modest amounts of Al from soils. Lake systems receiving high atmospheric inputs of acid and with initially higher alkalinity may also be impacted because of the titration of alkalinity by strong acid and the consequently increased mobilization of Al from soil to surface water.