Controls on surface water chemistry in the southern Blue Ridge and Piedmont provinces

Abstract

Using lake and stream data collected during the National Surface Water Survey (NSWS), and corrected for regional wet deposition and local cultural contamination, the role of weathering in supplying base cations CB, silica, and alkalinity is evaluated for the southern Blue Ridge (SBR) and southern Piedmont (SPM) Provinces of the United States. Silicate weathering can account for virtually all of the non-cultural (Na+K) leaving the catchments, consistent with the weathering of feldspars to kaolinite, and in some dilute montane catchments, gibbsite. The net export of divalent CB (Ca+Mg) represents 62 to 66% (median values) of total CB export for the regional NSWS index sample populations; the alkalis account for the remainder. The median percentage (Ca+Mg) is significantly lower (53%) for SBR special sites, consistent with their unusual vulnerability to acid deposition. Exchange-leaching currently accounts for < 17% of the divalent CB mobilized in these SBR and SPM catchments, possibly because of the high regional retention rates for both nitrate and sulfate.