Effects of land cover and geology on stream chemistry in watersheds of Chesapeake Bay

We measured the base-flow stream chemistry in all the major physiographic provinces of the Chesapeake Bay drainage basin. The spatial variation of stream chemistry was closely related to differences in geology and land cover among the sampled water-sheds. Some stream chemistry variables were strongly affected by geological settings in the watersheds while others were more influenced by land cover. The effects of land cover differed among chemical constituents and regions. Concentrations of Ca2+, Mg2+, pH, total alkalinity, and conductivity were mainly functions of carbonate bedrock, especially in the Great Valley. Nitrate-N and total dissolved N were closely related to cropland and increased as the percentage of cropland increased. The rate of increase varied from region to region with the highest in the Piedmont. Na+ and Cl-were mainly affected by the percentage of developed area in a watershed, especially in the Coastal Plain and Piedmont. We observed no significant effects of region or land cover on species of phosphorus because samples were collected under base now conditions and only dissolved forms were measured. Dissolved silicate (DSi) was not related to any other water chemistry variables. DSi increased as developed area decreased and cropland increased in the Coastal Plain, but these patterns were reversed in the Piedmont. There was no consistent pattern in the spatial variation of land cover effects on the reduced forms of N, dissolved organic P, dissolved organic matter, and K+.