Compilation of catchment mass balance studies in the eastern US through the 1980's suggested that Ca was the forest nutrient most susceptible to long-term depletion. In the 1990's, efforts were made to improve the accuracy of estimates of weathering flux, perhaps the most critical uncertainty in mass balance determination. Isotopic studies of Sr have been used to refine mass balance estimates in study areas where Sr isotopic composition of atmospheric inputs and weathering products are distinct. Alternatively, characterization of the Ca mass balance has been clarified by study of Na, a better marker of weathering reactions due to its limited participation in biomass uptake and soil exchange processes. At the Hubbard Brook Experimental Forest and Cone Pond Research Watershed, NH, where these techniques have been applied, atmospheric inputs and weathering flux are insufficient to account for hydrologic losses of Ca, implying depletion of available pools. Corresponding changes in soil quality have not been confirmed by direct measurement of soils. There are several possible reasons for this apparent discrepancy.