The geochemical mass-balance approach has been one of the more eective ways of interpreting biogeochemical processes in watersheds (catchments). Waters passing through a watershed interact with watershed materials, the vegetation, soils, regolith and bedrock, and reΒ―ect these interactions by changes in chemical composition and in other characteristics. The 18th century French chemist Lavoisier referred to natural waters as ``the rinsings of the earth''. This is an apt description, as all of the various contacts and reactions that water undergoes in the watershed are integrated in the solutions draining the watershed. The Β―ux of water and accompanying dissolved and suspended materials into and out of the watershed provides a frame of reference within which biotic and abiotic information can be interwoven to understand watershed ecosystem processes. Since Garrels and Mackenzie's 1967 application of the geochemical mass-balance approach to interpreting weathering reactions and their eects on the chemical composition of Sierra Nevada waters, the method has been widely applied in watershed biogeochemical research.
The papers in this volume describe the current status of the method and explore how it has been applied in some recent studies. Topics address the assessment of atmospheric deposition inputs to watersheds, methods used to quantify solute exports from watersheds, rates of mineral weathering and its eect on stream chemistry in forested systems and on bare rock, biogeochemical cycling and Β―uxes of nitrogen and sulfur in watershed systems, and application of the geographic information approach to watershed mass-balance studies. The collection of papers in this volume, although only a small sample, gives a sense of the variety of mass-balance applications in watershed and catchment studies and attests to their primary importance in elucidating the complex biogeochemical processes that govern the functioning of these systems. The composition of natural waters and the impacts of man's activities on water quality are strongly dependent on watershed processes. We hope that this volume will serve to extend our understanding of those processes and encourage new innovative applications of the mass-balance approach to the study of environmental systems.