Past, Present and Future of Geochemical Input-Output Budgets

Abstract

Geochemical mole-balance studies (also known as input-output budgets) invoke a simple conservationof-mass principle, the principle that 'some of it plus the rest of it equals all of it'. Quantitative molebalance modelling was introduced to geochemical literature by Gavels and MacKenzie (1967) in a study referred to in many textbooks; they presented a classical analysis of the natural processes responsible for the composition of springs and groundwater in a pristine area (Sierra Nevada, USA), which became known as the 'balance-sheet' method, and has been employed by numerous other workers. In the present paper we present a brief overview of mole-balance models (and associated environments) that have been used in the recent past, that are in use in the present, and which are under development for application in the near future. In studies of small and pristine watersheds, where most stream flow is fed by subsurface water percolating the soil and saprolite, the composition of springs and streams is essentially influenced by the atmospheric input plus the chemical and biological reactions occurring within the weathering profile (Taylor and Velbel, 1991). The budget for the watershed may be described by the equation: