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A dynamic model based on a simple nonlinear relationship between adsorbed and dissolved sulfate in soils is used to examine the implied long-term control exerted on drainage water sulfate concentration by mineral soil adsorption. The model assumes reversible adsorption and does not consider biological sulfur transformations or variable adsorption arising from changes in soil organic matter or pH. The model is used to simulate the effects of more than a century of increased atmospheric sulfur deposition into low-order, mountainous catchments. The response time of simulated drainage water sulfate concentration is a function of the hydrological retention time and the amount of sulfate adsorbed on the soil. Simulated catchments with small adsorption capacity respond at essentially the hydrological response time (months to a year). Simulated catchments with large adsorption capacity respond much more slowly (decades). These response times decrease dramatically as adsorbed sulfate accumulates. There has been a tendency to classify catchments as those that respond to changes in sulfur deposition and those that do not. Our results suggest that (at least for low-order, shallow soil catchments) responses to sulfur deposition are continuous, not dichotomous, and response times will decrease under prolonged atmospheric deposition. [ABSTRACT FROM AUTHOR]

Research in recent years has led to conceptualizations of the long-term responses of catchment surface water quality to acidic deposition. That research has focused attention on certain soil processes as likely keys to catchment responses (anion retention, cation exchange, primary mineral weathering, aluminum dissolution, and CO2 solubility). We present a mathematical model which uses quantitative descriptions of these soil chemical processes to estimate the long-term chemical changes that occur in the soil, soil water, and surface waters of catchments in response to changes in atmospheric deposition. The model is applied to a small forested catchment in the Shenandoah National Park, Virginia. Historical changes in surface water quality are reconstructed for the catchment for the last 140 years. The model indicates that alkalinity of surface waters in the catchment may have been reduced by as much as 50%. Water quality is forecast for the catchment under three different scenarios of future changes in atmospheric deposition. The model indicates that all but very large reductions in deposition will result in further deterioration of the catchment water quality. The process-oriented, lumped-parameter approach used is consistent with all currently available observations of water quality in the catchment. Due to the lack of long-term records of catchment water quality, strict verification of the model estimates and an assessment of the model validity is problematic. This is the case for all models of long-terrn catchment chemical responses to acid deposition. Nonetheless, the model provides a means of integrating the results of individual process level laboratory and field studies. Used this way, the model becomes a vehicle for examining the interactions and long-term implications of our conceptualization of the acidification process. [ABSTRACT FROM AUTHOR]

A technique is presented for using light-cured composite resin to create desirable contours on abutment teeth for the retention and support of removable partial dentures. The desired tooth shape is created on a diagnostic cast, captured in a clear temporary splint material, and formed in resin on the abutment teeth with the splint acting as a matrix.