PHYSICOCHEMICAL COMPUTER SIMULATION OF GROUNDWATER QUALITY: FUNDAMENTALS. FLUORINE-BEARING WATERS

Abstract

A method is proposed for the physicochemical computer simulation of rock-water systems that can be used in predicting the changes that take place in the composition of fresh water in the course of hydrogeochemical processes. The main physicochemical factors controlling the composition of natural waters are variations in the proportions of the reacting masses of the rock and water (R/W), the thermodynamic openness-closeness of the systems with respect to CO2 and O 2, the extraction rate of Cl and organic carbon from the rocks, and the temperature (pressure). The method of physicochemical computer simulation makes it possible to reproduce the physicochemical parameters under which the composition of natural waters was formed and can be utilized to predict variations in the water chemistry under the effect of natural and anthropogenic factors. Based on the synthesis of systematic temporal observations of groundwater quality at their currently exploited basins and the thermodynamic physicochemical simulation of geochemical processes that determine the main compositional features of groundwaters, we have identified (a) the geochemical processes that control an increase in the F concentrations of groundwaters during exploitative water withdrawal and the associated lowering of the groundwater level in the aquifer and the partial draining of the latter; and (b) the causes that made earlier predictions unreliable. The thermodynamic modeling of geochemical processes in systems of F-bearing rocks plus groundwater indicates that any lowering of the groundwater table results in the metamorphism of the water composition into HCO3-Na, Ca-free waters with high Na/Ca ratios. An increase in these ratios results in hydrogeochemical systems highly disequilibrated with respect to CaF 2. Their disequilibrated state is leveled off by geochemical processes that transfer significant F masses into the groundwaters. If geochemical phenomena of this type are ignored when the groundwater level lowers, erroneous predictions of the variations in the water quality during the exploitation of groundwater basins will be made which will result in the general worsening of the ecological state of the environment.