Behavior of organic compounds during infiltration of river water to groundwater. Field studies

Abstract

The behavior of organic micropollutants during infiltration of river water to groundwater has been studied at two field sites in Switzerland. In agreement with predictions from model calculations, persistent organic chemicals exhibiting octanol/water partition coefficients smaller than about 5000 moved rapidly with the infiltrating river water to the groundwater. The biological processes responsible for the “elimination” of various micropollutants (e.g., alkylated and chlorinated benzenes) occurred predominantly within the first few meters of infiltration. Alkylated benzenes were “eliminated” at faster rates than 1,4-dichlorobenzene. Anaerobic conditions in the aquifer near the river hindered the biological transformation of 1,4dichlorobenzene. Among the compounds that were found to be persistent under any conditions were chloroform, l,l,l-trichloroethane, trichloroethylene, and tetrachloroethylene. With respect to such chemicals, bank filtration is ineffective as a first step in the treatment of river water for water supplies. Since in many European countries a significant fraction of the groundwater is recharged through infiltration of river water (1,2), the impact of river pollution on groundwater quality is of major concern. In addition, many waterworks use natural or artificial bank filtration as a first step in the treatment of river water for water supplies (3, 4). Therefore, the behavior of organic pollutants during infiltration is of great interest. The transport and fate of organic pollutants in a river water-groundwater infiltration system is determined by several interacting processes, including advection, dispersion, (ad)sorption/desorption, hydrolysis, redox reactions, and biological transformations. In laboratory experiments, individual processes may be studied under controlled conditions (5,6), and mathematical models may be developed to predict the effect of a particular process on the transport and fate of a compound in the environment (7,8). However, comprehensive field investigations are needed to evaluate the applicability of laboratory studies and model calculations to natural systems. To date, most of the field studies on natural river water-groundwater infiltration systems have been conducted with respect to the use of bank filtrate for public water supplies (e.g., ref 3). These studies have usually been confined to monitoring selected water constituents in the river and in groundwater wells near the region of infiltration. The temporal and spatial variations in concentration of organic compounds along the infiltration path have not been thoroughly investigated. Consequently, the results of such investigations provide only very limited insights into the behavior of individual compounds during infiltration. In this paper, we report the results of two field studies aimed at investigating the transport and fate of organic micropollutants, including chlorinated hydrocarbons, alkylated benzenes, and chlorinated phenols during natural infiltration of river water to groundwater. In the near