An operational model for managing the effect of land treatment of wastewater on groundwater quality

Abstract

Some land uses, such as land treatment of wastewater, could be operated for protection of the underlying groundwater if appropriate monitoring information were available. It is suggested that leachate quality in the vadose zone be monitored for operational management, by means of lysimetry, in preference to observations of groundwater quality with transport lag of months or years. Leachate data comprise values of solute concentration and associated increments of soil drainage flux, often collected at irregular time intervals, which are highly variable in space and time. These characteristics pose a data processing problem for operational management analogous to the requirement for the techniques of statistical process control in the manufacturing industries. For credible management of potential environmental effects, it is desirable that the quality assurance technique is based on concepts of physical processes which can be openly debated in terms of assumptions and model parameter values. This paper describes a data processing method based on one-dimensional solute transport through the vadose zone to the groundwater surface and subsequently by streamtubes in the groundwater to the designated target region. Smoothing of the leachate data. for real-time comparison wilh maximum allowable values, is governed only by longitudinal dispersion within the vadose zone and streamtubes, The concept of stream tubes without transverse dispersion addresses issues of equitable use of allowable aquifer contamination by land use managers. The computational algorithm for continual updating of management information is derived from the mixing cell concept for simulating dispersion. A demonstration example is presented using data on nitrate-N concentration and drainage flux of leachate from land treatment of meat processing wastewater, for feasible values of dispersivity in the vadose zone, and underlying aquifer.