Zero-tension lysimeters: an improved design to monitor colloid-facilitated contaminant transport in the vadose zone

摘要

There is increasing evidence that mobile colloids facilitate the long-distance transport of contaminants. The mobility of fine particles and macromolecules has been linked to the movement of actinides , organic contaminants, and heavy metals through soil. Direct evidence for colloid mobility includes the presence of humic materials in deep aquifers as well as coatings of accumulated clay, organic matter, or sesquioxides on particle or aggregate surfaces in subsoil horizons of many soils. The potential for colloid-facilitated transport of contaminants from hazardous-waste sites requires adequate monitoring before, during, and after in- situ remediation treatments.A lysimeter is a device permanently installed in the soil to sample soil water periodically. Zero-tension lysimeters (ZTLs) are especially appropriate for sampling water as it moves through saturated soil, although some unsaturated flow events may be sampled as well. Because no ceramic barrier or fiberglass wick is involved to maintain tension on the water (as is the case with other lysimeters), particles suspended in the water as well as dissolved species may be sampled with ZTLs. Conventionally, ZTLs consist of shallow pans or troughs that are inserted laterally into the soil from an access pit or trench. But conventional design and installation of ZTLs leads to a number of problems. First, digging access pits or trenches to depths appropriate for sampling subsurface materials may be impractical or prohibitively costly. Second, disturbance of trench walls by digging equipment (e.g., smearing of the pit walls; fractures induced by the jarring of backhoe buckets) may alter physical conditions and limit interpretations and predictions for unsampled sites. Finally (and most importantly), the time and space required to install a conventional ZTL may place limitations on the number of replications possible at a given site and thereby limit appropriate monitoring of spatial variability .To address these problems, a ZTL design is proposed that is more suitable for monitoring colloid-facilitated contaminant migration. The improved design consists of a cylinder made of polycarbonate or polytetrafluoroethylene (PTFE) that is placed below undisturbed soil material. In many soils, a hydraulically powered tube may be used to extract an undisturbed core of soil before placement of the lysimeter. In those cases, the design has significant advantages over conventional designs with respect to simplicity and speed of installation. Therefore, it will allow colloid-facilitated transport of contaminants to be monitored at more locations at a given site.Zero-tension lysimeters are intended to capture samples of suspended colloids as they move in the vadose zone. The specific benefits of the proposed design are related to the simplicity of installation and the concomitant reduction of cost of monitoring. Because more ZTLs of the proposed design than of the conventional design can be installed to assess site variability, more accurate monitoring of contaminant transport before, during, and after remediation will be achieved. In addition, because of the improved spatial resolution in sampling and monitoring, if contaminants are mobilized, their source can be more easily identified than with piezometer-based monitoring methods.The improved zero-tension lysimeters described in this proposal have been installed at five sites contaminated with americium and plutonium at Rocky Flats Plant, Golden, Colorado. Rainfall simulation experiments have been performed to test the ability of the improved lysimeters to intercept mobile colloids and actinides and to compare their behavior with that of conventional zero-tension lysimeters.Zero-tension lysimeters have been installed near Ames, Iowa, where colloid- facilitated transport of heavy metals (Cu, Ni, Cd, and Zn) in municipal sewage sludge amendments is monitored. in water samples collected from the lysimeters, suspended colloid concentrations, heavy metals, dissolved organic carbon, electrical conductivity, and pH are determined. Rainfall simulation experiments designed to simulate the intensity of typical summer storms will be completed along with transport simulation trials designed to identify the initial pH and ionic strength levels of soil water that are most conducive to mobilization of organic and phyllosilicate colloids and associated metals.