Academic journal article Journal of Geoscience Education

A Simple Sand Column Laboratory Exercise to Illustrate Pollutant Hydrology in Groundwater Systems

Academic journal article Journal of Geoscience Education

A Simple Sand Column Laboratory Exercise to Illustrate Pollutant Hydrology in Groundwater Systems

Article excerpt

ABSTRACT

The transport of pollutants in groundwater systems is an important concept taught in contaminant hydrology. Yet, surprisingly, no relevant soil column experiments have been described in the pedagogical literature (Journal of Chemical Education, Chemical Education Research and Practice, or Journal of Geoscience Education). This experiment illustrates the transport of tracers in a sand column using the conservative (non-retained and non-reactive) tracer fluorescein, along with a step input of non-conservative (retained/adsorbed) cadmium ion. The experiment is easy to set up, uses inexpensive commercially-available glassware and sand, and is highly reproducible. Step inputs of conservative tracer elute in one pore volume (the volume needed to completely replace the water held in the saturated column), with cadmium eluting in three pore volumes. This experiment can be completed in two, three-hour laboratory periods.

INTRODUCTION AND PURPOSE

Cleanup of polluted groundwater sites is a primary goal of EPA s Superfund budget. While pollutant transport is extensively covered in groundwater modeling lectures, it is completely absent in published laboratory exercises. This is true of most experiments involving soils or sediments, as noted by Tran et al. (2001), who found that of the 92 environmental articles published in the Journal of Chemical Education from 1969 to 2000, only approximately 15 percent pertained to soil matrices. Iran et al. (2001) further notes the persistence of a "perception that ... soil chemistry is more complex and challenging than water chemistry". Indeed, the relative lack of simple soil experiments may result from the variety of complex chemical and physical properties that can be present in soil and sediment samples (i.e. mineralogy, surficial coatings, organic matter, pH values, and EH values). Even ifan educator develops and publishes an excellent soil experiment, others who follow the published procedure, inevitably with different soils, will likely ootain widely varying results.

Topics covered by the few relevant environmental experiments from the literature that involve soil matrices include various sorption reactions (Xia et al., 2003; Dolan et al., 1998), soil remediation projects (Roundhill, 2004; Harle et al., 2003), and the interaction of acid rain with soil weathering (Schilling et al., 2004). We describe two new laboratory experiments using soil matrices in our manual on instrumental and environmental chemistry (Dunnivant, 2004). One is an experiment to determine the distribution coefficient of a metal in a soil solution. Distribution coefficients are chemical parameters commonly used in modeling the fate and transport of pollutants in groundwater systems. In the second experiment, we describe the Soxhlet pollutant extraction procedure, in which commercially available sand provides the needed analyte recoveries and reproducibility. The new experiment described in this teaching exercise describes a very useful experiment for illustrating the fate and transport of a non-retained and retained chemical tracer in a simulated groundwater system.

The overall goals of these laboratory exercises are to experimentally illustrate fundamental concepts of contaminant hydrology, specifically step inputs of tracers, pollutant distribution coefficients and retardation factors, and pollutant breakthrough curves.

RATIONALE AND APPROACH

The lead author of this article (Dunnivant) has worked on two major groundwater laboratory and field investigations for the Department of Energy (DOE). When presenting the findings of these invesh'gations at national and international meetings, he was asked why there is not a teaching laboratory exercise for these important fate and transport concepts. The answer is simple-it is very difficult to find a pollutant and a geologic media that utilizes these concepts and allows the experiment to fit into the standard three-hour teaching laboratory time slot. …

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