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A Field Study of Solute Dispersion in a Shallow, Unconfined Aquifer¹

ABSTRACT

A natural gradient tracer test was performed to (i) characterize the patterns of variation of saturated hydraulic conductivity (K_s) and longitudinal dispersivity (_L) in a shallow, unconfined aquifer in a sandy soil on a sloping site and to (ii) examine scale effects on quantification of _L. Chloride tracer was injected into the saturated zone, and its downslope movement was monitored by means of multilevel samplers.

Results show that the mean value of K_s derived from the tracer test was in close agreement with values obtained independently on undisturbed core samples and from double-ring infiltrometer tests performed in situ. Results also indicate a nonuniform distribution of velocity on both vertical and horizontal transects at right angles to the mean direction of flow. Values of _L, calculated using a simplified solution of the two-dimensional advective-dispersive equation, show a nonuniform distribution of _L in the profile and reveal that increasing the scale of measurement vertically at right angle to the flow leads to larger measured values of dispersivity. The effect was observed with a change in scale from point measurements to averages taken over the 60- to 75-cm thickness of the profile's saturated zone. The frequency of sampling is also shown to have a significant effect on quantification of _L.

¹ This research was supported in part by the National Science Foundation (Grant no. CEE 80-18873).

² Consulting Engineer, Arthur D. Little, Inc., Cambridge, MA 02140; and Professor, Dep. of Plant and Soil Sciences, Univ. of Massachusetts, Amherst, MA 01003.

Received for publication June 26, 1981. Accepted for publication March 16, 1982.