Field study of tracer movement in a shallow alluvial aquifer

A field study was performed to observe movement of a conservative bromide tracer in an unconfined, shallow aquifer. The data collected were used to estimate the apparent hydraulic heterogeneity at the test site located in the Texas Southern High Plains. Although the hydraulic gradient does affect flow, anisotropy of transmissivity had a major effect on the speed and spreading of the tracer plume. The USGS method of characteristics model was used to assist in describing the geologic conditions of the test site. The transmissivity values were varied across the model grid used to discretize the area in order to match the model results to the field data. Results showed that zones of high permeable soil existed in close proximity with low permeable soils, and this situation could increase the mobility of a solute. Significant layering of geological materials prevented the calibrated two-dimensional model from perfectly describing the field data.