Seismic Refraction Case Studies At Environmental Sites

Seismic refraction was used to investigate two sites with groundwater contamination.<br>Although seismic refraction is not a viable technique for directly locating the<br>contaminants, it can provide very useful geologic information. The ground water at both<br>of these sites is contaminated with dense, non-aqueous phase liquids (DNAPL) that sink<br>through the groundwater until reaching an aquitard, a geologic layer that is relatively<br>impermeable. Seismic refraction was used on both of these sites to map the surface of<br>the aquitard and locate low areas in which DNAPLs might accumulate. Tricloroethene<br>contamination is present in the groundwater of a military installation located in a karst,<br>limestone setting. Fractures and solution cavities in the limestone dominate the<br>hydrogeology. The surface of the limestone is quite uneven as a result of dissolution and<br>glacial action. We collected detailed seismic refraction data in the region surrounding the<br>source to map the elevation of the limestone and locate low areas where the DNAPLs<br>might collect. The velocities of the limestone were also useful for indicating where the<br>most intense dissolution had created fractures and cavities. A mixture of several<br>DNAPLs contaminated the groundwater at the second site. There are no consolidated<br>sediments within 100 feet of the surface, but a very dense unconsolidated aquitard<br>separates the contaminated sand and gravel aquifer and the lower, clean aquifer used for<br>groundwater supply. Several techniques, ground probing radar, resistivity and seismic<br>refraction were used on this site, but seismic refraction was the most effective at locating<br>low spots in the clay layer. The seismic refraction data also revealed several areas with<br>very coarse-grained gravel that could serve as pathways for contamination to leave the<br>site.