INNOVATIVE USE OF A MODIFIED RISK-BASED SAMPLING APPROACH TO INVESTIGATE A NATIONAL PRIORITY SITE

A remedial investigation (RI) was conducted at a CERCLA site in southern California. A dual sampling approach was successful in characterizing the site. This approach facilitated the collection of representative characterization data comparable to that provided by a traditional RI or standard risk-based sampling approach. The case study is presented to highlight the success of the approach.

A sodium valve disposal area at an NPL site was investigated in 1994. The preliminary assessment concluded that valves and areas of soil contamination were not present. The site was eliminated from further investigation. Subsequently, several sodium-filled valves were discovered in a borrow area located near the suspected site. Other wastes included miscellaneous metal debris, Marston Matting and unidentified metal canisters. An RI investigation was initiated as a result of the discovery.

The major concerns were as follows: the site is approximately 5 acres in area; characteristics of the wastes was unknown; and the cost of sampling, analyses and investigation using a traditional RI approach would be prohibitive. As a result a modified risk-based sampling approach was developed to characterize the site.

A geophysical survey was conducted to identify anomalous areas. The site was then subdivided based on signature: areas where anomalies were not identified; anomalous areas identified using EM-31 in-phase (metal detection) data; and anomalous areas identified using EM-31 quadrature (conductivity) data. Portions of the site where anomalies were not identified were characterized using a statistical approach.

Eight anomalies were evaluated via trenching. Trenching proved to add value in several ways:

• trenches oriented approximately perpendicular to each other in the anomalies exposed long sidewalls for improved description of subsurface conditions,
• detailed identification of soil characteristics (or waste, if present) responsible for each anomaly was possible, and
• representative samples were collected from known soil horizons or wastes for chemical analysis, thereby reducing the sampling and analytical requirements.

Implementation of this dual approach resulted in a more efficient characterization, and significant cost and schedule savings compared to a traditional RI approach.