aerobic reactions must occur at rates that adequately protect the food chain and human health.

A recent study (McNulty, 1997) carefully retrieved, dated, and chemically analyzed sediment samples taken from different depths of the river bottom. In a highly contaminated section of river sediments, significant dechlorination of PCBs had occurred, but the dechlorination rate decreased dramatically after about a year. Even after years to decades, complete dechlorination of PCBs had not occurred. Furthermore, at a moderately contaminated section of sediments, only initial signs of dechlorination had developed. Thus, the observed footprints of PCB dechlorination, although encouraging, are insufficient in themselves to ensure that natural attenuation will be sufficient to decrease contaminant concentrations to meet regulatory standards.

The Electric Power Research Institute and Niagara Mohawk Power Company collaborated to assess how removing the contaminant source at a coal tar disposal site would affect natural attenuation of dissolved contaminants from coal tar remaining in groundwater (Taylor et al., 1996). The study area is located in a rural setting near South Glens Falls, New York. The site was used for disposal of tar from a single event that occurred in the early 1960s at a plant that manufactured gas. The site was ideal for study because the contaminant source and hydrogeology are relatively simple. Because the contamination resulted from a single disposal event, there was only one coal tar source, with no unknown residuals, making source characterization relatively straightforward compared to more complex sites. The hydrogeologic setting is an aquifer composed of coarse to fine silty sands, with a confining layer 6 to 9 m (20 to 30 ft) below ground surface preventing extensive downward migration of the contamination. The groundwater velocity is about 12 m (40 ft) per year.

Source removal commenced in May 1991 with installation of a sheet pile enclosure driven to a depth of more than 12 m (40 ft), well below the confining layer 7.6 m (25 ft). Despite the relatively simple hydrologic setting, locating and thoroughly removing the coal tar source material were technically difficult. The enclosed area was about 1,000 m² (0.25 acre), and approximately 7,200 m³ (9,400 yd³) of tar-contaminated soil and overburden were removed. In October 1991, the sheet pile was removed, and the excavated area was filled with clean native soil with a grain size similar to that of the removed soil.

Figure 3-16 shows an areal view of concentrations of naphthalene—one of the key constituents dissolved from the coal tar—in June 1990 just

Front Matter (R1-R14)

Executive Summary (1-19)

1 Introduction: Using Natural Processes in Groundwater Restoration (20-36)

2 Community Concerns about Natural Attenuation (37-64)

3 Scientific Basis for Natural Attenuation (65-149)

4 Approaches for Evaluating Natural Attenuation (150-211)

5 Protocols for Documenting Natural Attenuation (212-254)

Appendix A Acronyms (255-256)

Appendix B Presenters at the Committee's Information-Gathering Meetings (257-258)

Appendix C Biographical Sketches of Committee Members and Staff (259-264)

Index (265-274)