Conducting Subsurface Environmental Investigations in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 119

performed. Interpretations and conclusions must rely upon a firm foundation of
science.
• Evaluating the possible presence of chemicals in the environment often requires
searching for specific compounds at extremely low concentrations, routinely in the
microgram per liter (or kilogram) range. A microgram per liter is roughly equiva-
lent to a part per billion (ppb), which in time units equates to 1 s per 31 years. This
analogy demonstrates why these low quantities of evidence require great care to
ensure the integrity of the investigation.
• Well-defined activities characterize subsurface environmental investigations,
including: scientific inquiry; historical review; data collection; interpretation and
assessment; and in some cases, remedial action to lower or eliminate any environ-
mental risk considered unacceptable. During these activities, special emphasis is
placed on evaluating the presence or potential likelihood of uncontrolled releases
or migration of contaminants.
• Once discovered, the key to defining the nature and extent of contamination is
through a detailed understanding of the site geology and hydrogeology, and the
behavior of every contaminant detected. Geological and hydrogeological exper-
tise and experience are crucial for achieving a successful outcome for the site/area
being investigated.
Subsurface environmental investigations exist as a subset of a larger set of environmental
investigation types. Throughout the chapter when we refer to an environmental investiga-
tion, the reference is to a subsurface environmental investigation, unless otherwise noted.
4.2 Types and Description of Subsurface Environmental
Investigations, Studies, Plans, or Reports
Environmental investigations are conducted for many different purposes and objectives
but generally follow a step-wise, logic- and progressive-based approach, where the results
of one investigation are used to evaluate the need and scope of subsequent investigations
if necessary. The most common types of environmental investigations include
• Phase I environmental site assessment (ESA)
• Phase II investigation
• Geophysical investigation
• Remedial investigation
• Feasibility study
• Ecological and human health risk assessment
• Natural resource damage assessment (NRDA)
• Environmental impact statement (EIS)
• Remedial action plan (RAP)
• Confirmatory investigation
Each type of investigation is discussed in greater detail in the following sections.