The Phase I Environmental Site Assessment is generally considered the first step in the process of environmental due diligence .
Standards for performing a Phase I site assessment have been promulgated by the US EPA[1] and are based in part on ASTM Standard E1527-13.[2]
Phase II Environmental Site Assessment services include intrusive testing performed on air, soil and water of a property to determine whether environmental contaminants are present.
Phase III services include but are not limited to;

Identifying the extent of environmental contaminants found during the Phase II Environmental Site Assessment process.
Determining the amount of soil and groundwater impacted by environmental contaminants.
Completing a Site Notification Report to notify regulatory bodies of site contamination.
Developing a Remedial Action Plan (RAP) to remove environmental contaminants from the property.
Assessing options available for parties involved, including costs and timelines of remediation.

Purpose of a Phase III ESA

During a Phase II Environmental Site Assessment, intrusive testing is performed on a property to determine if environmental contaminants are present on a property.

If no contaminants are found, a Phase III Assessment is not necessary.
However, if contaminants are found, a Phase III Assessment takes the information from a Phase II and determines the extent of contamination of the property.

Phase III is a much more detailed investigation of the property, and is necessary to uncover additional information before remediation work can take place.

Phase III ESA should result in the parties involvement in a plan of remediation, which is approved by local, state and federal environmental agencies.

It should detail a full scope of options for all parties and time and costs involved with cleanup.

Expertise needed: Soil scientists . hydrologists, environmental engineers should conduct additional testing to determine the extent of pollution on the property.
Common tests may include:

Groundwater testing
Subsurface soil assessment
Sediment testing
Assessing pathways of contaminants in air , water and soil

Related Services:

Environmental Compliance Audits
Certification of Clean Up of Contaminated Site