Groundwater Remediation: A Practical Guide for Environmental Engineers and Scientists

By Nicholas P Cheremisinoff

Groundwater is one of the Earth’s most precious resources.  We use it for drinking, bathing, and many other purposes.  Without clean water, humans would cease to exist.  Unfortunately, because of ignorance or lack of caring, groundwater is often contaminated through industrialization, industry, construction or any number of other ways.  It is the job of the environmental engineer to remediate the contaminated groundwater and make what has been tainted safe again.Selecting the proper remediation strategy and process is the key to moving forward, and, once this process has been selected, it must be executed properly, taking into consideration the costs, the type of contaminants that are involved, time frames, and many other factors. 

This volume provides a broad overview of the current and most widely applied remedial strategies. Instead of discussing these strategies in a generic way, the volume is organized by focusing on major contaminants that are of prime focus to industry and municipal water suppliers. The specific technologies that are applicable to the chemical contaminants discussed in different chapters are presented, but then cross-referenced to other chemical classes or contaminants that are also candidates for the technologies. The reader will also find extensive cost guidance in this volume to assist in developing preliminary cost estimates for capital equipment and operations & maintenance costs, which should be useful in screening strategies.

The eight chapters cover all of the major various types of contaminants and their industrial applications, providing a valuable context to each scenario of contamination.  This is the most thorough and up-to-date volume available on this important subject, and it is a must-have for any environmental engineer or scientist working in groundwater remediation. 

• Language: English
• Publisher: Wiley
• Release date: Jun 13, 2017
• ISBN: 9781119407737

Nicholas P Cheremisinoff

Nicholas P. Cheremisinoff, Ph.D. (Ch.E.) is Director of Clean Technologies and Pollution Prevention Projects at PERI (Princeton Energy Resources International, LLC, Rockville, MD). He has led hundreds of pollution prevention audits and demonstrations; training programs on modern process design practices and plant safety; environmental management and product quality programs; and site assessments and remediation plans for both public and private sector clients throughout the world. He frequently serves as expert witness on personal injury and third-party property damage litigations arising from environmental catastrophes. Dr. Cheremisinoff has contributed extensively to the literature of environmental and chemical engineering as author, co-author, or editor of 150 technical reference books, including Butterworth-Heinemann’s Handbook of Chemical Processing Equipment, and Green Profits. He holds advanced degrees in chemical engineering from Clarkson College of Technology."

Book preview

Chapter 1

Conducting Groundwater Quality Investigations

1.1 Introduction

The volume is intended as a primer to address groundwater contamination often caused by legacy pollution or unintentional releases of chemicals to the subsurface. When groundwater has been adversely impacted, a variety of sciences, strategies, technologies and actions are needed to assess human and ecological risks from the contamination. The first step in assessing impacts requires a body of good practices that are recognized by industry on the whole and is referred to as the environmental site assessment.

Environmental site assessment practices are also commonly referred to as environmental audits. The practices for conducting an environmental site assessment began evolving in the United States in the 1970s. Throughout the 1980s environmental site assessment practices evolved further with the promulgation of the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) and the Resource Conservation and Recovery Act (RCRA), which required commercial facilities to identify, report and remediate recognized environmental conditions. Throughout the 1990s environmental site assessment practices were enhanced with more precise tools that aided in site characterization and quantification of recognized environmental conditions. Over the years additional analytical tools have evolved to aid environmental site assessment practices.

The goal of an environmental site assessment is to identify recognized environmental conditions. The term recognized environmental conditions means "the presence or likely presence of any hazardous substances or petroleum products on a property under conditions that indicate an existing release, a past release, or a material threat of a release of any hazardous substances or petroleum products into structures on the property or into the ground, groundwater, or surface water of the property."1

1.2 Evolution of Site Assessments

The control of hazardous substances and the prevention of the entry of these substances into the environment is the objective of several acts of U.S. Congress. Rules regulating various aspects of hazardous waste can be attributed to the Toxic Substances Control Act (TSCA); the Clean Water Act (CWA); the Clean Air Act (CAA); the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA); the Safe Drinking Water Act (SDWA); the Resource Conservation and Recovery Act (RCRA); and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). RCRA and CERCLA are the two that are most often associated with environmental site assessments.

RCRA was passed to control industrial and municipal solid wastes, including sludges, slurries, etc. The act also called for a tracking system to document the generation, transport, and disposal/storage of solid wastes. The discovery of a large number of uncontrolled and abandoned hazardous waste sites, such as at Love Canal, New York, prompted a much greater emphasis on the hazardous nature of the wastes. In the 1980s the regulations and resources of RCRA were primarily devoted to the control of hazardous wastes, with a lesser emphasis on nonhazardous solid wastes.

In 1980, legislation aimed at providing federal money for the cleanup of inactive waste disposal sites was enacted. The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), often called the Superfund Act, provides regulatory agencies with the authority to deal with inactive sites, funds to deal with hazardous waste emergencies and a means to assign the liability of cleanup to the responsible parties. It also provides monies (Superfund) to pay for the mitigation of hazards from abandoned sites when no responsible party can be found or when the responsible party refuses to take action. In addition, it empowers the government to seek compensation from responsible parties to recover funds used in mitigation actions.

Section 105 of the CERCLA requires that the National Contingency Plan (NCP), developed under the Clean Water Act, be revised to include procedures and standards for responding to releases of oil and hazardous substances. The revised plan reflected and effectuated the responsibilities and powers created by the act.

Subpart F of the NCP, Hazardous Substance Response, establishes a seven-phase approach for determining the appropriate extent of a response authorized by CERCLA when any hazardous substance is released or there is a substantial threat of such a release into the environment, or there is a release or substantial threat of a release of any pollutant or contaminant which may present an imminent and substantial danger to the public health or welfare2. Each phase sets specific criteria to establish the need for further action. The phases are:

Phase I – Discovery and Notification

Phase II – Preliminary Assessment

Phase III – Immediate Removal

Phase IV – Evaluation and Determination of Appropriate Response – Planned Removal and Remedial Action

Phase V – Planned Removal

Phase VI – Remedial Action

Phase VII – Documentation and Cost Recovery

This phased approach is the basis for implementation of all CERCLA-authorized Hazardous Substance Responses with which industry is obligated to comply.

The practice of conducting environmental site assessments began in the 1970s in the United States. These practices evolved over time, which is why it is important to place them within a historical context. As early as the 1970s specific property purchasers in the United States undertook studies resembling current Phase I ESAs, to assess risks of ownership of commercial properties which had a high degree of risk from prior toxic chemical use or disposal. Many times these studies were preparatory to understanding the nature of cleanup costs if the property was being considered for redevelopment or change of land use.

The evolution of best practices in conducting site assessments was driven by an expanding knowledge base on the fate and transport of harmful chemicals. Until the early 1960s, the question of whether or not groundwater was significantly affected by organic wastes was generally addressed by observing the subsurface breakdown of sewage and similar matter. There was a general belief that the easiest way to eliminate contamination was through the natural processes of separation, filtration, dilution, oxidation and chemical reaction. Soils were believed to serve the purpose of filtration, aid in chemical reaction by adsorbing some chemicals, while groundwater was generally believed to be an infinite medium, thereby diluting any harmful chemicals. Not until the mid-1960s did organic contaminants begin to receive attention.

Some properties are associated with groundwater contamination that can be characterized as being comprised of Dense Non-Aqueous Phase Liquids (DNAPLs). DNAPLs are characterized by their lack of noticeable taste or odor and their higher density relative to water. These properties render them difficult to detect and monitor. In contrast, petroleum spills float atop the water table and are usually volatile with distinctive tastes and odors. The rare discovery of DNAPL contamination before the development and ready availability of analytical techniques allowing the measurement of organic contaminants on the ppm to ppt level is not surprising.

Although appropriate analytical methods actively existed and were relied on by industry since the mid-1950s, there was no drive to investigate groundwater for the presence of chlorinated solvents. Analytical chemists instead concentrated efforts on alkyl benzene sulphonate (ABS) detergents and organic pesticides such as DDT and aldrin. The surreptitious nature of DNAPLs led them to be disregarded as groundwater contaminants until much later. Dissolved plumes caused by DNAPLs were not discovered until the 1970s. DNAPL (the free phase, not dissolved phase) was not discovered until the mid-1980s. This was partially because monitoring wells was not understood, as it is now, to be a poor method to detect DNAPL (i.e., it has rarely been reported in wells).

The discovery of DNAPLs was prompted by legislation introduced during the previous decade: Safe Drinking Water Act (1974), Resource Conservation and Recovery Act (RCRA, 1976) and the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA, 1980). These legislations required sampling of municipal wells specifically for chlorinated solvents, which were discovered in some drinking water systems. Unlike some other contaminants, such as methyl tert-butyl ether (MTBE), chlorinated solvents have high taste and odor thresholds, meaning that people don’t taste or smell the compounds in water until there is a relatively high concentration. Chlorinated solvents have taste thresholds around several hundred μg/L (i.e., ppb) whereas MTBE is nearly two orders of magnitude lower. Furthermore, taste thresholds are highly dependent on the individual.

The 1980s ushered in a vast cache of knowledge supported by reports and peer reviewed publications concerning groundwater investigations and DNAPLs. During this time period the evolution of vapor intrusion pathway (VIP) science also took place.

VIP refers to the migration of vapors from the soil zone into structures. The pathway starts from the groundwater to soil gas pathway. The origins of VIP may be traced back to the 1930s when petroleum exploration by soil gas analysis for hydrocarbons was first understood, but not from an environmental aspect. From the 1950s onward it was common practice to use volatile chemicals as root zone fumigants. This application added to the general knowledge of VIP, but there was no link to environmental concerns. In the 1960s vapor intrusion began to be understood as a risk associated with acute exposure or fire/explosion, mostly from petroleum wells. The American Petroleum Institute (API) published warnings, guidelines and best practices to reduce these risks associated with well drilling and exploration activities.

Beginning in the early 1960s and onward landfill gas surveys and radon surveys were steadily reported in the industry and in the scientific literature. In the 1970s VOC plume mapping by soil gas surveys began to evolve. By the late 1980s VOC plume mapping by soil gas surveys was a well-established and standard technique used in environmental investigations.

Throughout the 1980s vapor intrusion risk from acute exposure and chronic risks began to be considered in tandem where acute chemical risks were identified. Chronic exposure and risks via the VIP was recognized in the late 1970s/early 1980s which gave rise to OSHA’s focus on VOCs as inhalation carcinogens (1970s); and then in the early 1980s it was recognized as a mainstream topic of concern for residential indoor air quality. The most significant topic of VIP in the early 1980s concerned radon intrusion.

Along with the evolution of science, best practices and tools for industry, statutory evolution took place. In 1980 RCRA 261.31 F001 listing of spent degreasing solvents became an obligation. The U.S.EPA defined TCE and PCE mobility in groundwater along with the properties of volatility and carcinogicity, and further acknowledged the pathway of vapor intrusion into the basements of buildings as a human health risk.

In 1984 the U.S.EPA published a nationwide strategy for groundwater protection3. It stated that ground water contamination looms as a major environmental issue of the 1980’s. The attention of agencies at all levels of government, as well as that of industry and environmentalists, is now focused on this vital resource. As contamination has appeared in well water and wells have been closed, the public has expressed growing concern about the health implications of inappropriate use and disposal of chemicals. As concern has increased, so have demands for expanded protection of the resource.

In 1985 through Love Canal Enforcement actions the well-known, so-called Murphy Models were applied to assessing VIP into basements as part of performing risk assessments; and in 1986 RCRA OSWER4 Corrective Action directives required that investigations be conducted in environmental site assessments in order to characterize subsurface gasses from buried waste and hazardous constituents found in groundwater.

In 1989, RFI Guidance for Conducting RI/FS5 noted inter media transfer from groundwater to soil gas to air. In 1992 Air/Superfund guidance and best practices were published (U.S.EPA - Assessing Potential Indoor Air Impacts for Superfund Sites). This document includes case studies. In 1993 a further Air/Superfund guidance document was published (Options for Developing and Evaluating Mitigation Strategies for Indoor Air Impacts at Superfund Sites). This publication includes examples, case studies and best practices.

From the mid-1990s onward several states began to require VIP evaluations when conducting an environmental site assessment. These states were Massachusetts, Michigan, Connecticut, and Rhode Island. In later years more states added such requirements. In 1994 and again in 1995 the ASTM developed separate but complementary guidelines for conducting general Phase I and Phase II site assessments. In 1996 U.S.EPA published the NPL (National Priority List) Guidance document titled Soil Screening Guidance User’s Guide.

The ASTM developed the RBCA standard for petroleum releases that includes VIP. RBCA stands for Risk-Based Corrective Action, which is a generic term for corrective action strategies that categorizes a site according to risk and moves the site toward completion using appropriate levels of action and oversight. The most recent ASTM standard provides a framework for implementing a RBCA strategy. With this process, regulators and investigators can make sound, quick, consistent management decisions for a variety of sites using a three-tiered approach to data collection and site review contained in ASTM’s E1739 standard guide for Risk-Based Corrective Action applied at Petroleum Release Sites.

The RBCA helps to categorize sites according to risk, allocate resources for maximum protection of human health and the environment, and provide resources for appropriate levels of oversight. These actions are intended to assist sites to move forward quickly towards defining risks and mitigating them.

The ASTM RBCA standard, like the early ones established by the U.S.EPA in 1985, is intended to identify exposure pathways and receptors at a site; determine the level and urgency of response required at a site; determine the level of oversight appropriate for a site; incorporate risk analysis into all phases of the corrective action process; and enable selection of appropriate and cost-effective corrective action measures. RBCA is not a substitute for corrective action, but a tool for determining the amount and urgency of action necessary.

The ASTM standard (E1739) is based on a tiered approach to risk and exposure assessment, where each tier refers to a different level of complexity. The goal of all of ASTM’s tiers is to achieve similar levels of protection. The difference is that, in moving to higher tiers, more efficient and cost-effective corrective action results because the conservative assumptions of earlier tiers are replaced with more realistic site-specific assumptions. Additional site assessment data may be required as sites move to higher tiers. In contrast to earlier approaches to conducting site assessments which tend to be executed in steps, the approach taken today is more streamlined.

In 2001/2002 the U.S.EPA published Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway From Groundwater and Intrusion to Indoor Air Pathway from Groundwater and Soils.

Beginning circa 1980, the U.S.EPA began to steadily develop best practices for conducting environmental site assessments. These best practices were widely published and accessible to industry. By 1985 well-defined best practices were established, constituting the foundation for further refinements over the next decade. From about 1995 onward, further refinements to both technologies that aid in site assessments as well as more refined best management practices were devised and published by the American Society of Testing Materials (ASTM) and later further refined by such organizations as the World Bank Organization (WBO), ANSI, ISO, and others.

In 1985 U.S.EPA published a three-volume manual of best practices for industry to follow when conducting environmental site assessments. The first volume was titled: Characterization of Hazardous Waste Sites: A Methods Manual, Volume I – Site Investigations6. The following are excerpts from the publication, annotated in some instances with my comments. Overall the statements and recommended good industry practices are self-evident.

"At the first meeting of the Agency-Wide Steering Group for the Development of a Methods Manual for Characterization of Hazardous Waste Sites in August 1981, the scope of the planned Available Methods Manual was expanded from sampling and analysis to site characterization. The steering group agreed that sampling and analysis of hazardous wastes must be closely tied to sampling and analysis strategy. Before methods can be useful, they must be related to the purposes and objectives of sampling and analysis. Such an association leads to the necessity of considering all aspects of hazardous waste site characterization."

As early as 1981 the U.S.EPA recognized and recommended that proper site characterization requires that a strategy with clearly defined objectives be established in order to properly identify and characterize the environmental conditions of a property.

The objective of this manual is to provide field and laboratory managers, investigators, and technicians with a consolidated source of information on the subject of hazardous waste site characterization. The manual covers the range of endeavors necessary to characterize hazardous waste sites, from preliminary data gathering to sampling and analysis.

Because of the large number of subjects covered in this manual and the need to provide detailed methodology in the areas of sampling and sample analysis, this manual comprises three volumes: Volume I - Site Investigations; Volume II - Available Sampling Methods; Volume III - Available Laboratory Analytical Methods.

U.S.EPA’s 1985 multi-volume manual of practices provides guidance on information-gathering activities in support of the requirements specified in the National Oil and Hazardous Substances Pollution Contingency Plan. "The National Contingency Plan contains a seven-phase approach to implementing the authority of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). Each phase represents a level of response dependent upon the situation. Information must be obtained to determine the appropriate level of environmental response. Both remedial and enforcement actions under CERCLA require reliable site information. This volume describes approaches to obtaining this information and follows a semi-chronological order through subsequent phases of the National Contingency Plan. These steps range from preliminary data gathering,to site inspections,to large field investigations."

U.S.EPA’s manual described policies and procedures common to all data-gathering efforts, such as personal conduct, document control, and quality assurance. Sections included in the manual provided a framework for gathering the required information. U.S.EPA detailed what information is necessary, where that information can be found and how the information can be acquired in an environmental site assessment. Its manual presented topics such as investigative conduct, documentation and recordkeeping, quality assurance, site entry, etc., from the viewpoint of Agency policy. It stated that although its discussions were based on EPA policy, they were intended to serve as a guideline for anyone conducting a hazardous waste site investigation.

U.S.EPA stated that the following requirements constitute good practices: Persons conducting hazardous waste site investigations must develop and report the facts of an investigation completely, accurately, and objectively.

On p. 2–3 of EPA’s document control practices are discussed. "The purpose of document control is to assure that all project documents issued to or generated during hazardous waste site investigations will be accounted for when the project is completed. The purpose is achieved through a program which makes all investigation documents accountable. This should include serialized document numbering, document inventory procedures, and an evidentiary filing system. Accountable documents used or generated during investigations include: Project Work Plans, Project Logbooks, Field Logbooks, Sample Data Sheets, Sample Tags, Chain-of-Custody Records and Seals, Laboratory Logbooks, Laboratory Data, Calculation, Graphs, etc., Sample Checkout, Sample Inventory, Internal Memos, External Written Communication, Business Confidential Information, Photographs, Drawings, Maps, Quality Assurance Plan, Litigation or Enforcement Sensitive Documents, and Final Report."

EPA recognized that site investigations have the potential to generate large volumes of information and reports and that document control is an essential element to controlling information, and in support of any analysis applied towards remediation. It recommended that each document be assigned a serialized number and be listed, with the number, in a project document inventory assembled at the project’s completion. Volume II, Appendix D, provides further discussion of Document Control/Chain-of-Custody Procedures.

Beginning on p. 2–17 of Volume I U.S.EPA recommended good practices to be applied in environmental site assessments to ensure high quality and reliability throughout the assessment and in developing remedial actions.

Section 4 (beginning p. 4–1) of EPA’s 1985 good practices manual provides practices, protocols and stepwise procedures for data gathering in order to perform a proper environmental site assessment. EPA recommended that a task should be initiated to collect and review available information about the known or suspected hazardous substance site or release. EPA’s recommended practices constitute what is commonly referred to as a Phase I environmental audit.

In Section 5 beginning on p. 5–1 EPA provided detailed procedures, protocols and best practices for conducting site inspections. It defined these as being important components of Phase II, Preliminary Assessment and Phase IV, Evaluation and Determination of Appropriate Response - Planned Removal and Remedial Action. It stated that the major objective of a site inspection is to determine if there is any immediate danger to persons living or working near the facility. It explained in great detail the recommended practices, protocols and procedures for conducting these activities and stated that the primary items addressed during the site inspection are:

A determination of the need for immediate removal action;

An assessment of the amounts, types and location of stored hazardous substances;

An assessment of the potential for substances to migrate; and

Documentation of immediate threats to the public or environment.

The section covers various topics and best practices for conducting preliminary site investigations, Phase I site investigations, Phase II site investigations, and conducting remedial investigations. The recommended practices are detailed and stepwise. It stated for examples (p. 5–6) that Inspections of basins and vessels should verify structural dimensions and note the number and location of input or discharge lines. Any manways, hatches, or valve pits should be identified and monitored with the survey instruments. If the structures contain a material, an estimate of percent full (look for staff gauges or site glasses) and a description of the material should be noted. A general assessment of structural condition also should be included…. The presence of buried vessels is often only apparent upon discovery of small standpipes or vents protruding above the ground surface. All such pipes should be noted and marked with colored tape and/or flags. Closer investigation of the immediate vicinity of the vents often uncovers hatches or valve pits. Further investigation during the inspection should be limited to screening the vents and hatch seals with an OVA, HNu or other monitors …

On p. 5–7 EPA recommended that information regarding population size and distribution should be available from the preliminary assessment. In many instances this information, if obtained from state or regional agencies will be somewhat dated. It is important therefore to tour the area assessing the likelihood of significant demographic changes. Recently constructed housing developments, apartments, schools and public buildings may indicate that changes have occurred since the information was published. Such practices were recommended in order for the environmental site assessment to define the potential risks of hazardous substances on-site to neighboring off-site receptors.

Beginning on p. 6–1 EPA addressed the need and best practices for data evaluation. It wrote that "a data assessment is performed to ultimately assist in formulating response management decisions affecting later stages of the investigation. The data evaluation may also indicate data gaps which need to be filled either by further background research or additional site inspections (or an initial inspection if one has not yet been conducted) … The evaluation should encompass the scope detailed below:

the existence (or nonexistence) of a potential hazardous waste problem;

probable seriousness of the problem and the priority for further investigation or action; and

the type of action or investigation appropriate to the situation."

In 1996 the ASTM published its standard Designation: E 1528 – 96: Standard Practice for Environmental Site Assessments: Transaction Screen Process. It wrote, "The purpose of this practice, as well as Practice E 1527, is to define good commercial and customary practice in the United States of America for conducting an environmental site assessment of a parcel of commercial real estate with respect to the range of contaminants within the scope of the Comprehensive Environmental Response Compensation and Liability Act (CERCLA) and petroleum products …" It further defined the term Recognized Environmental Conditions: "In defining a standard of good commercial and customary practice for conducting an environmental site assessment of a parcel of property, the goal of the processes established by this practice is to identify recognized environmental conditions. The term recognized environmental conditions means the presence or likely presence of any hazardous substances or petroleum products on a property under conditions that indicate an existing release, a past release, or a material threat of a release of any hazardous substances or petroleum products into structures on the property or into the ground, groundwater, or surface water of the property. The term includes hazardous substances or petroleum products even under conditions in compliance with laws. The term is not intended to include deminimis conditions that generally do not present a material risk of harm to public health or the environment and that generally."

It further wrote, "Objectives guiding the development of this practice and Practice E 1527 are (1) to synthesize and put in writing good commercial and customary practice for environmental site assessments for commercial real estate, (2) to facilitate high quality, standardized environmental site assessments, (3) to ensure that the standard of appropriate inquiry is practical and reasonable …"

It also wrote, "This practice and Practice E 1527 are designed to assist the user in developing information about the environmental condition of a property and as such has utility for a wide range of persons, including those who may have no actual or potential CERCLA liability and/or may not be seeking the innocent landowner defense."

In 1997 the ASTM published its standard Designation: Designation: E 1903 – 97: Standard Guide for Environmental Site Assessments: Phase II Environmental Site Assessment Process. It wrote The primary objectives of conducting a Phase II ESA are to evaluate the recognized environmental conditions identified in the Phase I ESA or transaction screen process for the purpose of providing sufficient information regarding the nature and extent of contamination to assist in making informed business decisions about the property …

ASTM further stated in E 1903 – 97 "At the completion of a Phase II ESA, the environmental professional should be able to conclude, at a minimum, that either (a) the ESA has provided sufficient information to render a professional opinion that there is no reasonable basis to suspect the presence of hazardous substances or petroleum products at the property associated with the recognized environmental conditions under assessment, or (b) the ESA has confirmed the presence of hazardous substances or petroleum products at the property under conditions that indicate disposal or release. If the information developed in the ESA is insufficient for the environmental professional to reach either of these conclusions, the environmental professional may recommend additional iterations of assessment if warranted to meet the objectives of the user. If the environmental professional reasonably suspects that unconfirmed hazardous substance or petroleum releases remain but concludes that further reasonable assessment is not expected to provide additional information of significant value, he may recommend that further assessment is not warranted. In such circumstances, the recommendation for no further assessment should be accompanied by an explanation why a reasonable suspicion of releases remains and why further reasonable assessment is not warranted."

In 1998 the necessity of performing a Phase I ESA was underscored by congressional action in passing the Superfund Cleanup Acceleration Act of 1998. This act requires purchasers of commercial property to perform a Phase I study meeting the specific standard of ASTM E1527: Standard Practice for Environmental Site Assessments: Phase I Environmental Site Assessment Process. The most recent standard is Standards and Practices for All Appropriate Inquiries 40 Code of Federal Regulations, Section 312 which drew heavily from ASTM E1527-05 which has become known as ‘All Appropriate Inquiry’ (AAI). Previous guidances regarding the ASTM E1527 standard were ASTM E1527-97 and ASTM E1527-00.

1.3 Technology Limitations and Cleanup Goals

The basis of any groundwater remediation strategy needs to take into consideration the risks to receptors, current technology, regulatory requirements and trends, and cost considerations. Today, the EPA and many state regulatory agencies acknowledge there are limitations of existing technologies to completely remediate some impacted sites. This awareness has resulted in U.S. regulatory changes which are favorable to more site-specific and risk-based remediation objectives for industrial sites. As an example, in 1994, the EPA published guidance that allows for Technical Impracticability waivers for sites where complete remediation is impossible due to the site conditions or the presence of inaccessible DNAPLs. This guidance describes what technical evidence is required and what regulatory procedures exist for establishing more realistic remediation objectives for chlorinated solvent contaminated sites.

The EPA Brownfields initiative encouraged local governments, environmental regulators, and land developers to work together to establish realistic cleanup goals for contaminated industrial properties. Using risk assessment tools, the actual exposure pathways and receptors are identified for the proposed land use and cleanup goals and remediation activities are tailored to eliminate these risks. Both RCRA and CERCLA contain provisions for establishing alternate concentration limits or remediation goals based on industrial land use assumptions. It is not always reasonable nor practical to apply drinking water maximum contaminant levels (MCLs) as the basis for cleanup goals in all situations where there is little chance of human exposure to groundwater.

Many if not most state agencies now publish risk-based cleanup criteria for industrial sites and recognize mixing zone concepts which allow stable contaminated plumes to attenuate in place so long as surface water and drinking water resources are protected. ASTM has also been developing a risk-based corrective action (RBCA) standard for chlorinated solvents that is similar to the standard developed for fuel.

1.4 Conceptual Models

The nature and extent of a site’s groundwater contamination must be defined in part with a conceptual model. The investigator needs to develop a useful conceptual site model or update an existing one and determine what human or ecological receptors may be at risk and how to limit their exposure to the contamination.

An accurate conceptual site model is critical to evaluating the true risk of contamination, as well as the possibilities and limitations of site remediation strategies. A complete model should include a visual representation of contaminant source and release information, site geology and hydrology, contaminant distribution, fate and transport parameters, and risk assessment features such as current and future land use and potential exposure pathways and receptors.

The conceptual site model should be developed as a part of the site investigation or feasibility study phase of site remediation. Many interim remedial systems have been installed and are operating without a well-defined model, oftentimes leading to major cost overruns or inability to achieve cleanup goals within reasonable time periods. Some remedial systems were designed based on an initial model that requires updating based on recent operations and monitoring data. Changes in land use, or changes in the enforcement of institutional controls, can also alter the exposure and risk assumptions of the model. It is important to recognize that the conceptual site model is intended to be a dynamic representation of site conditions based on a continual influx of information from the site. The following are important elements of a conceptual site model.

1.4.1 Source and Release Information

The conceptual site model should include a description of the source of contamination and what is known about the timing and quantity of the release. Most site characterizations begin by locating areas where chemical contaminants were originally released to the subsurface. In many cases, the distinct source of contamination is known to be a former underground storage tank (UST), disposal pit, a leaking pipeline, a spill, etc. However, many industrial source areas are dispersed and sometimes difficult to delineate. For example, oil/water separators, and sanitary and storm sewers have historically received chlorinated solvents from process operations or various plant maintenance shops. At such sites, it may be impossible to pinpoint the exact source of contamination. Soil gas surveys can be used to locate dispersed source areas at sites with sandy, permeable soils. However, at sites with low permeability soils, locating dispersed sources will often require excavation and removal of contaminated soils along underground utilities. This level of intrusive characterization may not be possible along active utility corridors or within the building of large industrial complexes.

The timing and the amount of chemical contaminants released are equally difficult to estimate. Historical records on chemical uses are sometimes difficult to obtain, and if they exist are generally found in Phase I Installation Restoration Program documents developed in the early 1980s. Many sites as an example used chlorinated solvents like TCE which was used for decades at some operations before it was phased out in the early 1980s. Such chemicals may not have been widely used at a facility for nearly 20–30 years. This fact is important when evaluating the fate and transport of chlorinated solvents or any chemical contaminant and is especially important when estimating degradation rates based on the breakdown products of certain chemicals like the chlorinated solvents.

In sandy soils, the amount of chlorinated solvent remaining in the subsurface can be roughly estimated based on a comprehensive soil gas survey in a known source area. Average soil gas concentrations of chlorinated solvents can be equated to soil concentrations to estimate the mass of solvents in an impacted volume of soil. Likewise, average groundwater concentrations can be used to roughly estimate the amount of chlorinated solvent dissolved in a volume of impacted aquifer. But these methods are generally ineffective estimators of contaminant mass in low permeability soils or sites where chlorinated solvents exist as dense non-aqueous phase liquids (DNAPLs), and they are not applicable to many other types of chemical contaminants.

1.4.2 Geologic and Hydrogeologic Characterization

The conceptual site model must include a complete description of the site geology and hydrogeology. The descriptions should include at a minimum:

A general description of site geology including major soil strata that are impacted by or influence the migration of contaminants. Strata thickness, lateral extent, continuity and depositional features should be described.

Physical and chemical properties of subsurface materials such as sieve analysis, bulk density, porosity and total organic carbon.

Geologic or manmade features which may provide preferential migration of chemicals, DNAPLs, solvent vapors, or dissolved contaminants.

Depth to groundwater, seasonal variations, recharge and discharge information including interactions with surface waters.

Ranges of hydraulic gradients (horizontal and vertical).

Ranges of hydraulic properties (e.g., hydraulic conductivity, storage coefficient, effective porosity, seepage velocity).

Geochemical properties influencing the natural biodegradation of the chemical contaminants.

The conceptual site model will need to be updated to reflect current estimates of these properties based on site remediation experience. For example, the hydraulic properties of an aquifer can be more accurately estimated after a groundwater extraction system has operated several months. On sites where natural attenuation has been selected as the groundwater remedy, tracking the movement (or stability) of the contaminant plume provides essential information that can be introduced into an updated model.

1.4.3 Contaminant Distribution, Transport and Fate

A conceptual site model should include a summary of the chemical, physical, and biodegradation properties of key contaminants of concern and describe their distribution, movement, and fate in the subsurface environment. Descriptions should include:

Chemical and physical properties of the chemical contaminants that impact subsurface transport (e.g., partitioning coefficients, solubility, vapor pressure, Henry’s Constant, density, viscosity);

Estimates of the phase distribution of each contaminant (free-phase DNAPL, sorbed, in soil vapor, or dissolved) in the saturated and unsaturated zone;

Temporal trends in contaminant concentrations in each phase;

Geochemical evidence of contaminant natural attenuation processes (destructive and nondestructive).

1.4.4 Geochemistry Impacting Natural Biodegradation

Under certain conditions, geochemical parameters may favor natural biodegradation of some chemicals. Examples of these can be found with chlorinated solvents. Geochemical indicators such as dissolved oxygen, nitrate, iron, manganese, sulfate, methane, and hydrogen ion concentrations should be reported in the