Framework Document for Monitored Natural Attenuation of Inorganic Contaminants in Ground Water

Transcription

1 Framework Document for Monitored Natural Attenuation of Inorganic Contaminants in Ground Water Ronald Wilhelm United States Environmental Protection Agency Abstract This paper is intended to provide an overview of the U.S. Environmental Protection Agency s (EPA) ongoing efforts to produce a guidance document on the use of Monitored Natural Attenuation (MNA) as a remedial approach for radionuclide and inorganic contaminants 1 in ground water. The focus is to provide a tiered decisionmaking approach for determining whether monitored natural attenuation (MNA) is likely to be an effective remedial approach for these contaminants in ground water. This approach can be applied in accordance with the remedial investigation and feasibility study phases of a Superfund cleanup, under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA). This approach can also be used during the equivalent phases of a Resource Conservation and Recovery Act (RCRA) Corrective Actions (facility investigation and corrective measures study, respectively). The agency policy for the use of MNA in Office of Solid Waste and Emergency Response (OSWER) cleanup programs is described in the April 21, 1999 OSWER Directive titled, Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action and Underground Storage Tanks (Directive No P. This is an ongoing effort and it should be noted that the final guidance will likely differ from this overview due to its ongoing development and the EPA peer review process. This guidance is intended to provide a reasoned technical approach for evaluating MNA remedies. A draft is expected in late 04 and a final peer reviewed published report in Introduction and Background The OSWER Monitored Natural Attenuation (MNA) policy directive (Dir p, 1999) provided a definition for Monitored Natural Attenuation. The OSWER policy defined MNA as the reliance on natural attenuation processes to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods. The natural attenuation in-situ processes that are at work included a variety of physical, chemical, or biological processes that under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentrations in soil or groundwater. These processes include: biodegradation, dispersion, dilution, sorption, volatilization, radioactive decay, chemical or biological stabilization, transformation, or destruction of contaminants. 1 The term inorganic contaminants is used in this document as a generic term for metals and metalloids (such as arsenic); and also refers to radiologic as well as non-radiologic isotopes. 1

2 In 2001, the EPA s science advisory board (SAB) reviewed EPA s Research Program for Monitored Natural Attenuation. This review was published as EPA-SAB-EEC This report further defined MNA as naturally occurring processes in the environment that act without human intervention to reduce the mass, toxicity, mobility, volume or concentration of contaminants. Immobilization was identified as the primary process operative for contaminant metals and metalloids. In developing this guidance, EPA s MNA work group decided not to include the dispersion and dilution physical processes. However, radioactive decay, an important process for the radionuclide elements, will be covered in the guidance. The Need The guidance will identify and address the primary technical issues that are needed to address the scientific foundation for assessment and acceptance of MNA. They are the following: -Timescale of the attenuation process will be consistent with regulatory needs for the site remediation. -Stability of immobilized contaminant will be sufficient to resist re-mobilization due to changes in site geochemistry. -Assessment of the immobilization process should employ technically feasible and scientifically defensible analytical methods. -Employment of geochemical models is recommended to test the feasibility of the immobilization process. In addition, the scientific and technical needs that will provide the basis for a reasoned and logical framework for screening candidate sites and assessing the technical requirements to demonstrate viability will be developed. This approach will include the following scientific and technical needs: -Identification of contaminant-specific and site-specific immobilization processes -Determination of site-specific rate and capacity of immobilization process -Evaluation and codification of test methods to assess stability of immobilized contaminants -Development and application guidance on geochemical models for site characterization Framework Document Focus An important part of this effort was the creation of an EPA Framework Document Team and the selection of EPA and non-epa subject matter experts to develop this document. (See Members of the Framework Document Team section). The Framework Document Team s goal is to address the following topics: -Select inorganic contaminants, including radionuclides, in ground water (GW) -Provide a tiered approach for guiding decisions on the use of MNA 2

3 -Focus on the groundwater media; with the unsaturated zone treated as a source term -Address both the radiological and non-radiological inorganic elements MNA Decision Analysis: Tiered Approach In the tiered approach analysis, EPA will recommend that the remedy chosen meet the following benchmarks: -Actively demonstrate removal from ground water. This demonstration will require site specific data and the theoretical basis for the contaminant removal from ground water. -Identify/confirm primary mechanism(s) of removal. Mechanisms are identified for the selected contaminates in Table 1. -Demonstrate long-term capacity and stability of the major attenuation mechanisms and processes. -Design a monitoring program and define the regulatory triggers for MNA failure such as maximum concentration levels (MCLs) (see Table 2). -Establish a contingency plan if MNA fails the regulatory triggers and clean-up levels. Contaminant Selection Criteria The radioactive elements (radionuclides) proposed for inclusions in this Monitored Natural Attenuation guidance are: americium, cesium, cobalt, iodine, plutonium, radium, radon, strontium, technetium, thorium, tritium, and uranium. The non-radioactive elements and inorganics proposed for inclusions are: arsenic, cadmium, chromium, copper, lead, mercury, nickel, nitrate, perchlorate, selenium, and zinc. The selection of these contaminants by EPA was based on two criteria. First, the elements had to be one of high priority to the site remediation or risk assessment activities of EPA (EPA 1993, EPA 2002). Second, selection was based on chemical behavior considering chemical traits such as: toxicity, cations, anions, conservatively transported, non-conservatively transported, and redox sensitive elements (EPA, 1999a&b and 2004) (see Table 3). By using these characteristics of the contaminants, the general geochemical behavior of a wide range of radionuclide contaminants could be covered as well as the chemical classes that make up the Periodic Table. The classes would include: -nonmetals, -transition, noble metals, and lanthanides, -alkaline metals and earths, -actinides and transuranics. In addition, this selection accounts for many daughter and fission product contaminants in the radioactive decay chains. This factor is important as the decay of radioisotopes 3

4 can produce daughter products that may differ both physically and chemically from their parents. These daughter products may be more or less mobile and hazardous than their parents. The daughter products can decay by several different paths, emitting different types of radiation in simple steps or in complex decay chains. The selection of radionuclide contaminants for this document is representative of these characteristics. As stated above, the inorganic contaminants selected for this document include: arsenic, cadmium, chromium, copper, lead, mercury, nickel, nitrate, perchlorate, selenium, and zinc. The selection of these contaminants by EPA was based on several criteria. First, a 1994 booklet containing information regarding common chemicals found at Superfund sites across the nation was consulted (EPA, 1994). The most commonly found inorganic contaminants were included for consideration in this document, particularly those which tend to migrate into ground water. Another document specific to metal-contaminated Superfund sites (EPA, 1995) identified arsenic, cadmium, chromium, mercury, and lead as the primary contaminants of concern based on toxicity, industrial use, and frequency of occurrence at Superfund sites. Second, selection criteria was based on chemical behavior considering chemical traits such as: toxicity, ion charge (cation vs. anion), transport behavior (conservative vs. non-conservative), and redox chemistry to cover a broad range of geochemical behavior as mentioned above for the radionuclide elements. Finally, EPA regional staff was asked to nominate inorganic contaminants that occurred frequently or that were problematic in their regions. The above list of eleven non radioactive inorganic contaminants was selected from this process. Document Structure The guidance document will consist of two major parts. The first part will be on the regulatory policy, the scientific basis, and the developed framework for technical assessment of radioactive and inorganic contaminates at MNA sites. The second part will encompass the specific contaminant assessment of the MNA feasibility based on the current state of knowledge. In addition, a strong emphasis is placed on site characterization with regards to hydrology and biogeochemistry for solid and aqueous matrices. The proposed document structure is divided into two major parts. The first part of the document, Basis for Assessment, is designed to address the following four sections and topics: -Section I will provide a conceptual background for natural attenuation. -Section II will provide the scientific basis for natural attenuation. -Section III will discuss site characterization. -Section IV will discuss modeling and the tiered approach. Part two of the document is titled Element Chapters. This part is designed to address each individual element or inorganic contaminant. Each chapter will discuss the occurrence and distribution of that element or contaminate, the regulatory aspects governing the element or contaminant, the dominant geochemistry and attenuation processes, the subsurface site characterization that is needed, the long-term stability and 4

5 capacity processes, and the supporting tiered analysis approach to address the uncertainty of selecting MNA for that radionuclide or element. Framework Document Development As might be expected with such guidance, an extensive document review process will take place. The following EPA Program Offices will conduct an internal review: Office of Research and Development (ORD), Office of Solid Waste and Emergency Response (OSWER), Office of Air and Radiation (ORIA), and the Regional Groundwater Forum. Additional reviews will include, among others, the Interstate Technology Regulatory Council (ITRC). This guidance document is expected to be a working document with revisions, updates and additions as needed. This fact is particularly important as an increased understanding and knowledge of the elemental and contaminant chemistry expands with increased field experience and technology applications. MNA Performance Monitoring Categories The 1999 EPA OSWER Directive P, requires that assessment of the background contaminant levels and ambient geochemical indicators must be done. This directive also requires that process monitoring is performed to verify progress. This will assure that the chemical attention mechanisms exist and are functioning properly. Finally, the directive requires monitoring to detect any plume expansion that might occur. The performance monitoring objectives required by the OSWER directive include: Summary -Demonstrate that natural attenuation is present -Detect changes in environmental conditions -Identify potentially toxic and/or transformation products -Verify plume is non expanding -Verify no unacceptable impact downgradient -Detect new releases of contaminants to the environment that impact MNA -Demonstrate efficacy of institutional controls in place to protect receptors -Verify attainment of remediation objectives The U.S Environmental Protection Agency s ongoing effort to develop the Monitored Natural Attenuation guidance is underway with an expected completion timeframe of The document will follow the OSWER Directive (No P, 1999) on the use of Monitored Natural Attenuation at OSWER Superfund, Corrective Action, and Underground Storage Tanks sites. Natural attenuation processes that reduce the mass, toxicity, mobility, volume or concentration of contaminants are the preferred processes. The guidance will identify and address the primary technical issues needed to support the scientific foundation for assessment and acceptance of MNA. A reasoned and logical framework for screening candidate sites and assessing the technical requirements for selected contaminants, including radionuclides and inorganic elements, will be developed. A decision analysis based on a tiered approach that relies on demonstrating 5

6 removal from ground water, identifying the chemical mechanisms and long term capacity with an emphasis on a monitoring program is outlined. Selected radioactive elements have been proposed based on a number of occurrence and chemical criteria. The same selection process was used for the non-radioactive elements and contaminants. EPA created a Framework Document Team including both EPA and non-epa subject matter experts. An extensive document review process will take place in late 2004 with an expected publication timeframe in References USEPA (U.S. Environmental Protection Agency) Environmental Characteristics of EPA, NRC, and DOE Sites Contaminated with Radioactive Substances. EPA 402-R , U.S. Environmental Protection Agency, Office Radiation and Indoor Air, Office of Solid Waste and Emergency Response, Washington D.C. USEPA (U.S. Environmental Protection Agency) Common Chemicals Found at Superfund Sites. EPA/OSWER No. 9203, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington D.C. USEPA (U.S. Environmental Protection Agency) Contaminants and Remedial Options at Selected Metal-Contaminated Sites. EPA/540/R-95/512, U.S. Environmental Protection Agency, Washington, D.C. USEPA (U.S. Environmental Protection Agency). 1999a. Understanding Variation In Partition Coefficient, Kd, Values Volume I: The Kd Model of Measurement, And Application Of Chemical Reaction Codes. EPA 402-R A, U.S. Environmental Protection Agency, Office of Radiation and Indoor Air, Washington D.C. USEPA (U.S. Environmental Protection Agency). 1999b. Understanding Variation In Partition Coefficient, Kd, Values Volume II: Review of Geochemistry And Available Kd Values For Cadmium, Cesium, Chromium, Lead, Plutonium, Radon, Strontium, Thorium, Tritium ( 3 H), And Uranium. EPA 402-R B, U.S. Environmental Protection Agency, Office of Radiation and Indoor Air, Washington D.C. USEPA (U.S. Environmental Protection Agency). 1999c. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive p, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington D.C.. USEPA (U.S. Environmental Protection Agency) Monitored Natural Attenuation: USEPA Research Program-an EPA Science Advisory Board Review. EPA-SAB-EEC , U.S. Environmental Protection Agency, EPA Science Advisory Board, Washington D.C. 6

7 USEPA (U.S. Environmental Protection Agency) Common Radionuclides Found at Superfund Sites. EPA 540/R , U.S. Environmental Protection Agency, Office of Radiation and Indoor Air, Washington D. C. USEPA (U.S. Environmental Protection Agency) Understanding Variation In Partition Coefficient, Kd, Values Volume III: Review of Geochemistry and Available Kd Values for Americium, Arsenic, Curium, Iodine, Neptunium, Radium, and Technetium. EPA 402-R C, U.S. Environmental Protection Agency, Office of Radiation and Indoor Air, Washington D.C. Members of the Framework Document Team Chair Co-Chair Co-Chair Co-Chair Members Non-EPA Members Robert Puls, EPA, Office of Research and Development (ORD), National Risk Management R Laboratory (NRMRL) Ken Lovelace, EPA, Office of Solid Waste and Emergency Response (OSWER), Office of S Technology Innovation (OSRTI) Stuart Walker, OSWER/OSRTI Ron Wilhelm, Office of Air and Radiation (OAR), Office of Radiation and Indoor Air (ORIA) Robert Ford, ORD Rick Wilkin, ORD Steve Acree, ORD Ann Keeley, ORD Chunming Su, ORD Steve Mangion, ORD, EPA Region 1 Jim Ammonette, Pacific Northwest National Laboratory Paul Bertsch, University of Georgia Craig Bethke, University of Illinois Pat Brady, Sandia National Laboratory Doug Kent, U.S. Geological Survey 7

8 Tables Table 1. Parameters Influencing Contaminant Sorption Element Important Aqueous- and Solid-Phase Parameters (EPA, 1999b) Influencing Contaminant Sorption Cd [Aluminum/Iron-Oxide Mineral], [Calcium], Cation Exchange Capacity, [Clay Mineral], [Magnesium], [Organic Matter], ph, Redox, [Sulfide] Cs [Aluminum/Iron-Oxide Mineral], [Ammonium], Cation Exchange Capacity, [Clay Mineral], [Mica-like clays], ph, Potassium Cr [Aluminum/Iron-Oxide Mineral], [Organic Matter], ph, Redox Pb [Aluminum/Iron-Oxide Mineral], [Carbonate, Fluoride, Sulfate, Phosphate], [Clay Mineral], [Organic Matter], ph, Redox Pu [Aluminum/Iron-Oxide Mineral], [Carbonate, Fluoride, Sulfate, Phosphate], [Clay Mineral], [Organic Matter], ph, Redox Rn none Sr Cation Exchange Capacity, [Calcium], [Carbonate], ph, [Stable Strontium] Th [Aluminum/Iron-Oxide Mineral], [Carbonate], [Organic Matter], ph 3 H none U [Aluminum/Iron-Oxide Mineral], [Carbonate, Fluoride, Sulfate, Phosphate], [Clay Mineral], [Organic Matter], ph, Redox, [U] 8

9 Table 2. Radionuclide Maximum Concentration Levels (MCLs) Radionuclide Current MCL or UMTRCA a (pci/l) Am E-09 Cs E-09 H-3 20, E-09 I E-06 Pu E-07 Ra h 5.1E-09 Ra h 1.8E-11 Sr E-11 Tc E-05 U c 4.8E-06 U (or 30 µg/l) c 3.0E-02 U c 9.0E-02 U (or 30 µg/l) c 3.0E-02 Mass Equivalent to MCL, Proposed MCL or RBL (mg/l) a Federal Register, Vol. 65, No. 236, December 2, 2000; MCL is 4 mrem/yr to the whole body or an organ, combined from all beta and photon emitters; MCL is 15 pci/l, with the concentration level combined for all alpha emitters, except radon and uranium. b National Primary Drinking Water Standards, EPA 816-F July 2002 c Federal Register, Vol. 42, No. 65, March 2, 2000, Rules and Regulations; MCL standard is 30 µg/l for uranium; UMTRCA groundwater standard is 30 pci/l combined for U-234 and U-238. d Lead and copper are regulated by a Treatment Technique that requires systems to control the corrosiveness of their water. If more than 10% of tap water samples exceed the action level, water systems must take additional steps. For copper, the action level is 1.3 mg/l, and for lead is mg/l. e National Secondary Drinking Water Regulations (non-enforceable guidelines) f National Recommended Water Quality Criteria: 2002, EPA 822-R , November 2002; CMC = Criteria Maximum Concentration, CCC = Criterion Continuous Concentration g Risk Based Limits calculated for 30-year exposure duration and 1 x 10-6 risk. These were calculated using equation 11 in Risk Assessment Guidance for Superfund (RAGS): Volume I: Human Health Evaluation Manual (Part B, Development of Risk-based Preliminary Remediation Goals), (page 37). The equations were adjusted to account for radioactive decay. h MCL is 5 pci/l combined for Ra-226 and Ra

10 Table 3. Selected Chemical Behavior/Chemical Traits Element (EPA, 1999b) Radionuclide Primary Species at ph7 and oxidizing conditions Redox Sensitive Transport Through Soils at ph7 Cationic Anionic Neutral Not Retarded Retarded Cd X X X Cs X X X Cr X X X X Pb X X X X Pu X X X X X Rn X X X Sr X X X Th X X X 3 H X X X U X X X X X Am X X X As X X X (limited) Cm X X X I X X X X (limited) Np X X X X (limited) Ra X X X Tc X X X X 10