1、 Groundwater Remediation Strategies Tool Regulatory Analysis however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any
2、 federal, state, or municipal regulation with which this publication may conflict. All rights reserved. No part of this work may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission f
3、rom the publisher. Contact the Publisher, API Publishing Services, 1220 L Street, N.W., Washington, D.C. 20005. Copyright 2003 American Petroleum Institute iii ACKNOWLEDGMENTS API would like to acknowledge the following people for their contributions of time and expertise during this study and in th
4、e preparation of this report: API STAFF CONTACT Harley Hopkins, Regulatory Analysis and Scientific Affairs Department (RASA) MEMBERS OF THE SOIL AND GROUNDWATER TECHNICAL TASK FORCE Curtis Stanley, Shell Global Solutions (US), Chairman iv ABSTRACT This guide provides strategies for focusing remediat
5、ion efforts on 1) the change in contaminant mass flux1in different subsurface transport compartments (e.g. the vadose zone, smear zone or a zone within an aquifer of interest) and 2) the change in remediation timeframe. In this approach, groundwater flow and contaminant concentration data are combin
6、ed to estimate the rate of contaminant mass transfer past user-selected transects across a contaminant plume. The method provides the user with a means to estimate the baseline mass flux and remediation timeframe for various transport compartments and then evaluate how different remedies reduce the
7、mass flux and the remediation timeframe in each transport compartment. Results from one or more transects can be used to evaluate: Potential water quality impacts on downgradient water supply wells. The natural attenuation of the contaminant mass with distance downgradient of the source. The relativ
8、e benefits of remedies based on their anticipated reductions in mass flux from the source to the receptor. In addition to step-by-step instructions for the strategies, several utilities are provided including: Worksheets for estimating baseline mass flux and remediation timeframe and evaluating pote
9、ntial remedies. Tools for calculating mass flux. Resources on estimating remediation lifetime and evaluating remedy flux reduction / mass removal factors. Tools for evaluating how long it takes for an upgradient remedial action to affect a downgradient groundwater transect zone. 1Strictly speaking,
10、mass discharge v CONTENTS page 1.0 INTRODUCTION - MASS FLUX APPROACH. 1 1.1 Transport Compartments 1 1.2 Mass Flux and Remediation Timeframe . 2 1.3 Structure of This Document 2 1.4 Key Definitions . 2 2.0 GENERAL GROUNDWATER REMEDIATION PROCESS (FLOWCHART 1) 4 2.1 Preliminary and Detailed Site Char
11、acterization. 4 2.2 Baseline Mass Flux and Remediation Timeframe Evaluation Tool (Worksheet 1). 6 2.3 Remedy Evaluation Tool Using Mass Flux and Remediation Timeframe (Worksheet 2). 7 3.0 TOOLS FOR CALCULATING MASS FLUX 10 3.1 Groundwater Mass Flux Calculation - Transect Method. 10 3.2 Groundwater M
12、ass Flux Calculation - Solute Transport Model Method . 13 3.3 Groundwater Mass Flux Calculation - Extraction Well Method. 14 3.4 Control Point Concentration Calculation . 14 3.5 Vadose Zone to Groundwater Mass Flux Calculation. 15 4.0 TOOLS FOR ESTIMATING REMEDIATION LIFETIMES 17 4.1 Key Resources B
13、ooks 17 4.2 Key Resources Data Interpretation Methods . 17 4.3 Key Resources Models 17 4.4 Key Resources Field Tests 17 5.0 TOOLS FOR EVALUATING FLUX REDUCTION FACTORS AND MASS REMOVAL FACTORS . 18 5.1 Removal Technologies. 18 5.2 Containment Technologies. 19 5.3 Remediation References 19 6.0 TOOLS
14、FOR EVALUATING CHANGES IN GROUNDWATER MASS FLUX AFTER REMEDIATION 21 6.1 How to Use the Mass Flux vs. Distance Curves . 21 6.2 Constant Source. 22 6.3 Decaying Source 23 6.4 Step-Function Source. 25 vi CONTENTS (continued) page REFERENCES 27 APPENDIX A: MASS FLUX vs. DISTANCE CURVES .A-1 APPENDIX B:
15、 EXAMPLES B-1 Example 1: Baseline MTBE Mass Flux B-2 Example 2: Remediation with Soil Vapor Extraction . B-16 Example 3: Remediation with Multi-Phase Extraction. B-19 Example 4: Point-of-Use Control. B-27 BLANK WORKSHEETS Worksheet 1 Worksheet 2 API Groundwater Remediation Strategies Tool 1 1.0 INTR
16、ODUCTION - MASS FLUX APPROACH Potential impacts on groundwater receptors and the need for and relative benefits of alternative remedial measures may be evaluated on the basis of the mass flux of contaminants from the source zone to the receptor. This mass-based approach to site assessment and remedi
17、ation has been described by various researchers (Einarson Gallagher et al, 1995) and identified by USEPA as a key consideration in the evaluation of natural attenuation remedies (USEPA, 1998). Under this approach, groundwater flow and contaminant concentration data are combined to estimate the rate
18、of contaminant mass transfer (e.g., grams per day) past selected transects through an affected groundwater plume. Strictly speaking, this is a mass discharge rate; however the term “mass flux” is typically used to describe mass discharge, and this convention will be used in this document. Results fr
19、om one or more such transects can then be used to evaluate: i) potential water quality impacts on downgradient supply wells (as determined from a mass balance analysis of the supply well pumping rate), ii) the natural attenuation of the contaminant mass with distance downgradient of the source (as d
20、efined by the reduction in mass flux between transects), and iii) the relative benefits of alternative remedies (based on their anticipated reductions in mass flux from source to receptor). The Environmental Protection Agencys Natural Attenuation Seminar (USEPA, 1998) summarized the benefits of the
21、mass flux approach to evaluate groundwater impacts: “The reduction in the flux along the flowpath is the best estimate of natural attenuation of the plume as a whole.“ “The flux is the best estimate of the amount of contaminant leaving the source area. This information would be needed to scale an ac
22、tive remedy if necessary.“ “Flux estimate across the boundary to a receptor is the best estimate of loading to a receptor.“ Pankow and Cherry (1996) state that: “Therefore, the ultimate impact of plumes emanating from solvent DNAPL source zones can be evaluated in terms of the impact of relatively s
23、mall annual mass fluxes to the receptor such as water-supply wells or surface waters. In some cases, the fluxes present significant risk to human health and/or the environment, and extensive remedial action is warranted. In other cases, the fluxes are insignificant, and remedial action would provide
24、 little or no actual environmental risk reduction.“ In summary, the use of a mass flux approach is a powerful tool for risk management (Einarson and Mackay, 2001a), one that can be used to identify high-risk sites that require higher degrees of site investigation and corrective action. This is parti
25、cularly true for MTBE, as it is attenuated less in the subsurface than other plume constituents from fuel releases at many sites. 1.1 Transport Compartments Several researchers have identified how remediation efforts can focus on individual components of a release site. For example, Gallagher, et al
26、. (1995) developed a “Mass-Based Corrective Action“ approach where the masses in different “compartments“ (soil, smear zone, and dissolved plume) were estimated and the cost per pound to remediate these masses was estimated. The concept of different transport compartments is well suited for the mass
27、 flux approach, and the conceptual remediation framework described in this document is based on evaluating the vadose zone, smear zone, and several “transect zones“ in the dissolved plume. API Groundwater Remediation Strategies Tool 2 1.2 Mass Flux and Remediation Timeframe A logical extension of th
28、e mass flux approach is to use mass flux estimates with approximations of source masses to derive order-of-magnitude estimates of remediation timeframe. With this approach, remediation efforts can focus on the change in two key process variables: 1) The change in mass flux in different transport com
29、partments; 2) The change in remediation timeframe. Although estimating remediation timeframe involves considerable uncertainty, relative changes in remediation timeframe can be performed with some degree of accuracy. The conceptual remediation framework described in this document discusses methods t
30、o estimate source masses and remediation timeframes in different transport compartments. 1.3 Structure of This Document This document expands on a mass flux framework originally proposed by Einarson and MacKay (2001a) and provides tools for evaluating mass flux at affected sites. While the framework
31、 can be used for any constituent, it was originally developed with a focus on MTBE releases from petroleum release sites. This expanded mass flux framework consists of the following elements: null Groundwater Remediation Process Flowchart (Section 2, Figure 1); null Baseline Mass Flux and Remediatio
32、n Timeframe Tool (Section 2, Worksheet 1); null Remediation Evaluation Tool Using Mass Flux and Remediation Timeframe (Section 2, Worksheet 2); null Mass Flux Calculation Tools (Section 3); null Remediation Timeframe Tools (Section 4); null Resources for Evaluating Mass Flux and Mass Reduction Facto
33、rs (Section 5); null Tools for Evaluating Changes in Groundwater Mass Flux after Remediation (Section 6 and Appendix A); and null Method Examples (Appendix B). 1.4 Key Definitions Action Level: Typically a concentration-based standard in either groundwater, water being extracted from a water-supply
34、well, or a surface water quality standard. Blending: The mixing and dilution of mass flux in either: i) a water supply well that pumps both clean water and groundwater containing a site constituent; or ii) a stream that mixes constituents in groundwater with clean surface water. Control Point: Under
35、 a mass flux-based approach, the point where the mass flux of the constituent is to be managed. Examples include the intake of a well downgradient of a plume, or at the discharge point to a surface water body. Flow Area: The segmented area associated with a specific concentration measurement over wh
36、ich an individual mass flux estimate is calculated. Groundwater Transect Zone: The zone between two groundwater transects drawn across the dissolved constituent plume. Mass Flux: The mass per time moving across a control area in a transport compartment in units of mass per time. This is also called
37、the mass flowrate or the mass discharge rate. Note that some researchers refer to mass flux in units of mass per area per time. For this document, mass flux is used in a more general manner to mean mass per time crossing a transect. In this document, mass flux is represented by the symbol w. API Gro
38、undwater Remediation Strategies Tool 3 Source Zone: The zone that includes both the affected soils in the vadose zone and the smear zone. Transport Compartment: Either the vadose zone, smear zone, or a transect zone that has a constituent mass flux associated with it. Variables Used in Worksheets 1
39、and 2: Variables used in Worksheets 1 and 2 take the form: A_BCWhere: A indicates the parameter represented, either the mass flux (w), the timeframe (t), the flux reduction factor (rw), or the concentration (C); B indicates the chronology of the parameter, indicating its occurrence either before rem
40、ediation, as a baseline (b), or after remediation (ar); and C indicates the transport compartment, occurring at the vadose zone (vd), the smear zone (sm), the total source zone (ts), at one of four groundwater transects (gw-1, gw-2, gw-3, and gw-4), at the control point (cp), or at the point-of-use
41、(pou). API Groundwater Remediation Strategies Tool 4 2.0 GENERAL GROUNDWATER REMEDIATION PROCESS (FLOWCHART 1) This groundwater remediation process works by estimating the baseline mass flux and remediation timeframe for various transport compartments, and then evaluating how different remedies redu
42、ce the mass flux and remediation timeframe in each transport compartment. The effects of an upgradient remedial action (such as remediating the vadose zone) on downgradient transport compartments is also considered. The general remediation process is shown in Figure 1. 2.1 Preliminary and Detailed S
43、ite Characterization First, a preliminary site assessment is performed. If an action level is exceeded, then a detailed site assessment is conducted. It is recommended that characterization of MTBE sites be conducted using the methods presented in “Strategies for Characterizing Subsurface Releases o
44、f Gasoline Containing MTBE” (Nichols et al., 2000) (see box below). This document provides instructions on how to use risk-based decision making in the site characterization process. Excerpt from “Strategies for Characterizing Subsurface Releases of Gasoline Containing MTBE“ (Nichols et al., 2000, w
45、ww.api.org): Risk-informed decision making is a manage-ment strategy that adds exposure and risk considerations to the traditional technical, so-cial, and economic components of the correc-tive action process. The risk-informed ap-proach presented in this bulletin uses site-specific risk factors to
46、help determine the appropriate level of assessment at oxygenate release sites. It includes a review of the various risk factors associated with oxygenate sources, pathways, and receptors. Based on these factors, three levels of assessment are recognized. The standard level is appropriate for the gre
47、atest number of sites: it includes moderate sample spacing with some vertical characterization, as well as horizontal charac-terization. The limited level is appropriate at sites with fewer risk factors: it includes relatively large sample spacing with emphasis on horizontal characterization. The de
48、tailed level is warranted for sites with the most risk factors: it requires the highest level of effort for each characterization task, with relatively close sample spacing, and extensive vertical characterization of chemical concentrations and hydraulic properties. The appropriate level of assessme
49、nt is initially determined based on receptor infor-mation, since receptor data are typically easier to obtain than source or pathway data. Detailed information about receptors can nor-mally be obtained from a survey of nearby wells and land uses. Receptor characterization should consider current uses and probable future uses of affected groundwater. Once receptors are characterized and an initial level of effort is established, a subsurface investiga-tion may then be conducted to obtain detailed information about sources and pathways. The source and pathway d
