Ground Water Models Scientific and Regulatory Applications (1990) / Chapter Skim
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5 EXPERIENCE WITH CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM
5 EXPERIENCE WITH CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM
Pages 160-210
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From page 160... ...
. These two regulatory agencies deal with contaminant transport from historic or proposed disposal facilities and recognize the need to evaluate present conditions and predict potential migrations.
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From page 161... ...
U.S. NUCLEAR REGULATORY COMMISSION REGULATIONS AND GUIDANCE One of the USNRC's responsibilities is the licensing of facilities for the disposal of low-level and high-level radioactive wastes (see 10 CFR Part 61, relicensing Requirements for Land Disposal of Radioactive Waste," and 10 CFR Part 60, "Disposal of High-Leve!
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From page 162... ...
The codes must be based on sound physical, chemical, and mathematical principles and must be correctly applied and sufficiently documented. The applicant must supply the following: a complete description of the contaminant transport pathways between the engineered disposal unit and the site boundary and existing or known future ground water user locations; ~ estimates and justification for the physical and chemical input parameters used in the transport models to calculate radionuclide concentrations; a description of the contaminant transport models used in the analysis, including modeling procedures and complete documentation of the codes as required in NUREG-0856 (U.S.
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From page 163... ...
The program, as outlined in the USNRC guidance and the SRPs, appears to emphasize conservatism, although the regulations place equal emphasis on collecting adequate field information. Also, the program has not attempted to direct applicants toward particular computer codes because the codes the USNRC will use are not defined.
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From page 164... ...
The key EPA regulations and guidance affecting the use of contaminant transport moclels e.g., those in the SuperfuncI, hazardous waste management, ant} underground injection programs-are incluclec! in the following · ~ c .lscusslon.
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From page 165... ...
Superfund Is primarily directed at cleanup of inactive hazardous waste sites. Courts generally have resolved legal uncertainties and issues of statutory interpretation in favor of the government in order to hold private parties liable, because Enliven the remedial nature of .
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From page 166... ...
estimation, is then compared to levels of public health concern, e.g., a drinking water standard or a risk-based cleanup level (Zamuda, 1986~. EPA advises that "caution should be used when applying models at Superfund sites because there is uncertainty whenever subsurface movement is modeled, particularly when the results of the mode!
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From page 167... ...
Resource Conservation and Recovery Act Law and Regulations There are tens of thousands of facilities that handle hazardous waste and therefore must obtain a permit. The Resource Compensa
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From page 168... ...
The EPA definition of a hazardous waste determines "whether a waste, if mismanaged, has the potential to pose a significant hazard to human health or the environment due to its propensity to leach toxic compounds" (51 Fed.
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From page 169... ...
28,892 t1988~°~. The proposal therefore would incorporate a contaminant transport into the definition of hazardous waste.
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From page 170... ...
Guidance Contaminant transport modeling can be used for the same purposes as in the Superfund program. EPA's general exposure guidance concerning the use of models (above)
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From page 171... ...
regulations govern, among other wells, Class ~ wells, those wells used to dispose of hazardous waste below an underground source of drinking water. Class ~ wells are subject to regulations that specify minimum design construction and operating conditions and require continued monitoring of the nearby ground water to ensure that a present or future drinking water supply is not endangered (40 CFR Part 144)
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From page 172... ...
As a practical matter in this situation, the only choices other than using contaminant transport models would be to rely on the best professional judgment of a qualified hydrologist or provide for no exemptions to the ban on hazardous waste disposal. Because the statute provides for such exemptions, EPA has attempted to balance the scientific uncertainty.
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From page 173... ...
The Madison aquifer case illustrates the use of a variety of ground water flow modeling approaches to predict water-level declines from large well withdrawals in an aquifer system in which very little is known about the hydraulic properties of the aquifer (Konikow, 1976~. Accurately predicting ground water flow conditions is an essential first step in simulating contaminant transport in ground water, and this case study illustrates particularly well techniques that can be used to assess the reliability of predicted ground water flow conditions.
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From page 174... ...
Madison aquifer Evaluation of large ground water withdrawals from an aquifer using a ground water flow model when aquifer parameters are poorly known. Snake River plain A ground water transport model used to predict migration of chloride, tritium, and strontium-90 in basalts.
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From page 175... ...
The discussion that follows this case study highlights the conceptual problems that the committee foresees as a result of using the current generation of ground water transport models for this purpose. The S-Area case study examines the use of models to investigate the migration of a immiscible, denser-than-water fluid within an aquifer.
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From page 176... ...
EPA assumes that no attenuation occurs because this is a reasonable worst-case characteristic of saturated soil systems, and because the water table is near the bottom of many waste sites. The third step of the modeling process calculates the dispersion of the chemical compound in a drinking water aquifer in the vertical and horizontal directions perpendicular to ground water flow as a result of a continuous source of contamination.
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From page 177... ...
Y = distance from disposal site to compliance point (m) X = length of the disposal site measured in the direction perpendicular to the direction of ground water flow (m)
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From page 178... ...
The reasonable worst-case parameter values used by EPA are as follows: . Distance to well: EPA uses 500 It for the distance from the waste disposal facility to the drinking water well.
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From page 179... ...
This sludge ~ classified as a hazardous waste in 40 CFR §260.22, and Gould, Inc. petitioned to delist the waste based on the destruction and immobilization of hazardous compounds by its wastewater treatment system.
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7,906 [1988~) is that [uinlese the Agency is able to assure protection of human health and the environment without generic, conservative assumptions, the Agency will employ these assumptions.
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As described above, EPA has proposed using the EPASMOD to define hazardous wastes. Although EPA rejected the use of EPASMOD for delisting petitions in 1986, it also indicated that it might reconsider use of that mode!
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From page 182... ...
study were to (1) improve the conceptual mode} of ground water flow in the aquifer system; (2)
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From page 183... ...
of the Madison aquifer. For example, the important influences of temperature differences and aquifer discontinuities on ground water flow in the Madison were recognized and documented as a result of the mode!
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184 GROUND WATER MODELS 2000 500 400 300 200 100 \ 2 5 10 20 30 40 50 60 70 80 90 95 98 PERCENTAGE GREATER THAN FIGURE 5.5 Calculated probability distribution of drawdowns at the Niobrara well field. reevaluate the system in a true three-dimensional framework so as to better consider vertical components of flow.
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From page 185... ...
This case study demonstrates that models can be very useful tools for gaining an understanding of aquifer systems in which little is known about the hydraulic properties of the system and that these models can be invaluable for prioritizing field data collection activities so that a maximum amount of information can be obtained for a given expenditure. Accurately predicting ground water flow conditions is an essential first step in simulating contarrunant transport in ground water, and this case study illustrates, particularly well, techniques that can be used to assess the reliability of predicted ground water flow conditions.
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From page 186... ...
These formations contain a vast amount of ground water and make up the major aquifer in Idaho, which is known as the Snake River plain aquifer. Ground water flow is generally to the southwest at relatively high velocities (5 to 20 ft/day)
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From page 187... ...
Lewis and Goldstein (1982) report that eight wells were drilled during the summer of 1980 near the southern boundary to help fill data gaps and to monitor contaminants in ground water flowing across the INEL boundary.
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188 ' ~,': _ 43030, GROUND WATER MODELS 113° Disposal / Ponds Am
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CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM 189 Disposal / Ponds TRA _ ,~ _ 40030' ~ /ICPP :<,~/~11: ~67 40 RWMC _/ /: ,1 r CFA - 20 EXPLANATION Improved roads · Disposal well 0 1 2 MILES -20- Line of equal chloride concentration, t~-T ~ in milligrams per liter: 0 1 2 KILOMETERS interval varies. FIGURE 5.7 Model-projected distribution of waste chloride in the Snake River plain aquifer for 1980 (ICPP-TRA vicinity)
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From page 190... ...
7~ 0 1 2 MILES I L I 0 1 2 KILOMETERS FIGURE 5.8 Distribution of waste chloride in the Snake River plain aquifer (ICPP-TRA vicinity) , October 1980.
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From page 191... ...
The Tucson area, with a population of 517,000, is one of the largest metropolitan areas in the country that is totally dependent on ground water for drinking water, and the trichIoroethene contamination was viewed as a threat to the integrity of the water supply system. The area containing the contaminated ground water is listed on the National Priority List and is known as the Tucson Airport Area Superfund Sites (51 Fed.
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:x'\. _ BOUNDARY OF U\~ _~W ~/ _ AIR FORCE LAND N. I Scale miles FIGURE 5.9 Location of Tucson Airport.
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From page 193... ...
The remedial investigation made some general conclusions regarding the significance of individual source areas, but these conclusions were questioned by the potentially responsible parties (Arizona Department of Health Services, 1986~. As a result, EPA, Region ~X, asked CH2M Hill, an environmental consulting firm, to conduct an assessment of potential sources.
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From page 194... ...
The reasons given for this assumption were the following: the extraction system required to contain or withdraw contaminated ground water is not dependent on the levels of contamination but, rather, is nearly directly proportional to the area of contamination; ~ treatment costs are influenced more by volume of water treated than by actual levels of contamination; and ~ the quantity of contaminants released cannot be reliably estimated when only low levels of contamination are observed. The relative contribution of a source area was assessed with the following equation: where ( 1 m A )
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From page 195... ...
Mode} of Source Contributions The area of contamination resulting from a contaminant release from an individual source could not be determined with the information available on the distribution of contaminants. Rather, the area of contamination from a source was estimated using a two-dimensional numerical contaminant transport model.
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From page 196... ...
and A2 AT = 1 = area of affected aquifer under consideration GROUND WATER MODELS / Al A2 A3 / RC (A Ai )
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From page 197... ...
simulations provided results for only specific cases: that is, the best estimate of permeability in each zone and one contaminant release scenario. CH2M Hill stated that if multiple simulations were performed to take into account the plausible variations in permeability and trichIoroethene releases, a range of relative contributions could be developed for each source in a quantitative manner.
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From page 199... ...
The Tucson Airport Area Remedial Investigation, which was managed by the Arizona Department of Health under a cooperative agreement with EPA, was concluded in 1985. To date, over 100 monitoring wells have been drilled to identify, characterize, model, and monitor the contamination in the area (Environmental Protection Agency, 1988a)
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From page 200... ...
Ground water flow and transport models have been used extensively at all of these sites, and the use of these models is particularly well documented (Mercer et al., 1985; C Faust, affidavits in Civil Action Nos.
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CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM 201 These contaminant transport models have also been incorporated into legally enforceable documents and have been evaluated and approved by a court. For simplicity, this case study focuses primarily on the models used at the S-Area site, because they illustrate the complex processes that can be simulated with the current generation of ground water transport models.
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202 Before Rain Lagoon Leaks GROUND WATER MODELS Rain .: .
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From page 203... ...
was constructed with a 2-ft negative head difference (downward flow) between the water table and bedrock potentiometric level.
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From page 204... ...
204 GROUND WATER MODELS TABLE 5.5 Capillary Pressure and Relative Permeability Data for ADL Simulation 1 Capillary Water Relative Permeabilities Pressure (N/m2) a Saturation Water NAPL Fine sand and bedrock 103,425.0 0.00 0.00000 1.00000 103,425.0 0.10 0.00000 0.82000 103,425.0 0.20 0.00000 0.68000 27,580.0 0.30 0.04000 0.55000 10,343.0 0.40 0.10000 0.43000 7,585.0 0.50 0.18000 0.31000 7,447.0 0.60 0.30000 0.20000 7,309.0 0.70 0.44000 0.12000 7,171.0 0.80 0.60000 0.05000 7,033.0 0.90 0.80000 0.00000 6,895.0 1.00 1.00000 0.00000 Clay 206,850.0 0.00 0.00000 1.00000 206,850.0 0.10 0.00000 0.82000 206,850.0 0.20 0.00000 0.68000 165,480.0 0.30 0.04000 0.55000 134,453.0 0.40 0.10000 0.43000 110,320.0 0.50 0.18000 0.31000 93,082.0 0.60 0.30000 0.20000 82,740.0 0.70 0.44000 0.12000 75,845.0 0.80 0.60000 0.05000 72,398.0 0.90 0.80000 0.00000 68,950.0 1.00 1.00000 0.00000 aN = newton (i.e., kg-n~s2)
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From page 205... ...
hSR-tWT = 9 It ky fine sand = 10 5 cm/s Initial NAPL distribution ADL ~ 250 days SWANFLOW ~ 275 days Multiple points Model boundary nodes ~ jI "\ LFine sand ; ~ 0 20 40 60 NAPL DISTRIBUTION, percent Bedrock FIGURE 5.13 NAPL saturation profiles at one time for the two-layer simulation. The effects of a water-phase hydraulic gradient on NAPL migration were also examined via these simulations, where the clay layer was assumed to be musing.
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From page 206... ...
Discussion This case study illustrates the use of relatively complex models of ground water and NAP L flow to help design a remedial action for a hazardous waste site. Field studies have shown that both of these models were able to simulate observed field conditions.
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From page 207... ...
14. Hazardous Waste Management System; Identification and Listing of Hazardous Waste: Use of a Generic Dilution/Attenuation Factor for Establishing Regulatory Levels and Chronic Toxicity Reference Level Revisions.
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From page 208... ...
EPA/700-8-87-037, Hazardous Waste Ground-Water Task Force, Washington, D.C. Environmental Protection Agency.
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From page 209... ...
1985. Results of the Tucson Airport Area Remedial Investigation, Volume III, Contaminant Transport Modeling.
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From page 210... ...
Plan for the Computer Software Supporting the U.S. Nuclear Regulatory Commission's High-Level Waste Management Program.
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