1、STD.API/PETRO PUBL 4bb4-ENGL 1998 m 0732290 Ob06581 b7T m American Petroleum X Institute MIXING ZONE MODELING AND DILUTION ANALYSIS FOR WATERQUALITY-BASED NPDES PERMIT LIMITS Health and Environmental Sciences Department Publication Num ber 4664 April 1998 American Petroleum Institute American Petrol
2、eum Institute Environmental, Health, and Safety Mission and Guiding Principles MISSION The members of the American Petroleum Institute are dedicated to continuous eftorts to improve the compatibility of our operations with the environment while economically developing energy resources and supplying
3、high quality products and services to consumers. We recognize our responsibility to work with the public, the government, and others to develop and to use natural resources in an environmentally sound manner while protecting the health and safety of our employees and the public. To meet these respon
4、sibilities, API members pledge to manage our businesses according to the following principles using sound science to prioritize risks and to implement cost-effective management practices: o To recognize and to respond to community concerns about our raw materials, products and operations. PRINCIPLES
5、 o To operate our plants and facilities, and to handle our raw materials and products in a manner that protects the environment, and the safety and health of our employees and the public. o To make safety, health and environmental considerations a priority in our planning, and our development of new
6、 products and processes. o To advise promptly, appropriate officials, employees, customers and the public of information on significant industry-related safety, health and environmental hazards, and to recommend protective measures. o To counsel customers, transporters and others in the safe use, tr
7、ansportation and disposal of our raw materials, products and waste materials. o To economicdly develop and produce natural resources and to conserve those resources by using energy efficiently. 0 To extend knowledge by conducting or supporting research on the safety, health and environmental effects
8、 of our raw materials, products, processes and waste materials. o To commit to reduce overall emission and waste generation. o To work with others to resolve problems created by handling and disposal of hazardous substances from our operations. o To participate with government and others in creating
9、 responsible laws, regulations and standards to safeguard the community, workplace and environment. o To promote these principles and practices by sharing experiences and offering assistance to others who produce, handle, use, transport or dispose of similar raw materials, petroleum products and was
10、tes. STD=API/PETRO PUBL 4bb4-ENGL 1998 0732290 Ob06583 442 Mixing Zone Modeling and Dilution Analysis for Water-Quality-Based NPDES Perm it Limits Health and Environmental Sciences Department API PUBLICATION NUMBER 4664 PREPARED UNDER CONTRACT BY: BROWN AND CALDWELL PLEASANT HILL, CALIFORNIA more st
11、ringent limits are applied on a case-by-case basis if necessary to protect receiving water quality. Water-quality-based NPDES permit limits are set so that the fully diluted effluent will not exceed ambient water quality criteria. However, EPA and many states recognize that a receiving water can be
12、protected without requiring an effluent to meet water quality criteria at the point of discharge. Water-quality-based permits often include mixing zone allowances to account for the dilution that takes place around an outfall. A mixing zone may be established by computing a dilution factor or it may
13、 be delineated as a spatial area with fixed boundaries. In either case, it is an allocated portion of a receiving water in which a discharge is rapidly diluted. Water quality criteria may be exceeded within a mixing zone but must be met at its boundaries. Dilution credits are typically calculated us
14、ing a mathematical model based on discharge and receiving water conditions assumed to represent critical (Le., poor) mixing conditions. The dilution factor derived from the model is then used to calculate environmentally-protective NPDES permit limits. Conservatisms inherent in computing the dilutio
15、n factor will directly result in more restrictive effluent limits. While the concept of a mixing zone is straightforward, the actual determination of dilution credits for a specific discharge raises potentially difficult issues for both regulatory agencies and NPDES permittees. To participate meanin
16、gfully in permit development, dischargers need to be aware of the technical and policy options available to them, as well as the advantages and disadvantages of various mathematical modeling approaches for mixing zone analysis. STD=API/PETRO PUBL 4bb4-ENGL 1998 m 0732290 ObOb594 228 m ES-2 This guid
17、ance document was commissioned by the American Petroleum Institute (API) to summarize available information on the role of dilution analysis and mixing zone models in NPDES permitting. The following major topics are discussed: Regulatory basis for mixing zones. 0 Hydrodynamics of effluent dilution a
18、nd outfall diffuser design. Availability and strategic use of mixing zone models. 0 Dye tracer studies and other field study methods as alternatives or supplements to mixing zone models. This document is not aimed at experienced water quality modelers. Rather, it is intended primarily for the benefi
19、t of those who manage or evaluate mixing zone studies in the course of obtaining water-quality-based discharge permits. The goal is to equip this user group with information and strategies to successfully negotiate site-specific NPDES permits which account for available dilution in the environment.
20、Repulatorv Basis for Mixing Zones EPA has established its position on mixing zones through two major regulations and a series of guidance documents and policies issued over the last 25 years. The most significant of these are discussed in Chapter 2, with the following key themes emerging: 0 Mixing z
21、ones are consistent with the objectives of the Clean Water Act (CWA) and should be considered by regulatory authorities when “DES permits are developed. 0 Dischargers are not automatically entitled to a mixing zone. This is a discretionary activity on the part of the permitting authority and is subj
22、ect to EPA review and approval. State regulatory agencies can decide to limit or deny a mixing zone on a site-specific basis. Mixing zones should be limited when necessary to prevent lethality to passing aquatic organisms, bioaccumulation of pollutants, and significant risk to human health. STD-API/
23、PETRO PUBL Ybb4-ENGL 1998 0732290 Ob06595 lib4 ES-3 States should adopt written mixing zone polices as part of their water quality standards regulations. However, current EPA regulations do not specifj minimum technical requirements for state mixing zone policies. The practical result is that the ap
24、plication of mixing zones in NPDES permits varies widely across the United States. In addition to policy guidance, the EPA documents reviewed in Chapter 2 offer numerous technical recommendations on mixing zone implementation. These include specification of receiving water critical low flows for dil
25、ution calculations, default assumptions for effluent dilution in open waters such as large lakes, and benchmarks for mixing zone size and shape. Chapter 2 also includes a survey of the mixing zone policies of 14 states. These states were selected to cover the major petroleum refining centers of the
26、United States as well as to represent a wide geographic distribution and a variety of receiving water environments. This survey demonstrates that some states have very prescriptive mixing zone policies. Others allow substantial flexibility, offering dischargers an opportunity for technical input and
27、 negotiation during NPDES permit development. Often, but not always, input parameters and critical assumptions for mixing zone modeling are specified in state water quality standards or policy documents. However, in some states, permit writers may simply take these parameters uncritically from EPA g
28、uidance documents, textbooks, other precedents, or customary professional practice. Dischargers should be aware that those model inputs not set by state regulation or written policy are negotiable and can often be changed on the strength of good technical arguments or field data. Mixing Zone Physics
29、 and Outfall Diffuser Design Chapter 3 introduces several basic concepts which describe the physical These interaction of effluent discharges and receiving waters within mixing zones. include: 0 Classification of effluents as either “jets” or “plumes” and the dilution pattern for each type of discha
30、rge. I STD.API/PETRO PUBL 4664-ENGL 1998 0732290 0606596 OTO m ES-4 Distinctions between “near field” and “far field“ mixing processes and the factors dominating effluent dilution in each region. The role of effluent buoyancy in promoting mixing and the limiting effect of ambient density stratificat
31、ion on dilution of positively buoyant discharges. Relatively rapid and uniform vertical mixing of effluents in rivers. Lateral mixing, on the other hand, occurs over longer distances and is dependent on factors such as current speed, channel morphology, and the presence or absence of rapids. The imp
32、ortance of ambient currents and tidal effects for complete effluent mixing. A grasp of these fundamental principles is essential for the proper design of outfall diffuser systems, the selection of an appropriate mixing zone model for a given discharge, and the interpretation of model output. The bas
33、ic components of a high-energy effluent diffuser are introduced in Chapter 2, including the outfall pipe, the diffuser pipe, and smaller diameter discharge ports and diffuser nozzles. Equations governing diffuser hydraulics are presented, and design techniques are described to ensure equal flow dist
34、ribution along the entire length of a multiport diffuser. Codiguration of the diffuser ports and nozzles will also dictate the effectiveness of initial mixing. Important factors include size, spacing, and horizontal as well as vertical orientation relative to ambient currents. It is generally recomm
35、ended to direct diffuser nozzles perpendicular to the centerline of the diffuser pipe and in the direction of the strongest ambient current. For discharges to marine or brackish waters, it is also important that diffuser ports be sized to achieve sufficient exit velocities and prevent saltwater intr
36、usion under low flow conditions. Empirical design criteria are presented to address these concerns. Materials of construction and outfdl location are the two primary construction considerations for diffuser systems. Material selection must be based on the characteristics of both the effluent and the
37、 receiving water as well as risks associated with geotechnical conditions and physical exposure. Acceptable pipe materials include plastics olyvinyl chloride and high density polyethylene), fiberglass, welded steel, ductile iron, and reinforced concrete. Metal fittings must be made of marine-grade E
38、S-5 stainless steel or other corrosion-resistant materials. The diffuser should be constructed so that it is protected from physical hazards, floating debris, wave and current action, and, in some cases, seismic activity. Construction techniques are described to mitigate these .- risks. Availabilitv
39、 and StratePic Use of Mixing Zone Models The availability and strategic use of mixing zone models are discussed in Chapter 4 through Chapter 6. A consistent emphasis throughout these chapters is that users should strive for the least complex modeling approach possible to achieve the objective of env
40、ironmentally-protective and cost-effective NPDES permit limits. There is a trade- off between cost and model accuracy. Simple models tend to err on the side of overprotective permit limits, while more complex models bring additional costs related to data collection and consultant support. Each disch
41、arger must determine whether the costs of implementing a more rigorous model are likely to be recouped via a less restrictive discharge permit. Chapter 4 introduces several important technical issues to consider when selecting a mixing zone model. These include the following: Stages of mixing. “Near
42、 field” models focus on the mixing that occurs in the immediate vicinity of the outfall and may be useful when a mixing zone is physically defined in terns of area or volume around the discharge point. This type of mixing is controlled primarily by the momentum and buoyancy of the discharge itself.
43、“Far field” models consider the mixing that occurs farther away from the discharge point. This stage of mixing is dominated by ambient turbulence in the receiving water. Spatial resolution. Mixing zone models are available which consider water quality changes over zero (completely mixed), one, two,
44、or three dimensions in space. Model complexity increases with the number of spatial dimensions considered. Guidance is provided regarding situations where each type of model is - and is not - appropriate. Temporal resolution. Models can also be categorized in terms of how they consider changes in po
45、llutant concentrations over time. The advantages of steady state and time variable mixing zone models are discussed, along with the use of tidally averaged steady state models to evaluate mixing in dynamic estuarine systems. STD.API/PETRO PUBL 4bb4-ENGL 1998 0732290 ObOb598 973 9 ES-6 Deterministic
46、vs. probabilistic models. A deterministic mixing zone model predicts a single environmental response to a fixed set of inputs intended to represent critical mixing conditions in the receiving water. This approach may result in overly conservative dilution factors, especially when mixing is governed
47、by several independent variables. On the other hand, probabilistic models predict the distribution of environmental responses over a full range of input conditions. These types of models should be considered when the frequency of expected water quality criteria violations is a critical issue in NPDE
48、S permitting. Chapter 4 also presents ten mixing zone models developed and/or currently supported by EPA. These range from simple desktop calculations to sophisticated computer programs requiring significant input data and expert support. The structure, assumptions, complexity, output, and computer
49、hardware requirements for each model are described. The range of appropriate discharge and receiving water conditions is discussed for each model, along with typical errors in model application. Guidance is provided on the applicability of specific mixing zone models to typical discharge situations, including shallow and deep rivers, estuaries, and open waters such as large lakes and marine systems. Finally, three case histories illustrate use of the mixing zone models discussed in Chapter 4 in actual NPDES permitting situations. These examples show how information is factored into model
