第二单元What is this book about？这本书是关于什么的？The objective of this book is to introduce engineering and science students to the interdisciplinary study of environment problems；their cause，why they are of concern，and how we can control them. The book includes:这本书的目的是使理工科的学生了解跨学科间的研究环境问题;它们的起因，为什么它们受到关注，以及我们怎样控制它们。
这本书包括：Description of what is meant by environment and environmental systems描述环境和环境系统意味着什么Information on the basic causes of environmental disturbances关于引起环境干扰基础原因的基本信息Basic scientific knowledge necessary to understand the nature of environmental problems and to be able to quantify them理解环境问题本质，并能够定量计算它们所必要的基本科学知识Current state of the technology of environmental control in its application to water，air and pollution problems目前适用于水，空气和环境污染问题的环境控制技术的现状Considerable gaps in our current scientific knowledge of understanding and controlling many of the complex interactions between human activities and nature 我们目前的科学知识在理解和控制人类活动和自然之间复杂的相互作用的科学知识上存在相当大的缺陷Many environmental problems which could be eliminated or reduced by the application of current technology，but which are not dealt with because of society’s lack of will to do so，or in many instance because of a lack of resources to do so.许多环境问题可以应用现有技术消除或减少，但没有得到处理是因为社会缺乏这样做的意愿，或者像许多例子那样因为缺乏资源。
Interaction of Systems互动系统A number of different environmental problems are associated with water, air, or land systems. 许多不同的环境问题都涉及到水，空气，或土壤。
Many of these problems will apply only within one of these systems, justifying the breakdown into these categories.许多这些问题只适用于在其中一个系统，这为这些种类中的细目分类提供了充分的理由。
Such a classification is also useful for easier comprehension of related problems within one system.这种分类也有助于更容易理解一个系统里的相关问题。
Moreover, it is sensible because, for manager and administrative reasons, such subfields as air pollution, water supply, wastewater disposal, and solid waste disposal are often dealt with separately by governmental agencies.此外，这样做是明智的，因为是行政管理的原因，空气污染，水供应，固体废物处置和废水处理，往往是由政府机构分别处理的。
Unfortunately, many important environmental problems are not confined to an air, water, or land system, but involve interactions between systems.很遗憾的是，许多重要的环境问题不只局限于一个空气，水，或土壤系统，更涉及系统之间的相互作用。
A current example is the acid rain problem stemming from the emission of sulfur dioxide and nitrogen oxide gases into the atmosphere from the stacks of generating stations, smelters, and automobile exhausts。
目前的一个例子是酸雨问题，根源是许多发电站，冶炼，汽车排出的废气中进入大气的二氧化硫和氮氧化物气体。
These gases are then transported by air currents over wide regions.这些气体被气流运送到广阔的区域。
Rainfall “washes them out”, creating acid rain which is harmful to aquatic life, forests, and agricultural crops.雨水“将它们洗去”，产生了对水生生物，森林和农作物有害的酸雨。
Two examples of interaction between systems that cause major environmental disturbances are presented-the buildup of atmospheric carbon dioxide, a global problem, and the acid rain problem, normally of regional nature.两个有关于系统间相互作用引起的环境问题有：空气中的二氧化碳的增加的全球性问题以及具有第三单元Pollution prevention: Pollution may be generated during manufacturing, or when certain products are used commercially or by consumers.污染的防止：污染可能在生产过程之中产生，或者在当一个产品被用作商业用途或被消费者使用的时候产生。
This may be prevented in three ways: changing inputs/reducing reliance on toxic or hazardous raw materials; process changes /increasing efficiency/improved maintenance such as equipment modifications, better housekeeping, in-process closed-loop recycling; or changing outputs/reducing reliance on toxic or hazardous products.这样可能在三个方式上被阻止：改变投入/减少依赖有毒物或有害的原材料；改变过程/增加效率/提高类似设备修改的维修保养，更好的管理，在过程中闭环循环；或者改变产出/减小对有毒或者有害产品的依赖。
With the passage of the Hazardous and Solid Waste Amendments to the Resource Conservation and Recovery Act in 1984, the U.S. Congress established a national policy declaring the importance of reducing or eliminating the generation of hazardous waste.随着1984年危险废物和固体废物修正案资源保护和恢复法的通过，美国国会建立一个国家政策来表明回收或消除危险废物的产生重要性。
The Congress hereby declares it to be a national policy of United States that wherever feasible, the generation of hazardous waste is to be reduced or eliminated as expeditiously as possible. 美国国会宣称它是一项无论在美国哪里都可行的国家政策，要尽可能迅速地回收或消除危险废物的产生。
Waste that is nevertheless generated should be treated, stored, or disposed of soas to minimize present and future threat to human health and the environment.然而产生的废物还是应被处理、储存或处置，从而尽量减少现在和将来对人类健康和环境的威胁。
In 1990, the U.S. Congress passed the Pollution Prevention Act, establishing a hierarchy of waste management as national policy; stating that first pollution should be prevented or reduced wherever feasible; 在1990年，美国国会通过污染防治法，建立了一个全国废物管理政策。