Large number of public wells in The U.S. Essays -

Findings showed that naturally occurring contaminants, such as radon and arsenic, accounted for about three-quarters of contaminant concentrations greater than human-health benchmarks in untreated source water. Naturally occurring contaminants are mostly derived from the natural geologic materials that make up the aquifers from which well water is withdrawn. Man-made contaminants were also found in untreated water sampled from the public wells, including herbicides, insecticides, solvents, disinfection by-products, nitrate, and gasoline chemicals. Man-made contaminants accounted for about one-quarter of contaminant concentrations greater than human-health benchmarks, but were detected in 64 percent of the samples, predominantly in samples from unconfined aquifers. "Detections of contaminants do not necessarily indicate a concern for human health because USGS analytical methods can detect many contaminants at concentrations that are 100-fold to 1,000-fold lower than human-health benchmarks," said lead scientist Patricia Toccalino. "Assessing contaminants in these small amounts helps to track emerging issues in our water resources and to identify contaminants that may warrant inclusion in future monitoring." Scientists tested water samples for 337 properties and chemical contaminants, including nutrients, radionuclides, trace elements, pesticides, solvents, gasoline hydrocarbons, disinfection by-products and manufacturing additives. This study did not assess pharmaceuticals or hormones. Most (279) of the contaminants analyzed in this study are not federally regulated in finished drinking water under the Safe Drinking Water Act. The USGS also sampled paired source and finished (treated) water from a smaller subset of 94 public wells. Findings showed that many man-made organic contaminants detected in source water generally were detected in finished water at similar concentrations. Organic contaminants detected in both treated and source water typically were detected at concentrations well below human-health benchmarks, however. Additionally, the study shows that contaminants found in public wells usually co-occurred with other contaminants as mixtures. Mixtures can be a concern because the total combined toxicity of contaminants in water may be greater than that of any single contaminant. Mixtures of contaminants with concentrations approaching benchmarks were found in 84 percent of wells, but mixtures of contaminants above health benchmarks were found less frequently, in 4 percent of wells. This USGS study identifies which contaminant mixtures may be of most concern in groundwater used for public-water supply and can help human-health researchers to target and prioritize toxicity assessments of contaminant mixtures. The USGS report identifies the need for continued research because relatively little is known about the potential health effects of most mixtures of contaminants. Wells included in this study are located in 41 states and withdraw water from parts of 30 regionally extensive aquifers, which constitute about one-half of the principal aquifers used for water supply in the United States. Human-health benchmarks used in this study include U.S. Environmental Protection Agency Maximum Contaminant Levels for regulated contaminants and USGS Health-Based Screening Levels for unregulated contaminants, which are non-enforceable guidelines developed by the USGS in collaboration with the EPA and other water partners. Treated drinking water from public wells is regulated under the Safe Drinking Water Act. Water utilities, however, are not required to treat water for unregulated contaminants. The EPA uses USGS information on the occurrence of unregulated contaminants to identify contaminants that may require drinking-water regulation in the future.

Expository Essay Samples

Expository Essay SamplesWe've all been there before; you're sitting in your high school English class, studying the specific vocabulary of the New York Times bestseller list, the term 'active voice' comes up. It's about time that you could find some expository essay samples to help you out when it comes to writing.Unfortunately, some of the materials and websites on the internet fail to provide good expository essay samples. When this happens, the end result is not always what you wanted, as this article will tell you about.The first common problems with expository essays are due to the fact that some of the writers don't have the opportunity to use what they are reading. The writers will often cut out important parts because they don't want to repeat themselves. So, for example, if the author were to take out the writing advice from the book 'Mindset,' it would lose most of its impact.Other times, the writers aren't allowed to use their own writing samples, so they create a new book which could be based on other material. This results in people writing something that's very different from what the work was really about.In addition, expository essay samples are often hard to find. Sometimes, someone can simply put in the content, but there isn't enough space to highlight important information and give a complete review.The main reason for this is because the thesis statement is often only one paragraph long, and the details of each chapter will then be used to create the entire essay. It would be better if the writer would be able to put in the information and keep it short.So, how should you choose your expository essay samples? Well, as I mentioned earlier, there are a lot of resources on the internet, but it's better to go with those that are very reliable.Some of the websites that offer these expository essay samples have produced really well written expositions. They have had years of experience as writers, and the end result is usually a dissertation whic h really sells the ideas of the author.