|How Safe Is Your Drinking Water Supply?
The Role of ASTM Standards in Monitoring Ground-Water Quality
by Gillian L. Nielsen & David M. Nielsen
Every day you turn on your tap at home or at work and draw what you believe to be a supply of clean water that is safe for drinking or other needs. But have you ever considered where your water supply actually comes from? Is your water provided by a public water system? If so, is the supply from ground water, surface water or a combination? Can you taste chemicals such as chlorine that have been added to your water to make it safer? If you live in the country, do you have your own private well? If so, do you know anything about that well? Where is it? How has it been constructed? How much water is available from the well? Do you notice any seasonal changes in the taste or amount of water available from your well? Do you know whether your public or private water supply is being affected by some source of contamination? Despite our dependence on safe and abundant water, very few people can answer these basic questions about their water supply. As a consequence, most people do not understand how sensitive a resource their water supply is to contamination, particularly if it is from ground water.
Why Is Ground Water Such an Important Resource?
Ground water is one of our most valuable natural resources. Americans have long depended upon ground water for many uses, but its primary importance is as a source of drinking water. More than 150 million Americans nearly 53 percent of the population depend on ground-water resources to supply their drinking water on either a public distribution system or private home owner well basis. (1) More than 75 percent of the nations public water supplies, more than one-third of the nations largest cities, and nearly 95 percent of all domestic water needs in rural areas are served by ground-water resources. In addition, ground water is used extensively in the western and central states for irrigation, in the northern states for residential and commercial heating, in the southern states for air conditioning, and across the nation for a variety of industrial purposes. National reliance on ground water has increased dramatically over the past few decades at a rate nearly three times that of surface water, particularly in rapidly developing urban areas in arid climates (such as Las Vegas, Nev., and Phoenix, Ariz.). This reliance will continue to increase as national consumption and use of water increase in the future and will be underscored if surface water shortages caused by prolonged regional and national drought conditions continue to occur.
How Are Ground-Water Resources Contaminated?
Despite the overwhelming evidence of the vital importance of ground water, until relatively recently in our history, ground water has been provided with little protection from myriad potential sources of organic, inorganic, biological and radiological contamination. There are literally millions of point and non-point sources of contamination which, when they release waste materials or chemicals into the environment, can directly impact ground water quality. The massive national pollution cleanup efforts of the early 1970s focused heavily on surface water protection but largely ignored ground water. In fact, ironically, by strictly regulating discharges into surface water and air, some of our early environmental laws actually encouraged land disposal and underground injection of wastes, thus exacerbating contamination of ground- water supplies. Only since about 1980 has the public become aware of the dangers of ground-water contamination and the many ways that it can occur. Even so, new problems are discovered on a daily basis across the U.S., some resulting in the complete shut-down of large public water supplies. Such a situation occurred in Santa Monica, Calif., in 1997 when multiple underground storage tank leaks impacted first one, then two, then three of the citys public water supply wells, forcing the city to close an entire well field and turn to the city of Los Angeles for its water supply needs, at significant expense.
Out of Sight, Out of Mind?
Probably the major reason that ground water has historically received so little attention is that it is out of sight and its existence, movement and susceptibility to contamination is not readily understood by most people. Many people, for example, still believe that ground water occurs in veins or underground rivers that transport water over great distances quickly, or vast underground lakes that store unimaginable quantities of water that are readily tapped by wells for limitless water supplies. These images and beliefs are far from reality!
The Role of Ground-Water Monitoring in Protecting Ground Water Resources
That the nature and occurrence of ground water is misunderstood is especially regrettable because, in many ways, preventing ground-water contamination is crucial. Ground-water contamination is particularly serious because it is difficult to detect and monitor. A great deal of scientific effort and money must be expended in characterizing the subsurface, to discern the flow pathways that ground water follows through the geologic materials that make up the earths surface. Ground-water samples must be collected during site characterization to determine if contaminants are present. If contaminants are detected, ground-water monitoring wells or other sampling points must be constructed in the zones determined to be contaminated and additional ground-water samples must be collected for laboratory analysis to monitor the direction and rate of movement of contaminants. These monitoring points, and the samples collected from them, must provide the answers to many important questions that should be asked, and also provide the basis for many important and potentially far-reaching and expensive decisions.
For example, if contamination is detected, are concentrations so high that they affect the suitability of ground water for use as a drinking water supply, or its use in commercial or agricultural applications? Have federal drinking water quality standards, which are often in the parts-per-billion concentration range, been exceeded? If drinking standards such as these are exceeded, do levels represent any health hazard to people or livestock using the ground water as a drinking water supply? If there is a health risk, is it necessary to implement some type of site cleanup to remove or treat the contaminant source and restore the media impacted by that contaminant source to its ambient condition? If so, what method(s) will work best? If not, is a risk assessment justified to assess potential future impacts?
How ASTM Standards Help Ensure Accurate and Precise Information Is Obtained By Ground-Water Monitoring Programs
The success of ground-water monitoring programs in providing accurate answers to these and other important questions about ground-water quality hinges on our ability to ensure that the samples and data that are collected in the field during sampling events are representative of actual ground-water conditions (sampling accuracy). In addition, it is important to ensure that sampling techniques are consistent between sampling events (sampling precision) to successfully identify any changes in ground-water chemistry over time. It is critical to ensure that the samples have not been biased during sample collection, which can easily occur when the very low detection limits for critical water-quality parameters are considered.
To assist these critical efforts in the field, ASTM Subcommittee D18.21 on Ground Water and Vadose Zone Investigations has developed a number of key guides and practices to provide technical and procedural guidance to environmental professionals involved in the development and implementation of ground-water monitoring and sampling programs. These guides and practices address each of the many components of ground-water sampling events to help ensure that the highest quality samples are submitted to laboratories for analysis. The quality of information obtained from the lab analyses can only be as good as the quality of the samples collected and submitted from the field.
Because samples are often analyzed at the parts-per-million or parts-per-billion level, even minor errors in sample collection and handling procedures can introduce either positive or negative bias in ground-water sample chemistry. Simple things, like poor housekeeping practices followed during sampling events, can result in the introduction of surface contaminants into monitoring wells and into samples, which can result in the detection of artificially high contaminant levels in samples (positive bias). Positive bias can make it appear that ground-water quality is worse than it actually is and may result in the expenditure of significant financial resources to clean up a site which may not be contaminated. Negative bias, which may be caused by something as simple as improper placement or operation of a sampling device, may result in either the failure to detect contaminants entirely or in the detection of contaminants at concentrations well below actual concentrations. This can make it appear that the water is safe for consumption when, in fact, it may not be.
The guides and practices produced by D18.21 help ground-water sampling field personnel avoid introducing error and bias into the ground-water sampling process. To assist samplers in maximizing efficiency while in the field, and to get a sampling event started on the right track, the procedures outlined in D 5903, Guide for Planning and Preparing for a Ground-Water Sampling Event, should be followed. This guide provides samplers with organizational checklists to help make it easier to get all of the necessary sampling and support equipment ready for the trip to the field. By following this guide, samplers avoid delays in the field which can easily result from not having all of the required field gear, field quality control samples, or sufficient or correct sample containers for the parameters being sampled.
Once in the field, samplers who do not follow correct field practices can introduce negative and/or positive bias to samples being collected in virtually every step of the sampling process. To assist in making critical decisions regarding how wells should be purged of water prior to sampling and which purging and sampling device is appropriate for the parameters to be sampled, samplers can consult several guides. These include D 6634, Guide for Selection of Purging and Sampling Devices for Ground-Water Monitoring Wells; D 6452, Guide for Purging Methods for Wells Used for Ground-Water Quality Investigations; and D 6771, Practice for Low-Flow Purging and Sampling for Wells and Devices Used for Ground-Water Quality Investigations. These standards address important issues such as the effects that some sampling devices may have on sample chemistry, the importance of using a consistent sample delivery rate, and chemical compatibility between the materials of construction of the sampling equipment and the chemical parameters being monitored. They also address correct placement of portable or dedicated sampling devices within a water column and proper operation of devices to avoid introduction of artifactual turbidity into samples (positive bias) or loss of volatile constituents from samples (negative bias). Practice D 6771 provides field personnel with critical information on implementing an improved but often misunderstood sampling method called low-flow purging and sampling, including detail on determination of optimal pumping rates for wells and definition of field parameter stabilization.
After a well has been purged, the samples that are collected often require pretreatment at the wellhead at the time of sample collection. Even when enclosed in a sample bottle, a ground-water sample is a dynamic, living and breathing environment that is highly subject to change with regard to its physical and chemical properties. Pretreatment methods ensure that the representative chemical and physical properties of the sample are retained. Two standards, D 6517, Guide for Field Preservation of Ground-Water Samples, and D 6564, Guide for Field Filtration of Ground-Water Samples, provide procedures for samplers to follow to achieve this objective. Samplers who follow the procedures outlined in D 6517 will preserve samples properly to ensure that sample bias attributed to phenomena such as microbial degradation, exposure to sunlight, precipitation of dissolved metals due to oxidation, and sorption onto sample container walls is minimized or eliminated. During ground-water sample filtration, samplers can introduce both negative and positive bias to samples. Guide D 6564 provides much-needed guidance on how to precondition sample filtration equipment to prevent positive sample bias attributed to contributions from the filter media, and how to avoid the negative bias associated with vacuum filtration methods.
It is important that samplers keep accurate records to document what has been done in the field during sample collection. These records contain details about what the samplers observed, what was actually done in the field during sample collection, and what the results of field parameter measurements were. Standard D 6089, Guide for Documenting a Ground-Water Sampling Event, provides a checklist of the details that should be recorded during a sampling event. A draft Guide on Packaging and Shipping Environmental Samples for Laboratory Analysis should be balloted by the subcommittee in 2003. This draft standard will provide guidance on how to properly pack and ship hazardous and unregulated samples in compliance with local, Federal and International shipping regulations to ensure safe delivery of the samples to the lab for analysis. When all of the aforementioned standards are incorporated into site-specific sampling and analysis plans, the acquisition of representative samples and the accuracy and precision of sampling efforts can be assured.
Sampling accuracy, sampling precision, bias, parts-per-million, parts-per-billion! Whats a sampler to do? All of this to think about while in the field dealing with inclement weather, pressure to get ground-water samples collected quickly and efficiently and the concern about making sure the analytical results to be reported make sense. These analytical results must accurately reflect the true chemistry of the ground-water being sampled so we can accurately answer the question, Do you know how safe your drinking water supply is?
(1) US EPA, 1997, Water On Tap: A Consumers Guide to the Nations Drinking Water, Office of Water (4601), EPA Publication # 815-K-97-002, Washington DC, 22 pp.
Copyright 2003, ASTM