Contaminants are grouped into two broad categories: chemical and microbial. Both chemical and microbial contaminants may be naturally occurring or may be caused by human activity. Chemical contaminants include metals, pesticides, synthetic chemical compounds, suspended solids, and other substances.
TABLE 5.2
Public water systems, by size and population served, 2003
| Very small 500 or less | Small 501–3,300 | Medium 3,301–10,000 | Large 10,001–100,000 | Very large >100,000 | Total | ||
| CWS | # systems | 30,417 | 14,394 | 4,686 | 3,505 | 361 | 53,363 |
| Pop. served | 5,010,834 | 20,261,508 | 27,201,137 | 97,706,485 | 122,149,436 | 273,329,400 | |
| % of systems | 57% | 27% | 9% | 7% | 1% | 100% | |
| % of pop | 2% | 7% | 10% | 36% | 45% | 100% | |
| NTNCWS | # systems | 16,785 | 2,786 | 97 | 16 | 2 | 19,686 |
| Pop. served | 2,327,575 | 2,772,334 | 506,124 | 412,463 | 279,846 | 6,298,342 | |
| % of systems | 85% | 14% | 0% | 0% | 0% | 100% | |
| % of pop | 37% | 44% | 8% | 7% | 4% | 100% | |
| TNCWS | # systems | 85,366 | 2,657 | 96 | 29 | 4 | 88,152 |
| Pop. served | 7,315,647 | 2,602,706 | 528,624 | 619,248 | 12,269,000 | 23,335,225 | |
| % of systems | 97% | 3% | 0% | 0% | 0% | 100% | |
| % of pop | 31% | 11% | 2% | 3% | 53% | 100% | |
| Total # systems | 132,568 | 19,837 | 4,879 | 3,550 | 367 | 161,201 | |
| CWS = Community water system: A public water system that supplies water to the same population year-round. | |||||||
| NTNCWS = Non-transient non-community water system: A public water system that regularly supplies water to at least 25 of the same people at least six months per year, but not year-round. Some examples are schools, factories, office buildings, and hospitals which have their own water systems. | |||||||
| TNCWS = Transient non-community water system: A public water system that provides water in a place such as a gas station or campground where people do not remain for long periods of time. | |||||||
Microbial contaminants include bacteria, viruses, and microscopic parasites.
The health effects of drinking contaminated water can occur either over a short or long period. Short-term, or acute, reactions are those that occur within a few hours or days after drinking contaminated water. Acute reactions may be caused by a chemical or microbial contaminant. Long-term, or chronic, effects occur after water with relatively low doses of a pollutant has been consumed for several years or over a lifetime. Most chronic effects are caused by chemical contaminants.
The ability to detect contaminants improved considerably in the late twentieth century. Scientists can now identify specific chemical pollutants in terms of one part contaminant in one billion parts of water. In some cases, scientists can measure them in parts per trillion. One part per billion (ppb) is equal to one pound in 500,000 tons or approximately one teaspoon in five Olympic-size swimming pools. Although these measurements appear very tiny, such small amounts can be significant in terms of health effects.
Chemical Contaminants
All drinking water contains minerals dissolved from the earth. In small amounts, some of these minerals are acceptable because they often enhance the quality of the water (for example, giving it a pleasant taste). A few, such as zinc and selenium, in very small amounts, contribute to good health. Other naturally occurring minerals are not desirable because they may cause a bad taste or odor (such as excessive amounts of iron, manganese, or sulfur) or because they may be harmful to health (for example, boron).
A wide variety of contaminants may cause serious health risks in water supplies. Not all contaminants are found in all water supplies: some water supplies have no undesirable contaminants. Other supplies have no contaminants that have health significance. Contaminant presence is frequently the result of human activity and may have long-term consequences. While harmful levels of microorganisms generally make their presence known quickly by causing illness with fairly obvious symptoms, the effects of some toxic chemicals may not be apparent for months or even years after exposure. Some chemical pollutants are known carcinogens (cancer causing agents), while others are suspected of causing birth defects, miscarriages, and heart disease. In many cases, the effects occur only after long-term exposure.
ARSENIC.
Arsenic is a naturally occurring element in rocks and soils, and is soluble in water. Arsenic has been recognized as a poison for centuries. Recent research, however, has shown that humans need arsenic in their diet as a trace element. Too much arsenic, though, can contribute to skin, bladder, and other cancers after prolonged exposure. Because of this risk, the National Research Council recommended lowering the current maximum contaminant level (MCL) of 50 ppb for arsenic in drinking water.
In January 2001 the EPA proposed a new MCL that required public water supplies to reduce arsenic to five ppb by the year 2006. The EPA estimated the cost of implementation at $379 to $445 million. The Association of State Drinking Water Administrators (ASDWA) and several organizations and states challenged the new level. Serious concerns exist about the extent of arsenic occurrence in drinking water, the health effects of arsenic at the three to twenty ppb level (the level found in U.S. supplies), the validity of the EPA's cost estimates (including waste disposal), and the EPA's approach to regulating non-transient non-community water supplies. The ASDWA has recommended that the EPA consider an MCL of twenty ppb, as this is the level where the EPA currently believes that the cost-benefit ratio is maximized. A maximized cost-benefit ratio is one in which the most benefit (in this case, the greatest reduction in the number of illnesses) can be achieved for the least cost per illness reduction. Money expended beyond the maximized cost-benefit ratio can result in additional illness reduction but at increasingly higher costs per illness reduced. For example, if at the maximized cost-benefit ratio an estimated 1,000 illnesses are avoided at a cost of $30 million, the avoidance of an additional 1,000 illnesses may cost an additional $75 million because of the need for additional preventive measures and treatment to achieve the additional reduction.
Most of the burden of implementing the new arsenic MCL will fall on the systems least likely to be able to afford it. According to a 2000 study by the U.S. Geological Survey (USGS), an estimated 845 public water supplies exceed twenty ppb arsenic. The vast majority of these systems served 10,000 people or less.
This new standard, adopted by the administration of President Bill Clinton three days before leaving office, was promptly suspended at the beginning of the Bush administration, outraging health experts and concerned members of the public. In April 2001 the EPA announced that it would work with the National Academy of Sciences and the National Drinking Water Advisory Council to review the science and the cost estimates supporting the proposed MCL. The effective date of the new rule was to be delayed until February 2002 to allow time to complete the full assessment and to afford the public ample opportunity to provide further input. Amid pressure from health officials and environmentalists the proposed standard was set at ten ppb. The new standard MCL was adopted on February 22, 2002. All public water systems are expected to comply with the new standard by January 2006.
LEAD.
Lead is a toxic metal that can cause very serious health problems if inhaled or ingested. Children are particularly at risk because their developing bodies absorb and retain more lead than adult bodies. According to EPA reports, low-level exposures can result in lowered I.Q., impaired learning and language skills, loss of hearing, reduced attention spans, and poor school performance. High levels damage the brain and central nervous system, interfering with both learning and physical development. Pregnant women are also at risk. Lead is believed to cause miscarriages, premature births, and impaired fetal development.
The major sources of lead exposure are deteriorated paint in older houses and dust and soil that are contaminated by old paint and past emissions of leaded gasoline. Plumbing in older homes can also contribute to overall blood lead levels. Blood lead levels in children aged one to five have declined sharply since 1976. The dramatic decline in blood lead levels between the late 1970s and the early 1990s is believed to be the result of the phase-out of leaded gasoline and the resulting decrease in lead emissions, although other exposures have also decreased. In December 2000 new data from the Centers for Disease Control and Prevention's (CDC) National Health and Nutrition Examination Survey (NHANES) III, Phase 2 and the NHANES 1999 showed that the average blood lead levels in children aged one to five declined again from the early 1990s to 2000. Some of the decline may be attributed to the continued decrease in air lead levels and lead emissions, although most of this decline occurred before 1995. The construction of new housing and the demolition or rehabilitation of old housing may be contributing to the continuing blood lead level decline. Other contributing factors, according to the EPA's America's Children and the Environment: Measures of Contaminants, Body Burdens, and Illnesses (February 2003), are EPA regulations reducing lead levels in drinking water; legislation banning lead from paint and restricting the content of lead in solder, faucets, pipes, and plumbing; and the elimination or reduction of lead in food and beverage containers and ceramic ware, and in products such as toys, window blinds, and playground equipment.
Unlike many water contaminants, lead has been extensively studied for its prevalence and its effects on human health, and for ways to eliminate it from the water supply. Lead is rarely found in either surface water or groundwater sources for drinking. Lead usually enters the water supply after it leaves the treatment plant or the well.
Lead contamination is most often a problem in older houses and other buildings. Until about 1930, many buildings in the United States had lead piping in their interior plumbing and lead pipes for the service connections that tied buildings and residences into the public water supplies, and lead solder was commonly used to connect pipes. Copper pipes replaced lead in many residences, but the practice of using lead solder to join the pipes continued. Lead solder is believed to be the primary cause of most lead in residential water supplies today. Corrosion, a reaction between the water and the lead pipes, solder, faucets, and fittings, is the process by which lead enters the water. Low pH (acidity), low calcium or magnesium levels in the water, and dissolved oxygen presence can all contribute to corrosion. The common practice of grounding electrical equipment to water pipes also accelerates corrosion.
No MCL had been established for lead as of April 2005. In 1991 the EPA replaced the old MCL of five ppb with a treatment technique (TT). The TT requires community and non-transient non-community water systems to sample tap water for lead. When more than 10% of these samples exceed the EPA's action level of fifteen ppb, the water supplier is required to take action. Systems exceeding the action level must:
- Install corrosion control to reduce or prevent the amount of lead leaching into drinking water.
- Conduct source water monitoring and install treatment if it is determined that the source level is contributing to the lead level at the tap.
- Conduct public education so that citizens will learn about the dangers of lead and steps that they can take to reduce their exposure to lead.
- Replace lead service lines owned by the system at the rate of 7% of the original number each year. This step is required only if the tap samples for lead are still above the action level after corrosion control and source water treatment are installed.
NITRATES.
Nitrates and nitrites are nitrogen-oxygen chemicals that combine with organic and inorganic compounds. Primary sources are fertilizer, human sewage, and livestock manure, especially from feedlots. Because they are soluble, nitrates have a high potential to migrate into groundwater.
Nitrates in drinking water can be an immediate threat to children under six months in age. In some babies, high levels of nitrate react with the red blood cells to reduce the blood's ability to transport oxygen. This can cause an anemic condition commonly known as "blue baby." The MCL for nitrates has been set at ten parts per million (ppm). When nitrate levels exceed this limit, a water supplier must notify the public and must provide additional treatment to reduce levels to meet the standards. Young livestock such as calves are also susceptible to blue baby. The number of community water systems with MCL violations for nitrates declined between 1980 and 1998. The highest number of systems in violation was registered in 1985, with 340 community water systems having levels over the limit.
According to the EPA's report Factoids: Drinking Water and Ground Water Statistics for 2003, in fiscal year 2003, nitrate violations were reported in 503 community and non-transient non-community water systems. The number of people served by suppliers cited for nitrate violations in 2003 was 1.9 million.
Microbiological Organisms as Contaminants
Microorganisms (bacteria, viruses, and protozoa) are found in untreated surface water sources used for drinking water. Groundwater does not contain these microorganisms unless they have been introduced through pollution of the aquifer. Unless the treatment system fails or contaminated water is introduced accidentally into the distribution system, treated drinking water is normally free of microorganisms or they are present in very low levels. When a water source or system is contaminated with human or animal wastes, some of the microorganisms may be pathogens (disease-causing microorganisms). The resulting illnesses can have symptoms that include headache, nausea, vomiting, diarrhea, abdominal pain, and dehydration. Although usually not life threatening, they can be debilitating and uncomfortable for victims. Extended illness or death may occur among individuals who are immunocompromised, that is, have weakened immune systems. Immunocompromised persons include infants, pregnant women, the elderly, human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome (AIDS) patients, those receiving treatment for certain kinds of cancer, organ-transplant recipients, and people on drugs that suppress their immune system.
Historically throughout the world, pathogens have been the major cause of disease associated with drinking water. In the United States in the early 1900s, the diseases cholera and typhoid were commonly associated with drinking water from public supplies. The practice of water treatment was begun to address this problem by reducing the number of pathogens present in water supply systems below an infective dose. The infective dose is the number of a particular microorganism required to induce disease, and is different for different microbes. For example, one Cryptosporidium protozoa is believed to induce disease while the infective dose for some types of Salmonellae bacteria is believed to be 10,000 to 100,000 organisms.
TURBIDITY.
Turbidity is a measure of the relative clarity of water. Turbidity is caused by suspended matter or impurities that make the water look cloudy. These impurities may include clay, silt, finely divided organic and inorganic matter, plankton, and other microscopic organisms. Figure 5.1 shows the types of activities in a watershed that cause turbidity in a water source.
Turbidity (excessive cloudiness in water) is unappealing and may represent a health concern. It interferes with the effectiveness of disinfection. Disinfection is the practice of killing pathogens in water by adding certain chemicals (for example, chlorine or ozone) to water or subjecting the water to ultraviolet light. Microorganisms can find shelter in the particulate matter, reducing their exposure to attack by disinfectants. Although turbidity is
FIGURE 5.1
Activities within a watershed that can increase turbidity
The EPA regulates turbidity using a TT based on the cloudiness of the water. Turbidity is measured in NTUs. An NTU is a unit of measurement from a nephelometric turbidimeter, a specialized instrument that measures cloudiness by the light scattered at an angle of ninety degrees from the incident light beam. (See Figure 5.2.) Water systems that filter their water must ensure that the turbidity goes no higher than one NTU (0.5 NTU for conventional or direct filtration) in at least 95% of the samples collected in any month. At no time can turbidity exceed five NTUs. Currently, turbidity may never exceed one NTU and must not exceed 0.3 NTU in 95% of daily samples in any month.
COLIFORM BACTERIA.
Coliform bacteria are a group of closely related, mostly harmless bacteria that live in soil, water, and the intestines of animals. These bacteria are generally divided into two groups: total coliform and fecal coliform. The total coliform group includes all coliform bacteria. The fecal coliform group is a subgroup found in the intestines and fecal waste of warm-blooded animals. There are a few organisms in the fecal coliform group that can be harmful to humans, particularly to children and to people with weakened immune systems.
The total coliform group is an indicator used to assess drinking water quality. This practice began in the early 1900s. It is based on the assumption that since coliform are always present in sewage from warm-blooded animals (including humans), and pathogens may be present in this same sewage, the presence of coliform bacteria may indicate the potential presence of pathogens. The most common problem caused by pathogens is gastroenteritis, a general illness characterized by diarrhea, nausea, vomiting, and cramps. While gastroenteritis is typically not harmful to healthy adults, it can
FIGURE 5.2
Nephelometric turbidimeter
Testing the water for each of a wide variety of potential pathogens is difficult and expensive. Testing for total coliform, by comparison, is easy and inexpensive. For this reason, total coliform are used to indicate whether a water system is vulnerable to pathogens. Presence of total coliform in the water distribution system may indicate that the disinfection process was faulty, that a break or leak has occurred in the distribution piping, or that the distribution pipes need to be cleaned. No more than 5% of the drinking water samples collected monthly from a water supplier may be positive for total coliform. All samples that are positive for total coliform are analyzed for the presence of the fecal coliform group or Escherichia coli (E. coli), a specific member of the fecal coliform group, both of which are more sensitive indicators of sewage pollution.
GIARDIA LAMBLIA AND CRYPTOSPORIDIUM.
Giardia lamblia and Cryptosporidium are microscopic protozoa that can infect humans and warm-blooded animals. They are frequently found in surface waters contaminated with animal or human wastes. Both organisms have a life stage protected by an outer shell called a cyst, which allows them to exist outside a host's body for a long period. If living cysts are ingested, they can cause an intestinal illness, the symptoms of which are nausea, vomiting, fever, and severe diarrhea. The symptoms last for several days, and a healthy human can generally rid his or her body of the organisms in one or two months. These two organisms are the most frequent cause of waterborne illness in the United States today.
Since 1996 the EPA has required water suppliers using surface water or groundwater under the direct influence of surface water to disinfect their water to control Giardia at the 99.99% inactivation and removal level. Groundwater is considered to be under the direct influence of surface water when the geologic formations (usually limestone or fractured bedrock) in which the aquifer lies do not provide adequate natural filtration.
A smaller parasite than Giardia, Cryptosporidium shows more than fifty-fold the resistance of Giardia to chlorine, the most commonly used drinking water disinfectant. Because of its high resistance to chemicals typically used to treat drinking water, it must be physically removed by filtration. As of 2002, water systems serving 10,000 or more people are required to provide filtration and achieve 99% removal or inactivation of Cryptosporidium.
Cryptosporidium was responsible for what many people view as the nation's worst drinking water disaster. In April 1993 residents of Milwaukee were infected with Cryptosporidium in the city water supply, which had been turbid for several days. For a week, more than 800,000 residents were without drinkable tap water. By the end of the disaster, fifty people had died and 403,000 people had been infected. In addition to the human suffering, the disease outbreak cost an estimated $37 million in lost wages and productivity. This outbreak was in large part responsible for the adoption of the EPA's Surface Water Treatment Rule.
User Comments Add a comment…