Microbiological Contaminants

5.3.7 Microbiological Contaminants

Microbiological contaminants are microorganisms potentially harmful to humans or ani- mals. Jointly called pathogens, they include parasites, bacteria, and viruses. Although the previous sections illustrate the seriousness of contamination with organic chemicals and inorganic substances, pathogens are by far the most widely spread water contaminants. As pointed out by researchers from the Johns Hopkins University, water-related diseases are a human tragedy, killing millions of people each year, preventing millions more from leading healthy lives, and undermining development efforts. About 2.3 billion people in the world suffer from diseases that are linked to water, and some 60 percent of all infant mortality is linked to infectious and parasitic diseases, most of them water related (Hinrichsen et al., 1997).

Where proper sanitation facilities are lacking, waterborne diseases can spread rapidly. Untreated excreta carrying disease organisms wash or leach into freshwater sources, con- taminating drinking water. Diarrheal disease, the major waterborne disease, is prevalent in many countries where sewage treatment is inadequate—human wastes are disposed of in open latrines, ditches, canals, and water courses, or they are spread on cropland. An estimated 4 billion cases of diarrheal disease occur every year, causing 3 to 4 million deaths, mostly among children (Hinrichsen et al., 1997).

Although surface water is the primary recipient and host of pathogen contamination, shallow groundwater is also greatly affected in many regions with poor or nonexistent sanitation. However, some pathogens, such as parasites Giardia and Cryptosporidium, are naturally present in surface water bodies and are not necessarily associated with poor sanitation practices. For this reason, the USEPA has instituted specific water-treatment requirements for public supply systems using “groundwater under direct influence” (GWUI) of surface water.

The increased efforts to reclaim and reuse wastewaters and gray waters pose addi- tional public health concerns. These concerns are heightened by the fact that the indicators of the “sanitary quality” of waters, i.e., the total- and fecal-coliform bacteria, are unreli- able indicators of the presence of a number of key pathogenic agents including enteric viruses and cyst-forming protozoans. Wastewater reclamation does not have a specific meaning in terms of the degree of treatment for different reclamation projects. Some reuses of wastewater are allowed with very little additional treatment beyond the con- ventional sewage treatment, of addressing enteric viruses and cyst-forming protozoans.

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Even after what is considered to be good conventional domestic wastewater treatment and chlorination (chloramination), domestic wastewaters released to surface waters in nearby streams, lakes, estuaries, or coastal marine waters, still contain large numbers of enteroviruses and pathogenic protozoans that can readily cause human disease upon ingestion and, to a lesser extent, body contact with these waters (Lee and Jones-Lee, 1993).

In addition to nonsanitary practices at the land surface, pathogens can enter ground- water systems in natural surface water-groundwater interactions, or via artificial aquifer recharge using surface water and treated wastewater. Once in the subsurface (aquifer), their survival and transport will depend on various biogeochemical interactions with native groundwater, porous media, and native microorganisms. Whereas some bacteria and parasites cannot survive more than several weeks in the saturated zone regardless of the native conditions, some viruses are known to survive for months or even years.

Pathogens can cause an adverse effect after an acute (short-term) exposure such as ingestion of just one glass of water. They can also cause epidemics and chronic diseases. In the early 1990s, for example, raw sewage water that was used to fertilize vegetable fields caused outbreaks of cholera in Chile and Peru. In Buenos Aires, Argentina, a slum neighborhood faced continual outbreaks of cholera, hepatitis, and meningitis because only 4 percent of homes had either water mains or proper toilets, while poor diets and little access to medical services aggravated the health problems (Hinrichsen et al., 1997).

Bacteria are microscopic living organisms usually consisting of a single cell. Water- borne disease-causing bacteria include E. coli and Shigella. Protozoa or parasites are also single-cell organisms. Examples include Giardia lamblia and Cryptosporidium. A virus is the smallest form of microorganism capable of causing disease. A virus of fecal origin that is infectious to humans by waterborne transmission is of special concern for drink- ing water regulators. More than 120 different types of potentially harmful enteric viruses are excreted in human feces and are widely distributed in type and number in domes- tic sewage, agricultural wastes, and septic drainage systems (Gerba, 1999; from Banks and Battigelli, 2002). Many of these viruses are stable in natural waters and have long survival times, with half-lives ranging from weeks to months. Because they may cause disease even when just a few virus particles are ingested, low levels of environmental contamination may affect water consumers. From 1971 to 1979, approximately 57,974 people in the United States were affected by outbreaks of waterborne pathogens (Craun, 1986; from Banks and Battigelli, 2002). Outbreaks of waterborne disease attributed to enteric viruses are poorly documented, even though viruses are commonplace in natu- ral waters contaminated with human feces. Illnesses in humans caused by waterborne viruses range from severe infections such as myocarditis, hepatitis, diabetes, and paral- ysis to relatively mild conditions such as self-limiting gastroenteritis. Currently, enteric viruses are included in the National Primary Drinking Water Standards issued by the USEPA, while several other groups are on the contaminant candidate list (CCL). Studies of possible groundwater contamination with viruses are still very rare (USEPA, 2003c), but because of their presence on the CCL, and the new groundwater rule promulgated by USEPA in 2006, the interest of the scientific community has increased.

Giardia (Fig. 5.15) was only recognized as a human pathogen capable of causing wa- terborne disease outbreaks in the late 1970s. Its occurrence in relatively pristine water as well as wastewater-treatment plant effluent called into question water system definitions of “pristine” water sources. This parasite, now recognized as one of the most common causes of waterborne disease in humans in the United States, is found in every region of

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F IGURE 5.15 Left: two Giardia intestinalis cysts in a wet mount under DIC microscopy; image taken at 1000× magnification. The cysts are oval to ellipsoid and measure 8 to 19 μm (average 10 to 14 μm). Right: Giardia intestinalis trophozoites are pear shaped and measure 10 to 20 μm in length. (Photographs courtesy of the Center for Disease Control (CDC) Parasite Image Library.)

the United States and throughout the world. In 1995, outbreaks in Alaska and New York were caused by Giardia. The outbreak of giardiasis in Alaska affected 10 people and was associated with untreated surface water. The outbreak in New York affected an estimated 1449 people and was associated with surface water that was both chlorinated and filtered (USEPA, 2003c). The symptoms of giardiasis include diarrhea, bloating, excessive gas, and malaise.

The infectious dose for Cryptosporidium is less than 10 organisms, and, presumably, one organism can initiate an infection. As late as 1976, it was not known to cause dis- ease in humans. In 1993, 403,000 people in Milwaukee, WI, became ill with diarrhea after drinking water contaminated with the parasite, resulting in the largest waterborne disease outbreak ever documented in the United States (Tiemann, 1996). For the 2-year period of 1993 to 1994, the Center for Disease Control reported that 17 states identified 30 disease outbreaks associated with drinking water. Since then, attention has been focused on determining and reducing the risk of cryptosporidiosis from public water supplies. Crypto is commonly found in lakes and rivers and is highly resistant to disinfection. Groundwater under the influence of surface water, and groundwater in highly transmis- sive karst and gravel aquifers, is also susceptible to contamination with parasites such as Giardia and Cryptosporidium. People with severely weakened immune systems are likely to have more severe and more persistent symptoms than healthy individuals.

In a nationwide study by the USGS, microbiological data were collected from 1205 wells in 22 study units of the National Water-Quality Assessment (NAWQA) program during 1993 to 2004. The samples of untreated groundwater were analyzed primarily for concentrations of total-coliform bacteria, fecal-coliform bacteria, and E. coli, and for the presence of coliphage viruses (Embrey and Runkle, 2006).

Nearly 30 percent of the 1174 wells analyzed tested positive for coliform bacteria. With at least one well in each study unit or principal aquifer testing positive, fecal-indicator bacteria were geographically widespread.

Samples were collected from 423 wells to test for the presence of coliphage viruses, which are considered indicators of the potential presence of human enteric viruses. Col- iphage were present in samples from four of the 11 study units—the Central Columbia Plateau-Yakima, Georgia-Florida, San Joaquin, and Trinity, representing the Columbia Plateau, Floridan, Central Valley, and Coastal Lowlands aquifers, respectively. Overall,

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coliphage viruses were present in less than 4 percent of domestic and public wells used for drinking water supply.

Wells used for domestic supply made up the largest class of water use, with total- coliform concentrations analyzed in 405 wells and E. coli concentrations analyzed in 397 wells, followed by public supply wells and unused wells with 227 and 37 analyses of total-coliform bacteria, respectively. Total coliforms were detected in untreated water from 33 percent of domestic wells and 16 percent of public supply wells; E. coli were detected in 8 and 3 percent of domestic and public supply wells, respectively. Although median concentrations were <1 CFU/100 mL for all classes of water use, as defined in this report, the overall distribution of total-coliform concentrations was significantly higher in domestic wells than in public supply wells.

Generally, coliform bacteria were detected more frequently and in higher concen- trations in wells completed in sandstone or shale, and in sedimentary, carbonate, and crystalline rocks than for wells in unconsolidated materials, in semiconsolidated sand, or in volcanic rocks. More than 50 percent of sampled wells completed in carbonate rocks (limestone and dolomite) or in crystalline rocks (schist and granite) tested positive for coliform bacteria. The Floridan, Piedmont and Blue Ridge, Ordovician, and Valley and Ridge aquifers, all of which had high detection rates or concentrations of coliform bac- teria, are composed of these fractured and porous rocks. The lowest rates of detections (less than 5 percent) were for wells in the Basin and Range and Snake River aquifers. Materials in these aquifers are primarily unconsolidated sand, gravel, and clay, or basalt with interbeds of sand, gravel, or clay.

The depths of public supply wells (median of 427 ft below land surface) and of the wells in the Basin and Range aquifer (median depth of 400 ft) might explain, in part, the relatively low detection frequencies of the coliform bacteria observed in these samples. A thick unsaturated zone increases the potential for natural attenuation of microorganisms, preventing the transport of bacteria into the groundwater. Fifty percent of wells in principal aquifers with median depths of sampled wells ranging from 100 to 200 ft below land surface tested positive for total-coliform bacteria, whereas only 9 percent of wells in principal aquifers with median depths of sampled wells greater than 200 ft tested positive.