Los contrastes sectoriales y empresariales en la industria urbana y rural.

Although the consumption of reticulated water has been shown to be protective of human health (Eberhart-Phillips et al., 1997; Hoque, Hope, Scragg, & Kjellstrom, 2003) there are also incidences where the consumption of contaminated reticulated drinking-water has led to outbreaks of disease (Baldursson & Karanis, 2011; Craun et al., 2010; Hrudey & Hrudey, 2004, 2007; Schuster et al., 2005). These outbreaks can affect a large number of people in a relatively short period of time. An example of this occurred in Milwaukee, Wisconsin, in March-April of 1993. In this outbreak an estimated 400,000 people contracted acute gastrointestinal illness (AGI) from consuming the town drinking-water supply that was believed to be contaminated by

Cryptosporidium (Mackenzie et al., 1994; Hrudey & Hrudey, 2007).

Reticulated supplies can become contaminated at any point in the supply network: at the source, the treatment plant, or in the distribution zone. For groundwater supplies, heavy rainfall can cause faecal contamination to be washed into the groundwater source (Nichols, Lane, Asgari, Verlander, & Charlett, 2009; Thomas et al., 2006). Periods of drought can cause cracks to occur in the ground which open up new water flow channels (Nichols et al., 2009). Human interventions, such as the placing of septic systems too close to groundwater sources, can also lead to source water contamination (Schmoll et al., 2006).

Untreated reticulated groundwater systems are of particular concern as these systems have no treatment barriers present to remove or reduce any contamination that may

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be present in the source water. In Norway, from 1984-2007, 26% of all waterborne outbreaks were attributed to untreated groundwater (Kvitsand & Fiksdal, 2010). In Finland from 1988-1999, nine of the 14 waterborne outbreaks recorded were associated with un-disinfected groundwaters (Miettinen, Zucheus, von Bonsdorff, & Vartiainen, 2001).

The importance of having an effective treatment system in the water supply chain is highlighted by the number of outbreaks that occur due to treatment system failures. In America, from 1971 to 2006, there were 338 waterborne disease outbreaks recorded in community water systems (those systems that supply water year round to at least 15 residents). Of these outbreaks, 154 (45.6%) were attributable to deficiencies in the treatment system (Craun et al., 2010). A review of worldwide waterborne protozoal disease outbreaks determined that there were 104 outbreaks associated with drinking-water systems over a period of 100 years from early last century until 2004. Deficiencies in water treatment systems were the most cited reason for these protozoal outbreaks. Deficiencies included poorly operated treatment systems, the use of insufficient barriers to ensure removal of contamination present in the source waters, and inadequate disinfection (Karanis, Kourenti, & Smith, 2007).

Contamination of water supplies can also occur in the distribution system. Studies have shown that the use of open storage reservoirs can lead to an increase in bacterial densities (Kerneis et al., 1995; LeChevallier et al., 1996; Lee et al., 2006). Biofilm formation in distribution pipes can lead to changes in microbiological water quality (LeChevallier et al., 1996). Contaminants can enter distribution pipes via breaks in the pipes, from cross-contamination, or from back-siphonage. Just over half of the 89 waterborne outbreaks caused by distribution deficiencies in the USA between 1971 and 1998 were found to be caused by cross-connections or back- siphonage (Craun & Calderon, 2001).

Not only can contamination of water supplies occur in the distribution network but drinking-water can also become contaminated in household plumbing (Lautenschlager et al., 2010; Pepper et al., 2004). A considerable number (n=55, 16.1%) of the 338 waterborne outbreaks occurring in community water supplies in the USA from 1971 to 2006 were due to deficiencies in household plumbing.

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Examples of these plumbing deficiencies included problems with cross-connections and contamination of equipment used to distribute water, such as drink-mixing machines (Craun et al., 2010).

The cause of an outbreak may not be related to a problem in just one part of a water supply network but may be caused by multiple barrier failure. An example of an outbreak believed to be caused by multiple barrier failure occurred in Walkerton, Canada, in May 2000. The first barrier to fail occurred when the source water became contaminated with cattle manure. The second barrier to fail occurred when an insufficient dose of chlorine was added to the water supply at the water treatment station. This led to inadequately treated water being distributed to consumers. Lastly, monitoring of the supply was poor and the skill level of the water treatment plant operators was inadequate. The failure of these barriers led to an outbreak in which an estimated 2,300 people became sick and seven died after consuming contaminated drinking-water (Hrudey, Payment, Huck, Gillham, & Hrudey, 2003).

3.5

Water testing

3.5.1 Advantages

In New Zealand, microbiological monitoring of water supplies forms an important part of the multiple barrier framework in which drinking-water suppliers must work (Nokes, 2008a). Monitoring acts as a method of demonstrating that the water supplied to the consumer is safe to drink and that the mechanisms utilised to prevent and reduce contamination in the supply system are working effectively (Stevens, Ashbolt, & Cunliffe, 2003). Water testing is quick and easy to perform and can provide an indication of any changes in water quality over time (WHO, 2011).

3.5.2 Disadvantages

Microbiological monitoring, though, should not be relied on as a sole means of ensuring a safe supply of water as there are drawbacks to monitoring (Neumann et al, 2005; Nokes, 2008a, b). Samples taken are small (often only 100 ml) and may not be representative of drinking-water quality (Neumann et al., 2005). Samples only represent the water quality at that point in time and water quality can change quickly. Samples can take time to analyse and by the time the results of monitoring

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are known contaminated water may have already been supplied to consumers (Nokes, 2008a; Stevens et al., 2003; WHO, 2011). Testing water in the distribution zone does not always give a clear indication of the water quality at the household tap, as microbiological water quality can change in household pipes (Lautenschlager et al., 2010; Pepper et al., 2004). Monitoring also relies on the effectiveness of remedial actions, such as boil water notices, and these are not always effective (Angulo et al., 1997; Craun & Calderon, 2001).

A further disadvantage of water quality monitoring is that the presence of indicator organisms is not always correlated with disease outbreaks (Craun, Berger, & Calderon, 1997), with the presence of pathogens (Wu, Long, Das, & Dorner, 2011), or with endemic levels of gastrointestinal disease (Payment et al., 1993; Strauss, King, Ley, & Hoey, 2001). A review of waterborne outbreaks in America from 1983-1992 determined that coliform bacteria were only present in half of the water supply systems prior to an outbreak occurring (Craun et al., 1997). Furthermore, a study to determine the rates of gastrointestinal disease in families supplied with conventionally treated tap water determined that rates of gastrointestinal illness were not correlated with the presence of faecal indicators (Payment et al., 1993).