Tap water comes from groundwater or surface water sources (rivers, reservoirs, etc.). Cities, which generally draw their water from distant but substantial rivers or groundwater sources, have the means to monitor and treat water quality. This is not always the case for small municipalities, which rely primarily on groundwater—generally of higher quality than surface water—but cannot afford the cost of sophisticated treatment if the source becomes polluted.

Contamination of distributed water can be of natural origin or result from human activities.

In both cases, this pollution is of the following types:

• microbiological: bacterial, viral, or parasitic,

• or chemical.

Numerous sources of pollution can explain the presence of contamination at the consumer’s tap. The diversity of water resources, varied treatment processes, and the distribution network are just a few examples.

Microbiological contamination

Microorganisms capable of contaminating water sources mostly originate from human or animal feces, which may contain pathogens harmful to humans. These pathogens can cause acute gastroenteritis among consumers. This is the case, for example, with Cryptosporidium, which parasitizes the intestines of humans, calves, and many mammals. The excreted parasites then end up in water sources and, in the absence of effective filtration, can enter the water distribution network. Unlike bacteria, parasites are resistant to certain types of disinfectants used in drinking water treatment, particularly chlorine. Bacterial agents such as Campylobacter and Salmonella, or viral agents such as norovirus and rotavirus, can also contaminate tap water following contamination of the water source or backflow of wastewater.

Other enteric pathogens causing symptoms other than acute gastroenteritis can be responsible for water contamination and transmitted through it: hepatitis A or E viruses, or Helicobacter pylori, which causes stomach ulcers and stomach cancer.

Finally, certain environmental bacteria can colonize water systems and cause disease, particularly in immunocompromised individuals. In these cases, infection is not linked to ingestion but occurs through contact with mucous membranes or open wounds (Pseudomonas aeruginosa) or through inhalation (Legionella pneumophila).

Cyanobacteria, also known as blue-green algae, colonize certain waterways. Their blooms are likely to increase with climate change.

Chemical Pollution

Water pollution can also be caused by chemical pollutants. These pollutants, which can have harmful effects on the health of water consumers, may:

• stem from human activities: industry, agriculture,

• be naturally present in the ground: arsenic, selenium, antimony…,

• be produced during the treatment or transport of drinking water: disinfection byproducts, lead, etc.

Arsenic can have various sources. Most of the excessive concentrations found in consumers’ tap water are attributable to geology (underground resources in volcanic massifs, for example), but certain industries or agricultural activities can contribute to environmental pollution.

Starting in the 1950s, industrial development and the intensification of agriculture were accompanied by the massive use of chemicals, causing increasing environmental pollution. Industrial pollution (heavy metals, solvents, petroleum products, etc.) has been significantly reduced through the efforts of the Water Agencies, established in 1964, which levy pollution taxes; the proceeds help fund the construction of wastewater treatment plants (WWTPs) as well as sewer systems to ensure collection and transport to the WWTPs, and pollution control measures. However, residues from past industrial pollution remain in many soils and subsoils. On the other hand, water and soil pollution from nitrates and pesticides remains a current issue.

The presence of pharmaceutical residues in surface water is increasingly being highlighted. Pharmaceutical residues originate primarily from urban wastewater discharges, healthcare facilities, and intensive livestock farming.

Furthermore, studies show endocrine (hormonal) disruptions in aquatic fauna, which result, for example, in an imbalance in sex ratios.