Thanks to the international media coverage of the water crisis in Flint, Michigan, the residents of towns and cities are beginning to realize that the need for regular water sampling is not limited to rural areas. While, in decades past, many people assumed that the laws and regulations which are typically in place to ensure the safety of urban water supplies would protect them from threats posed by contamination, today we know that the system is highly fallible. A simple change—in Flint’s case switching water sources from Lake Huron to the local Flint River—can cause massive problems, and these problems often go undetected until it’s too late.
As the situation in Flint demonstrates, water treatment and processing is not a perfect science; the condition of the water which enters city water treatment plants can greatly impact the quality of the water that ends up flowing into residences. According to news outlet The Verge, “Most of the contaminants now in Flint’s drinking water were introduced during or after processing.” In an attempt to rid the water of bacterial contaminants (which can cause diseases like hepatitis and Legionnaire’s disease), additional chlorine was used. Chlorine “Reacts with organic material in the water to produce carcinogenic byproducts such as trihalomethanes; it also makes water more acidic, which corrodes pipes.” While federal law mandates the addition of anti-corrosive agents to drinking water as a means to prevent pipe corrosion, cities are often lax about following this mandate. In Flint, absolutely no anti-corrosive agent was used, and as a result, lead and other dangerous metals were released into the city’s water supply as ageing pipes broke down. Because few people were aware of the need for regular water testing even in urban areas, by the time they realized there was a problem with the water in Flint, hundreds of children had already been poisoned, causing irreversible brain damage.
The quality of city water is threatened by multiple factors; some of these threats include: Road salt, which washes into the water supply in cold climates and is highly corrosive to pipes; fertilizers, which frequently enter city water supplies as a result of agricultural activity upstream; dumps and landfills, which leech contaminants into groundwater, and bacteria, which is common in slow-moving water or water that is being contaminated by poorly-treated sewage. Derelict industrial sites also frequently seep chemical contaminants into urban water supplies, and finally, there are natural hazards to consider, like Radon and minerals.
Radon. Though many think of radioactivity as a man-made phenomenon, radioactive Radon gas occurs naturally in many areas of North America, and it has been known to contaminate both rural and urban water supplies. When inhaled via water vapour (e.g. while showering) Radon can significantly increase a person’s risk of lung cancer.
Lead. As described above, lead (which is often present in water pipes) enters water after treatment, making it a particularly insidious contaminant as there is no way to test for it at water treatment facilities. Lead contamination is not limited to cities with an industrial past like Flint; it can occur anywhere water has a low PH value.
Testing for lead regularly at home is extremely important as there is new evidence linking even low-level lead exposure with cognitive deficits. According to Bruce Lanphear, a researcher at Simon Fraser University, “Data linking lead-contaminated water with increases in the prevalence of children having blood lead levels over 10 μg/dL suggest that testing water for lead contamination should be done routinely in older cities.”
Nitrate. Nitrate, which results from fertilizer pollution, can form Nitrite in the human digestive system, a compound which inhibits the blood’s ability to carry oxygen around the body. This is particularly dangerous to infants.
Fluoride. Fluoride, which is intentionally added to city water in order to prevent dental decay, can be toxic in significant amounts. As the level of fluoride varies from city to city, it’s recommended that city residents verify that the fluoride in their area’s water supply falls within acceptable limits.
Bacteria. Water-borne pathogens, which usually arise from Coliform bacteria strains, can cause illnesses ranging from intestinal infections to dysentery, hepatitis, typhoid fever, cholera, E. Coli infection, and Legionnaire’s Disease. While bacterial contamination is usually thought of as being an issue confined to rural wells, city water can also be affected; Legionnaire’s Disease, which can result in death, has broken out in a number of urban centers in recent years, including Flint, Chicago, Quebec City (Canada), and New York City.
Minerals. The term “hard water” is used to describe water that is contaminated with minerals like Calcium, Manganese, Magnesium, Sodium, and Iron. While hard water is not usually a human health hazard, it can be extremely destructive to appliances and result in hundreds of dollars’ worth of damage to dishwashers, laundry machines, etc. It also makes fabric feel stiff and rough and can irritate skin and damage hair. 
Some minerals, it should be noted, do carry a health risk: Copper contamination, which is common in city water, can cause intestinal problems, vomiting, diarrhoea, liver damage and kidney disease.
Arsenic. Arsenic can contaminate water supplies relatively easily as it is present in the natural environment, as well as being a common industrial byproduct. In fact, a 2000 NRDC study revealed that as many as 56 million people in 25 states were drinking water with arsenic at levels that posed a high risk of cancer. 
As a general rule, water testing should be carried out at least once per year. However, if your water begins to look, smell, or taste different than it usually does, it’s strongly advised that you have it tested immediately, regardless of when you last had a sample taken.
 How the Flint River got so toxic, http://www.theverge.com/2016/2/26/11117022/flint-michigan-water-crisis-lead-pollution-history
 Exposure on Tap: Drinking Water as an Overlooked Source of Lead, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2831942/
 Hard Water Hardness Calcium Magnesium Water Corrosion Mineral Scale, http://www.water-research.net/index.php/water-treatment/tools/hard-water-hardness
 Copper in Drinking Water: Health Effects and How to Reduce Exposure, http://www.health.state.mn.us/divs/eh/water/factsheet/com/copper.html
 What’s in Your Drinking Water? https://www.nrdc.org/stories/whats-your-drinking-water