City Water Systems

Colorado’s municipal water supply draws from two primary categories of source water: surface water and groundwater. Surface water includes rivers, streams, lakes, and reservoirs that collect snowmelt and rainfall from across the state’s mountain ranges and high plains. The majority of Front Range communities depend heavily on surface water originating in the Rocky Mountains. The Colorado River, South Platte River, and Arkansas River basins serve as critical watersheds, and large storage reservoirs such as Dillon Reservoir, Gross Reservoir, and Horsetooth Reservoir hold water during high-flow periods so it can be released and treated throughout the year.

Groundwater, drawn from underground aquifers through wells, serves as the primary or supplemental supply for many communities, particularly on the Eastern Plains and in areas south of the Denver metro region. The Denver Basin aquifer system -- comprising the Dawson, Denver, Arapahoe, and Laramie-Fox Hills aquifers -- supplies water to numerous towns and water districts. Unlike surface water, these aquifers recharge slowly over geological timescales, which means withdrawals can outpace natural replenishment. Some communities blend groundwater with surface water to balance supply reliability and water quality characteristics.

Seasonal variability plays a significant role in Colorado’s water supply. Approximately 80 percent of the state’s precipitation falls as snow in the mountains, and the spring and early summer snowmelt season determines how much water flows into reservoirs for the rest of the year. During drought years, reservoir levels can drop substantially, prompting water providers to implement conservation measures and, in some cases, draw more heavily on groundwater reserves. This cycle of abundance and scarcity shapes how utilities plan infrastructure, manage storage, and allocate water across competing agricultural, municipal, and environmental needs.

Before water reaches your faucet, it passes through a multi-stage treatment process designed to remove contaminants, pathogens, and sediment. While specific methods vary by utility, most municipal treatment plants in Colorado follow a conventional sequence that has been refined over more than a century of water engineering.

1. Screening. Raw water entering the treatment plant first passes through coarse screens and bar racks that remove large debris such as branches, leaves, trash, and aquatic vegetation. Some facilities also use finer mesh screens to catch smaller organic material. This initial step protects downstream equipment from damage and clogging.
2. Coagulation and Flocculation. Chemical coagulants -- commonly aluminum sulfate (alum) or ferric chloride -- are rapidly mixed into the water. These chemicals neutralize the electrical charges on fine suspended particles, causing them to clump together. The water then moves into flocculation basins where gentle, slow mixing encourages these small clumps to combine into larger, heavier aggregations called floc. The goal is to create particles large enough to settle out of the water by gravity.
3. Sedimentation. The water flows into large, quiet settling basins where the heavy floc particles sink to the bottom over a period of several hours. The accumulated sediment, known as sludge, is periodically removed and disposed of according to environmental regulations. Clarified water is drawn from the top of the basin and directed to the next stage. Some modern plants use inclined plate settlers or dissolved air flotation to accelerate this process.
4. Filtration. Even after sedimentation, fine particles, microorganisms, and residual floc remain in the water. Filtration passes the water through layers of granular media -- typically sand, anthracite coal, or activated carbon -- that physically trap remaining particles. Some utilities use membrane filtration, which pushes water through extremely fine synthetic membranes capable of removing particles down to the sub-micron level. Filters are periodically backwashed to flush out accumulated material and restore flow capacity.
5. Disinfection. Disinfection is the critical step that inactivates or destroys bacteria, viruses, and protozoan parasites such as Giardia and Cryptosporidium. Most Colorado utilities use chlorine or chloramines (a combination of chlorine and ammonia) as their primary disinfectant. Chloramines are favored by many larger systems because they produce fewer disinfection byproducts and provide a more stable residual throughout the distribution network. Some plants also employ ultraviolet (UV) light or ozone as supplementary disinfection barriers. A small, carefully controlled residual of disinfectant remains in the water as it enters the distribution system to prevent microbial regrowth during transit.
6. Corrosion Control. Before treated water is sent into the distribution system, many utilities adjust its chemistry to reduce its tendency to corrode pipes. This typically involves adding orthophosphate or adjusting pH and alkalinity levels. Orthophosphate forms a thin protective mineral coating on the interior surfaces of metal pipes, which helps prevent lead, copper, and iron from dissolving into the water. Corrosion control is one of the most important steps for protecting water quality between the treatment plant and the point of use.

Once water leaves the treatment plant, it enters a complex network of pipes, pumps, and storage facilities known as the distribution system. This infrastructure is responsible for delivering treated water at adequate pressure and volume to every connected home, business, and fire hydrant in the service area. The distribution system is often the largest and most expensive asset a water utility owns, comprising hundreds or even thousands of miles of buried pipe.

Storage tanks and reservoirs are positioned throughout the service area, often on elevated terrain or as tall standpipes, to maintain consistent water pressure. These tanks fill during periods of low demand (typically overnight) and release water during peak usage hours in the morning and evening. They also provide an emergency reserve for firefighting and can buffer the system during short-term treatment plant interruptions.

Booster stations are pumping facilities placed at strategic points in the network to increase water pressure where gravity alone is not sufficient. In Colorado’s hilly terrain, particularly along the Front Range foothills, booster stations are essential for pushing water uphill to neighborhoods at higher elevations. Without them, homes at the far reaches or highest points of the system would experience low pressure or intermittent service.

Pressure zones divide the distribution system into distinct areas, each maintained at a target pressure range (typically 40 to 80 pounds per square inch). Pressure-reducing valves and booster pumps work together to keep each zone within safe operating limits. Too little pressure results in poor service and potential backflow contamination, while excessive pressure can stress pipes and increase the risk of leaks and main breaks.

Main breaks occur when a section of buried pipe cracks, splits, or fails at a joint. Causes include age-related deterioration, ground movement, temperature fluctuations, corrosion, and pressure surges. When a main break happens, the surrounding area may experience a temporary loss of service or a boil-water advisory until repairs are completed and water quality is confirmed through testing. Utilities prioritize rapid response to minimize both water loss and service disruption.

Hydrant flushing is a routine maintenance practice in which utilities open fire hydrants to move water rapidly through sections of the distribution system. This helps clear accumulated sediment, refresh stagnant water in low-use areas, and verify that hydrants are functional for emergency use. Residents near flushing operations may temporarily notice discolored water or reduced pressure, both of which typically resolve within a few hours.

Much of Colorado’s municipal water infrastructure was built during periods of rapid population growth in the mid-twentieth century. In older cities like Denver, Pueblo, and Colorado Springs, portions of the distribution system date to the early 1900s. While well-maintained pipes can last many decades, all materials eventually degrade. Cast iron mains from the 1920s through 1960s are increasingly prone to corrosion and breakage. Concrete-lined pipes from mid-century can experience lining deterioration that exposes the underlying metal. Even newer materials installed in the 1970s and 1980s are now approaching or exceeding their expected service life.

The scale of the maintenance challenge is substantial. Colorado’s population has grown rapidly, with many Front Range communities doubling or tripling in size over the past few decades. This growth has meant that utilities must simultaneously maintain aging legacy infrastructure while extending new pipe to serve expanding development. Capital improvement budgets must balance the urgent need to replace deteriorating mains against the demand for new capacity. Nationwide, the American Society of Civil Engineers has consistently rated drinking water infrastructure as needing significant investment.

For homeowners, the practical consequence of aging infrastructure is that the pipe carrying water to your property line may be decades old, and the service line connecting the main to your home may be older still. Utilities are actively mapping, inspecting, and prioritizing replacement of the most vulnerable segments, but full system renewal is a generational effort. Understanding that your water has traveled through infrastructure of varying age and condition helps explain why water quality at the tap can sometimes differ from water quality at the treatment plant.

The pipes that make up a municipal distribution system are not uniform. Over the decades, utilities have used a range of materials, each selected based on the engineering standards, cost considerations, and supply availability of its era. Here are the most common materials found in Colorado water systems:

• Cast Iron. Widely installed from the late 1800s through the 1960s, cast iron was the standard material for water mains for generations. It is strong and durable but susceptible to internal corrosion and tuberculation (the buildup of rust nodules inside the pipe), which can reduce flow capacity and affect water color and taste over time.
• Galvanized Steel. Common in residential service lines and indoor plumbing from the 1930s through the 1960s, galvanized steel pipes are coated with a layer of zinc to resist corrosion. Over time, however, the zinc coating wears away and the underlying steel corrodes, leading to restricted flow, discoloration, and the potential for accumulated mineral deposits inside the pipe walls.
• Copper. Copper became the preferred material for residential plumbing and service lines starting in the 1960s and remains widely used today. It resists corrosion well in most water chemistries, though aggressive or acidic water can cause copper to dissolve at elevated levels. Proper corrosion control treatment by the utility helps protect copper pipes throughout the system.
• PVC (Polyvinyl Chloride). PVC pipe has been increasingly used for water mains since the 1970s. It is lightweight, resistant to corrosion, and relatively inexpensive to install. PVC does not interact chemically with water in the same way metals do, which means it does not contribute metals to the water supply. However, it can become brittle over time and is more susceptible to damage from ground movement in certain soil conditions.
• Legacy Lead Service Lines. Lead was used for service lines (the pipe connecting the water main to individual buildings) in many cities through the 1950s, and in some locations into the 1980s. Lead is a well-documented health concern, and utilities across Colorado are actively inventorying and replacing lead service lines in accordance with updated federal regulations. Corrosion control treatment reduces the amount of lead that dissolves from these pipes, but replacement is the long-term solution. If your home was built before 1960, it is worth confirming with your utility whether your service line has been assessed.

Water leaving the treatment plant meets stringent quality standards, but its characteristics can shift as it travels through the distribution system and into your home’s plumbing. This does not necessarily indicate a problem -- it reflects the physical and chemical realities of moving water through miles of pipe. Several factors can contribute to these changes:

Corrosion interaction. As water moves through metal pipes, it interacts chemically with the pipe walls. Even with effective corrosion control treatment, low levels of iron, copper, zinc, or (in legacy systems) lead can dissolve into the water over time. The degree of interaction depends on the pipe material, the age and condition of the pipe, the water’s pH and mineral content, and how long the water sits in contact with the pipe surface. Water that has been sitting in pipes for several hours -- such as overnight or during vacation periods -- has had more contact time and may carry slightly higher levels of dissolved metals than water that is flowing regularly.

Sediment disturbance. Over time, fine mineral particles, iron oxide, and manganese deposits can accumulate on the interior surfaces and at the bottom of larger distribution mains. Normally, these deposits remain undisturbed. However, sudden changes in flow direction or velocity -- caused by main breaks, hydrant use, nearby construction, or utility maintenance -- can stir up this settled material, resulting in temporarily discolored or cloudy water. While visually unappealing, these events are typically short-lived and resolve once normal flow conditions are restored.

Temperature shifts. Water temperature changes as it moves from the treatment plant through buried mains and eventually into your home’s indoor plumbing. Seasonal ground temperatures, sun exposure on above-ground pipes, proximity to hot water heaters, and the length of indoor pipe runs all influence the temperature of water at the tap. Temperature matters because warmer water is generally more chemically reactive, which can increase the rate of corrosion and affect the taste and odor of treated water. In summer months, water sitting in pipes exposed to heat may taste or smell slightly different than the same water in winter.

Understanding these factors helps explain why water quality testing at the tap -- not just at the treatment plant -- provides the most complete picture of what is actually being delivered to your household.