Aren’t Stormwater and Rainwater the Same Thing?
It seems logical to say that the terms stormwater and rainwater are synonymous. After all, they both refer to water falling from the sky. In the context of collecting and managing precipitation, however, the terms refer to distinct collection sources and therefore, the associated water quality.
In the realm of collection, management, and re-use of precipitation, rainwater is defined as runoff collected directly from roof surfaces, typically operated on a small scale by individual homeowners for individual use, for example. Because roof surfaces generally host lower levels of pollutants than ground-level surfaces, rainwater collected in this context may require relatively limited treatment before it’s safe to use.
In contrast, stormwater is rainwater plus anything the rain carries along with it. This is an important distinction, since stormwater is typically collected after it has traveled over many different types of urban surfaces, picking up pollutants from all of them. These pollutants may include random trash, chemical residues, oil and petroleum products, fecal bacteria from animal waste, and toxic metals like copper, lead, and zinc.
While stormwater is substantially more polluted than rainwater, it also comprises most of the overall precipitation that ends up in an urban or suburban environment. Uncontrolled and unmanaged stormwater creates significant problems in developed areas, posing threats to infrastructure, local water quality, and human populations. However, when it’s collected, treated, and released or re-used in an environmentally sensitive manner, stormwater represents a valuable resource that can help communities around the world respond to the challenges of water scarcity and changing weather patterns.
Why is Stormwater a Problem?
Historically, most industrialized countries have treated stormwater runoff as a nuisance to be disposed of as quickly and efficiently as possible. In developed regions where hard surfaces prevail, precipitation is no longer able to soak naturally into the ground where it would be absorbed by vegetation or seep deeper and rejoin the water table. Instead, water is forced to flow over pavement, parking lots, and sidewalks, where it joins other water flows and increases in energy as it travels.
The volume of water we’re talking about is not inconsequential. In fact, a single inch of rainfall on one mile of a narrow, two-lane road can produce 55,000 gallons of stormwater runoff. When that volume of stormwater is funneled through a narrow drain, the destructive energy carried by that mass of fast-moving water easily erodes and damages river and stream banks, destroying both natural habitat and valuable land.
For as long as modern cities have existed, it’s been apparent that large volumes of uncontrolled stormwater are dangerous to humans, property, and the environment. Flooding, erosion, damage or destruction of roadways and other infrastructure, and direct threats to public health are all typical consequences of poorly managed stormwater in urban environments. With these threats clearly in mind, the accepted approach to stormwater, from time immemorial, has been to focus on moving stormwater as quickly as possible out of the urban environment to nearby surface waters, with little concern about ecological impact.
Traveling over so many different surfaces and picking up a wide variety of pollutants, stormwater is a damaging source of ‘nonpoint source’ pollution. This type of pollution is particularly difficult to manage since there’s not a single source or location where it can be eliminated. In fact, the effects stemming just from the nutrient load carried into local waterways make up one of the country’s most widespread, costly, and formidable environmental problems.
It’s clear that pollutants picked up by stormwater turn largely clean rainfall into highly contaminated runoff. This is an unfortunate situation, as global access to relatively clean fresh water is becoming increasingly limited. Unfortunately, existing stormwater management systems across the country are expected to manage ever-increasing volumes as cities and developed areas continue to expand, without adequate system improvements and maintenance. As weather patterns change and our cities experience more and more severe storm events, the systems simply cannot keep up. An estimated 10 trillion gallons of untreated stormwater runoff enters US waterways every year. The impact on the local environment and our drinking water supplies cannot be overlooked.
The Damaging Effects of Stormwater
It’s hard to exaggerate the severity and variety of effects that poorly managed stormwater has on property, the environment, and human lives.
- Widespread flooding results when heavy rainfall accumulates too quickly for the existing infrastructure to capture and channel it away. When a stormwater system overflows, water is left to pool and flow on the surface, inundating roads and property. In many older urban areas such as Chicago, Pittsburgh, and Detroit, stormwater systems are connected to domestic sewage and industrial wastewater pipes. In these cases, flooding from an overwhelmed stormwater system also carries untreated sewage and industrial waste onto streets, across residential yards, and into local surface waters.
- Eroded streambanks occur when high volumes of water travel quickly along surface water channels. When banks are eroded, the stream channels widen, and valuable property may be lost. In addition, the sediment released as the banks are eroded is carried downstream, where it clogs waterways, fills lakes and reservoirs, and damages or destroys aquatic ecosystems, including the animals and plants that live there.
- Aesthetics and recreational value are impaired when stormwater escapes into the local environment. Contaminated, sediment-filled water full of trash and debris invariably generates foul odors, which makes natural areas unappealing. Swimming, fishing, and even boating is quickly curtailed, reducing the city (and state’s) recreational opportunities. Worse, this kind of damage isn’t conducive to a quick fix.
- Public health threats are associated with stormwater flooding, especially when a combined sewer overflow (CSO) event occurs. The combination of untreated or partially treated human waste containing harmful bacteria and viruses, toxic industrial waste, and even debris, together form a priority water pollution concern for approximately 700 municipalities across the US with combined sewer systems. This contaminated stormwater can infiltrate groundwater and overwhelm local surface waters that provide drinking water for urban populations.
- Public Safety is threatened during stormwater flooding - drownings and injuries from moving or submerged debris are common.
- Water treatment and wastewater treatment demand is more difficult to manage, and plants are often unable to operate at the necessary capacity. This means that inadequately treated (hence, unsafe) water is often released into the environment, with all its deleterious effects. It also means that municipal governments are forced to pay more for treating polluted water while also contemplating how to expand and improve their systems to accommodate extreme storm events that may happen only a few times a year.
Economic Impacts arising from impaired fisheries, contaminated shellfish, reduced tourism and the effect on recreation related businesses can be difficult to recover from and affect not only individual businesses and livelihoods, but tax income for the municipalities that are faced with the increased costs of dealing with the problem to begin with.
