Stormwater, traditionally thought of as a flood management problem, is getting attention as a resource with the potential to increase water supplies by up to 630,000 acre-feet per year. But implementation of stormwater projects face challenges by high costs and water quality concerns.
Traditionally, stormwater has been viewed as a flood management problem that called for shunting off the floodwaters as quickly as possible from urban areas to waterways in order to protect public safety and property. However, as drought has put more pressure on water supplies and groundwater basins become depleted, stormwater is increasingly being seen as a potential way to recharge groundwater basins and augment local water supplies. The potential benefit of putting that stormwater to use is substantial: According to an NRDC/Pacific Institute report, capturing the stormwater that falls in the urban areas of San Francisco Bay Area and Southern California could potentially increase supplies by as much as 630,000 acre-feet per year.
20th CENTURY VIEW: URBAN STORMWATER AS A FLOOD CONTROL AND WATER QUALITY PROBLEM
When rain falls on the urban landscape, impervious surfaces such as streets, sidewalks, rooftops, and parking lots prevent the water from soaking into the ground where it can infiltrate through the soil to recharge groundwater basins. That water has to go somewhere, so it flows across the urban landscape and heads downhill, picking up animal waste, trash, fertilizers, pesticides, metals, chemicals, and other contaminants; it flows through storm drain systems untreated, ultimately directly discharging into rivers, streams, and the ocean where it can pollute waterways, close beaches, and potentially impact fish and wildlife as well as other beneficial uses.
The conventional urban stormwater management approach has been more about responding to flood control concerns resulting from the urbanization of the watershed. This approach has been generally successful in preventing flood damage, but it also has come with several disadvantages: natural waterways have often been straightened and lined with concrete resulting in the loss of habitat and impacting natural physical and biological processes; constituents and contaminants that are picked up by the flowing water are discharged into waterways affecting water quality, and water that could potentially be used to recharge aquifers or for other uses is otherwise lost.
Not only can runoff cause water quality problems; it can contribute to increased downstream flood risk as impervious surfaces increase the volume and velocity of runoff, resulting in stream bank erosion and potential downstream flooding. Even when it isn’t raining, substantial amounts of urban slobber resulting from overirrigation of landscapes, car washing, industrial, and other uses flows into storm systems each day.
21st CENTURY VIEW: STORMWATER AS A RESOURCE
Increasing regulation of stormwater discharges along with the need to recharge depleted groundwater basins have led many communities to view stormwater as a resource to be captured and managed, rather than a problem to be whisked away as fast as possible.
While early stormwater regulatory efforts focused on implementing Best Management Practices to control pollutants, more recent efforts have taken a regional or watershed approach, combining the efforts of multiple agencies and organizations to control stormwater while providing other community benefits. Managing stormwater runoff at the watershed level allows communities to pool economic resources to obtain greater benefits for water supply, flood control, pollution control, open space, and the environment. Groundwater recharge and stormwater retention sites can be designed to provide wildlife habitat, parks, and open space.
Newer techniques for urban development utilize low impact development and green infrastructure projects to manage stormwater on-site. Low impact development (LID) refers to the practice of using site design techniques such as bioswales, rain gardens, or retention basins. LID techniques can improve water quality and augment water supplies either by harvesting the water for other uses or by allowing water to infiltrate into the groundwater aquifers. Green infrastructure expands the low impact development approach to the community scale, aiming to maintain or enhance the predevelopment hydrology of urban and developing watersheds.
In areas that do not overlay groundwater aquifers or where the soil and geologic conditions are unsuitable for infiltration, parks or other open spaces can be used to capture stormwater in cisterns or detention basins that can be later used for on-site irrigation or other non-potable uses. Buildings and residences can capture rainwater that can be used for landscape irrigation or toilet flushing. However, water savings from on-site or residential stormwater collection and reuse is dependent on tank size as well as the amount and timing of precipitation relative to water demand. In much of the arid West, the timing and intensity of rainfall can limit the capacity of residential-scale stormwater collection to meaningfully reduce potable water reuse.
STORMWATER QUALITY AND GROUNDWATER RECHARGE
The use of stormwater for recharging groundwater basins can potentially pose risks to groundwater. Stormwater quality is highly variable and could contain microorganisms, metals, organic chemicals, and sediments which are a concern for groundwater recharge. The actual threat to groundwater quality depends on several factors, including soil type, source control, pretreatment, the solubility of pollutants, how the recharge basins are maintained, current and past land use, and depth to groundwater.
Studies have found that most contaminants will be treated through the process of water infiltrating through the soil profile, although some pollutants, such as chlorides, are more likely to cause problems than others and must be carefully considered when planning groundwater recharge projects. To guard against these impacts, land uses, contributing areas, and collection materials can be selected and utilized to minimize contaminants. Surface practices, such as a bioinfiltration facility, bioswales, or filters can capture groundwater contaminants. In some cases, pre-treatment of stormwater or diversion of polluted waters away from infiltration areas may be necessary.
WHAT IS THE POTENTIAL FOR STORMWATER CAPTURE AND REUSE?
In June of 2014, the NRDC and the Pacific Institute conducted an analysis of stormwater potential focusing on the urbanized areas of Southern California and the San Francisco Bay Area using GIS analysis of selected land uses, impervious surface cover, and average annual precipitation; dry weather runoff was also considered as was rooftop rainwater harvesting for non-potable uses. The analysis estimated that stormwater capture could increase water supplies by 420,000 to 630,000 acre-feet per year.
CHALLENGES FOR STORMWATER PROJECTS
Multiple agencies are often involved in managing stormwater across a region or watershed, and even though downstream communities can be affected by activities upstream, there remains a lack of integration among these agencies to address stormwater issues. To complicate matters, land use planning is generally not done at the watershed scale. Thus many agencies spend millions of dollars annually with very little interagency coordination.
Perhaps the most significant barrier to stormwater projects is their high cost. In highly urbanized areas, the cost of purchasing land for facilities, as well as constructing, operating, and maintaining treatment facilities can be quite substantial. Local agencies usually have a limited ability to pay, and the provisions of Prop 218 have made raising fees difficult for most municipalities.
The Center for Watershed Protection A source for best practices in stormwater and watershed management. As national experts in stormwater and watersheds, the Center for Watershed Protection takes a practical approach to complex, technical watershed management issues; providing collaborative leadership across disciplines and professions with a commitment to protect, restore and enhance our streams, rivers, lakes, wetlands and bays.
Low Impact Development …
The Low Impact Development Center, a non-profit national research organization that focuses on sustainable stormwater management solutions for urban and developing areas.
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CA WATER COMMISSION: Stormwater Capture and Aquifer Recharge Posted on: April 24, 2019 Stormwater, traditionally thought of as a flood management problem, is getting attention as a resource with the potential to increase water supplies by up to 630,000 acre-feet per year. But implementation of stormwater projects face challenges by high costs and water quality concerns. Traditionally, stormwater has been viewed as a flood management problem that called for shunting off the floodwaters as quickly as possible from urban areas to waterways in order to protect public safety and property. However, as drought […]
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