Unique 20-year study assesses ecosystem response to different types of stormwater management Completed

Urban development is a well know stressor for stream ecosystems, leading to increased amounts of pollution, changes in streamflow characteristics, and degradation of aquatic life in streams. Managing stormwater runoff from development is primarily accomplished by installing stormwater practices to capture and filter stormwater before it drains into streams and rivers. Stormwater practices called green infrastructure, which use vegetation and soils to capture and treat runoff, are increasingly being used by local governments. Practices include features such as rain gardens, bioretention, sand filters, wetlands, and infiltration tree pits.

These practices can be expensive, so in addition to reducing water pollution levels, local governments and other practitioners would like to get additional benefits in stream health and flood mitigation. Therefore, more understanding is needed for the multiple benefits of stormwater practices.

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The USGS Study

The U.S. Geological Survey partnered with Montgomery County, Maryland to conduct an innovative study to document what happens when agricultural land is converted to suburban development with green stormwater infrastructure practices incorporated into the design of the neighborhood. This study is unique because it is one of the first to evaluate the impacts of different arrangements of stormwater practices on a suite of factors that influence stream health through long-term monitoring lasting 2 decades.

The study examined datasets collected in five sites: three treatment sites that underwent suburban development, an urban control site with a few large stormwater practices, and a forested site. The treatment sites had smaller stormwater facilities incorporated into the neighborhood design. The study design allowed comparison of an urban watershed with centralized stormwater management against those with stormwater practices distributed across treatment watersheds (see Figure 1). The watersheds were monitored before development, during construction, and after development. The monitoring included changes in water quantity, water quality, topography, and aquatic life in the streams. 

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Primary Findings 

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Results from the study indicate that distributed stormwater management is advantageous compared to centralized stormwater management.

Hydrologic benefits were greater with distributed stormwater infrastructure. 

• Runoff amounts and peak flows were reduced with a high density of stormwater practices distributed throughout a neighborhood compared to having a few large stormwater facilities. Benefits were pronounced for small rain events (< 0.8 inches of rain). Less runoff and lower peak flows would reduce the impacts of flow disturbance on aquatic life in the stream and help mitigate flooding.

• Baseflow increased temporarily during construction, which may buffer instream habitats from periods of low streamflow which can put aquatic life under stress. 

Water-quality benefits were mixed.

• Nitrate concentrations declined in surface and groundwater during development but remained elevated (> 1 mg/L) because of past agricultural land use.

• Specific conductance increased during construction and after development likely due to the new roadways and other impermeable surfaces.

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Topography changed substantially during construction with substantial excavation on ridges and filling of valleys. Change also occurred in the riparian zone around the stream, such as filling in part of the stream valley and burial of headwater stream channels.

Improvement but not full recovery of aquatic life. The index of biological integrity for benthic macroinvertebrates rebounded in some cases from initial development, but sensitive macroinvertebrates did not fully recover in treatment watersheds (Figure 2). Declines in aquatic life began in the years preceding construction. 

Implications

Building stormwater friendly neighborhoods.

Some of the primary lessons learned: 

• Building neighborhoods with fewer impermeable surfaces and distributed stormwater practices may be an effective approach to protect stream health, especially from hydrologic changes. Adding pollutant source reduction plans could help lessen water quality changes.

• The stormwater practices provided improvements to multiple stream conditions, including hydrology and biological conditions. However, nutrient reductions were influenced by past land use and conductivity increased due to more impervious surface. 

• Monitoring water quality, hydrological and biological conditions can provide insights into the multiple benefits of stormwater practices. 

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For more information

The results of the study “Lessons learned from 20 y of monitoring suburban development with distributed stormwater management in Clarksburg, Maryland, USA” was recently published in the journal Freshwater Science and is available here.

An infographic of the study results was also prepared, click on this image for a full-size version.