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Pathways to Progress in Tackling Stormwater Runoff in Near-Urban Agricultural Areas

Posted by Kevin Hyde | February 18, 2021
Culvert draining into a pool of water with oily slick on the surface
Stormwater pollution impacts many of the things Puget Sound residents hold dear. Photo: Washington State Department of Ecology.

Stormwater runoff, particularly from roadways, is one of the leading sources of water pollution in Puget Sound. Stormwater pollution impacts people and ecosystems in different ways. Many of the things Puget Sound residents hold dear, like swimming along rivers and beaches, harvesting and eating shellfish, and fishing for salmon, are directly affected by stormwater pollution. The Puget Sound Partnership works with many groups to tackle this complex problem, and polluted stormwater runoff is a focus of the Toxics in Fish Implementation Strategy, a recovery plan that aims to reduce the levels and impacts of contaminants on Puget Sound aquatic life.

Jordan Jobe, with Washington State University Extension, points out in a recent article that stormwater pollution also has implications for local food systems and farmers. She highlights the need to better understand the impact of stormwater runoff on agricultural viability in near-urban agricultural areas, where it may impact soil health, or contaminate crops.

Climate change is expected to further exacerbate the effects of stormwater pollution. The recent “Shifting Snowlines and Shorelines” report from the University of Washington’s Climate Impacts Group predicts more of Washington’s winter precipitation will probably be rainfall instead of snow. This will increase the volume of stormwater runoff in both urban and near-urban areas. The combination of increasing development (and associated impervious surfaces) and increasing precipitation is a double whammy. There are established green stormwater infrastructure (GSI) solutions available to deal with stormwater runoff, such as rain gardens, or densely planted riparian buffers or trees planted along roads or train tracks. However, these might not adequately deal with climate change-driven increased stormwater runoff in near-urban agricultural areas.  Jobe believes if springtime rainfall events do become heavier in the future, it will likely overwhelm existing stormwater infrastructure, lead to worse local flooding problems, and put more untreated stormwater onto agricultural land. She explained that additional water saturating farmers’ fields during the springtime planting period would present significant challenges, even without considering whatever contaminants may be found in that stormwater.

Though on-farm mitigation methods like these could be an option, it would be preferable to see what could be implemented at the source of the runoff, so that farmers are not asked to give up some of their valuable land to manage contaminants coming from surrounding development. Harvesting stormwater or planting trees near the source of the runoff could help address the problem, but more research is needed to determine how such practices could best be implemented.

Farm field with houses at the far edge, and Mt Rainier in the background
Farmland in the Puyallup River watershed, where agriculture is mixed with urban development. Photo: Kevin Hyde.

Jobe, along with her WSU colleagues Ani Jayakaran and Courtney Gardner, outlined for me what the research to understand these impacts might entail. One pathway is focused on nitrogen cycling microbes in soils that are critical for nitrogen assimilation in crops. These microbes are particularly sensitive to many toxic pollutants carried by stormwater and are used as indicator organisms for the presence of these stressors. Some of these toxic compounds—such as antimicrobials like triclosan and triclocarban—have an obvious and well-documented effect on nitrogen cyclers, while others are subtler. Understanding how polluted stormwater interacts with soil microbes is key to understanding how stormwater may be impacting agricultural sustainability and viability in the floodplain and may offer clues to mitigate these impacts.

One of the biggest challenges facing Puget Sound is addressing the pressures of development and climate change. In the Puyallup River area and other floodplains, standard GSI practices based on infiltration are not appropriate because the seasonal high-water table is within a few feet of the ground surface. Therefore, stormwater tends to be managed by more traditional means like stormwater detention ponds and stormwater ditches. All these practices can lead to increased connections between polluted stormwater and the groundwater table if there is not enough vertical separation between runoff sources and the subsurface water regime. Capturing stormwater at its source, reusing it after treatment, and relying on evapotranspirative processes are possible avenues to manage stormwater without impacting agricultural landscapes. Jayakaran explained that he is quantifying the amount of water trees transpire, a possible solution for managing stormwater where standard infiltration practices are not feasible.

Highway bordered by fields
A five-lane highway runs alongside farmland in the Puyallup River watershed. Photo: Kevin Hyde.

While the effects of stormwater runoff are more pronounced in an area like the Puyallup River watershed, the issue is also relevant to rapidly developing counties in wetter regions across the country, especially within floodplains. Floodplains are flat and tend to have higher quality soils, which makes them great places to farm. They are also ideal places to locate houses, transportation networks, and other infrastructure. As development in other urban areas around the nation occur, the unintended impacts of stormwater runoff on agricultural soils will become increasingly problematic. Our region has a rich agricultural legacy, as illustrated in part by the vibrant local farmers’ markets in our region. It’s critical we work together to understand and tackle the impacts of stormwater pollution on agricultural land and the local food systems we cherish.

This article is also posted on AgClimate.net.

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