Understanding the unique behavior of water in urban areas begins with understanding two fundamentals. First, hydrogen bonding between atoms makes water molecules stick to one another (cohesion) and other charged surfaces (adhesion). Second, water is polar, making it a “universal solvent.”
These properties are essential to life on Earth but sometimes make water difficult to manage. Water sticks together, flowing along paths of least resistance, dissolving substances and carrying them to new places. In human-altered systems, this can pose a variety of challenges.
Madison has had its share of water management issues. In Aug. 2018, the city received record levels of rainfall, producing a 100-year flood — a flood with a one percent chance of occurring in any given year — wrote assistant professor of civil and environmental engineering Daniel Wright.
The storm and flooding “clearly demonstrated Madison’s vulnerability to extreme rainfall” — a product of urbanization, increasing heavy storm frequency in the Midwest and the maintenance of high water levels in Lake Mendota, Wright wrote in a report.
The Yahara Watershed, consisting of the Yahara River and surrounding water bodies — and covering more than a quarter of Dane County — exists alongside a blend of agricultural and urban life. Runoff packed with excessive nutrients, such as phosphorus and nitrogen, has led to “unsightly and unsafe blue-green algae [cyanobacteria] blooms in the lakes,” UW-Limnology wrote.
As rain and meltwater run into waterways, they can pick up minerals, nutrients and chemicals. Substances like fertilizers, livestock manure and everyday chemicals may not normally be harmful, but when concentrated within these waterways, they can have ill effects on the natural environment and human health.
The University of Wisconsin-Madison is researching new ways to mitigate the effects of urban runoff using green engineering, also known as bioengineering, according to Nick Balster, professor of soil science and explorer of the interactions between soil, plants and organisms.
Ecosystem services — clean air and water, food, flood control and recreation — are among the countless ecosystem services provided by the land. Bioengineering seeks to capitalize on these services by developing natural solutions to environmental and human health concerns without sacrificing economic efficiency.
“Traditional engineering practices capable of handling heavy, episodic rain events can often be costly,” Balster said. Green infrastructure acts as a buffer and provides an alternative to debating the cost effectiveness of larger storm sewers.
“My research has primarily revolved around residential rain gardens and the interactions between plants and soil,” he said.
Rain gardens are small depressions, often supporting native plants, in residential yards. These products of bioengineering are designed to collect, filter and infiltrate (absorb) stormwater, as well as offer an aesthetic improvement to a home.
Working at the residential scale has allowed him a more “localized” approach to understanding the potential of bioengineering in urban systems, according to Balster. The gardens connect citizens to their land and water use habits, hopefully preventing damage to the surrounding community. An effective rain garden can then be replicated throughout a community, increasing the sum total of their impact.
“Certain plants are going to provide different ecosystem services because of their morphology, physiology and interaction with the soil,” he said. Some plants encourage deep infiltration of groundwater, while others increase evapotranspiration back into the atmosphere, and if slow infiltration is the goal, there is a plant community for that as well.
The robust, deep roots of native prairie species pull water out of the soil structure and transpire it. In doing so, pore space in the soil becomes available for the next rain event, although different species do this to different effects, he has found.
Gardeners can achieve changes in hydrology by manipulating the plant species present in their rain garden. The soils and plants “are talking to one another” and constantly changing over time — changes Balster continues to ponder.
Engineering a path for water in an urban environment requires creativity and good predictive modeling, Balster said. Engineers must understand how heavy rain events will interact with a system and where overflows will be buffered when the system is overwhelmed. Even so, he said, “these are complex systems. Emergent properties are going to arise because there are variables we cannot predict or control.”
While the size of a stormwater sewer and pitch of a gutter system can be controlled for example, the size and frequency of rain events cannot. Rain gardens encourage adaptive management — a strategy aimed at improving management practices over time through new insights and human interaction.
“People tend to gloss over the word ‘garden’ in rain garden,” Balster said, noting that homeowners are regularly interacting with these systems, which can provide their tremendous aesthetic quality.
As opposed to a monoculture lawn, rain gardens help provide biophysical diversity to urban ecosystems, he said. The planting of native species in gardens can attract pollinators, birds and animals. Preliminary evidence has also shown rain gardens to provide greater habitat for soil organisms including macrofauna, earthworms and insects.
What humans do to the environment has feedback on their quality of life, Balster said. Feedback can occur directly through flooding spaces and contaminating water bodies or indirectly through affecting recreational opportunities or not having sufficient water to grow plants.
Rather than distancing himself from the natural world and its dilemmas, Balster chooses to think of himself as part of the system and remain connected to the land. He believes this begins by approaching the world with reverence and curiosity.
“If I respect my role within the system, I am then naturally curious as to why things happen. That drives me to ask new questions and engage in understanding ways to more responsibly interact and adapt to change, which is guaranteed to come,” Balster said.