How Does Your Water Flow? Comparing soil infiltration rates between natural areas and turf grass in urban parkland

By Brittany Janney (SUST Major class of ’18) and Michelle Giles (SUST Major class of ’18)

This past summer we participated in the Urban Ecology Field Lab program at the Field Museum of Natural History. The Field Lab is an interdisciplinary research program where students get to design and implement a research project under the expert guidance of ecological and social scientists at the Field Museum. This year, 8 students participated in the program and split into three teams working on different topics related to urban ecology in Chicago: one focused on community interactions with green space, a second examined plant/pollinator relationships on restored sites, and our team looked at the relationship between soil and water infiltration rates across different land use types.

This is the second year a Roosevelt Sustainability major has participated in the program. Last Year Diana Ramirez (BA ’17) blogged about her experience working on the social science element. Brittany, Michelle, and visiting scholar Disha Sinha from New Delhi, India homed in on soil and water infiltration, which has big implications for storm water management.

Our research was conducted in a diverse natural area in Chicago known as the Burnham Wildlife Corridor, located on the south lakefront. This area covers 600 acres of land and features 100 acres of natural areas alongside turf grass and beaches interwoven through the narrow lakefront landscape (CPD, n.d.). The natural area restoration contains three terrains native to Illinois: prairie, savanna and woodlands. The combination of restored natural areas directly juxtaposed with turf grass sites made it the ideal location to compare infiltration rates between the various landscapes.

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The Chicago Park District Natural Areas program focuses on planting native vegetation in some areas instead of traditional turf grass in order to provide habitat for pollinators and improve stormwater management. In this image, you can see the difference between native plants on the left and turf on the right.

The goal of our research was to experiment on restoration sites of various ages and type to determine the factors that significantly influence water infiltration rates. The aim was to provide actionable data for the main land manager within the Chicago region, the Chicago Park District (CPD). CPD has already undergone a transition to a more natural area focused management and presently 16% of Chicago Park District is managed as a natural area (Freer, 2018). The soils and grounds that support the plant life of Chicago will see changes as the number of natural areas grow. Our study aims to shed some light on how these changes are making a difference for stormwater management.

Approaches to Managing Stormwater

Traditional stormwater management is designed to remove and direct water from wherever it lands towards treatment facilities. This has typically been accomplished through massive hardscape designs including tunnels and reservoirs. This management is not consistent with the natural hydrological processes of soil infiltration, which would naturally slow the absorption and retain that same stormwater in place (Redfern, et al. 2016). This disruption can exacerbate urban heat island effect by drying out landscapes and can result in flooding despite stormwater management hardscapes (Couttes, et al. 2012).

The Environmental Protection Agency has shifted to a model of “Green Infrastructure” encouraging installations of green roofs, land conservation, bioswales and other elements that mimic nature by soaking up the water instead of simply exporting it (EPA, 2018). In Chicago, stormwater management is an issue of utmost importance. Chicago could see precipitation rates that the Intergovernmental Panel on Climate Change predicts could change from -1% up to +19% by the end of the century (CMAP p. 16, 2013). The Chicago Green Infrastructure Vision encourages a patchwork of natural areas to support storm management in a more holistic fashion (Mengler & Williamson, 2012).

Since vegetation can be so impactful, the Metropolitan Water Reclamation District (MWRD) has also supported green infrastructure efforts by getting rid of legislative barriers (CMAP, 2011) and approving grants for green infrastructure throughout the region (MWRD, 2018). The city of Chicago has combined sewer systems meaning that wastewater from homes and storm water funnel through the same pipes. This can result in combined sewer overflows (CSO events) that force the MWRD to release untreated water into rivers (MWRD, 2015). While the MWRD is increasing its hardscape capacity with the Deep Tunnel and Reservoir Project to contain 20.55 billion gallons of water by 2029 (MWRD, 2015), they are highly in support of preventative green projects to keep that storm water in the soil instead of the sewer.

The Chicago Nature & Wildlife Plan Update defines natural areas as “managed open space dominated by native vegetation that supports wildlife and provides ecosystem services.” (Chicago Advisory Committee, 2011). Along with the co-benefits of providing wildlife habitat and aesthetically appealing landscapes, natural areas can help manage stormwater as well and are a key tool discussed throughout the Chicago Metropolitan Agency for Planning Green Infrastructure Vision for the city (CMAP, 2013). Presently, the Chicago region boasts over 3,000 acres of natural areas. Yet the Natural Areas Directory map (City of Chicago, 2011) shows over 900 additional acres are available for the creation of more natural areas, which will help Chicago become more climate ready and resilient per the goals of the Chicago Nature & Wildlife Plan Update (Chicago Advisory Committee, 2011). 

Methodology and Research Objectives

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Our study sites in the Burnham Wildlife Corridor

Given this interest in green infrastructure in Chicago, our research objective was to compare water infiltration rates between restored natural areas of different ages and traditional turf grass areas within 1-200 meters from one another. In other words, are natural areas better at managing stormwater than traditional turf grass parks? 

We hypothesized that the restored natural areas, containing mostly plants native to prairies and savannas, will have higher rates of water infiltration than nearby turf grass sites based on the deep perennial root structures characteristic of restored sites.

In lay terms, we poured water into the ground and measured how long it took to completely penetrate the soil. We hypothesized that water would take longer to penetrate the soil in areas that were turf grass instead of native vegetation, thus showing that restoring areas to native plants would enable the landscape to better manage heavy rain events.

In more scientific terms, our research utilized the United States Department of Agriculture’s recommendations for single-ring infiltrometer with a few modifications to better meet the site needs. A six inch (16 centimeter) PVC ring vessel was fixed approximately two cm into the ground to contain the water. A small garden shovel and rubber mallet were used to get the ring to stay in the ground without leaks. Our team repeated the test (pouring a fixed amount of water into the ring) six to eleven times to find constant rate of infiltration (Jarrett, n.d.).

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Conducting “pours” on turf site (Standing: Disha, Left: Brittany, Right: Michelle) 

Additionally, we took soil cores at three locations per site to access compaction and perform a crude qualitative analysis of sediment type, moisture content and color. The image on the left shows the healthier soil found in restored sites versus the compacted soil found in turf grass areas. Water moves more easily through the soil on the left.

Pill bugs were numerable enough to make mention of at two of our five testing locations in 41st St Bioretention and 47th St Restoration. These sites both had soil cores that showed dark, earthy and organic matter. Since infiltration rates can benefit from a healthy biotic community, we believe there is a connection here between decomposer presence and the time it takes water to infiltrate. More recording and literature review would be necessary to include this information in future studies.

Research Results

Due to the statistical significance in our p-value of 0.03, our hypothesis was supported. We have found natural areas to have higher infiltration rates and are therefore highly valuable in storm management. Their increased absorption may also have added benefits of reducing heat island effects and vegetation has the ability to support wildlife. Natural areas are in line with the Environmental Protection Agency’s Green Infrastructure recommendations and are a valuable solution for stormwater management.

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It was an honor and a privilege to conduct this research. Not only did we gain valuable experience, but our results were geared towards affecting land management policy. It is extremely rewarding to have been a part of the Urban Ecology Field Lab and has made us both very passionate about the future of green infrastructure in Chicago and the power of plants.

 

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1 Response to How Does Your Water Flow? Comparing soil infiltration rates between natural areas and turf grass in urban parkland

  1. SUSTatRU says:

    Reblogged this on Roosevelt University Green Campus and commented:

    SUST seniors Brittany Janney and Michelle Giles (class of ’18) took part in the Field Museum’s Urban Ecology Field Lab this summer. This post describes their research project’s methods and results. Great example of the research opportunities afforded to RU students through great partners like the Field Museum — not to mention the endless possibilities of studying urban ecology here in the Chicago region!

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