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Soil Remediation with Compost

April 2011 (volume 6 - issue 4)

Contributed by Nicholas Olson, John S. Gulliver (Department of Civil Engineering, University of Minnesota) and John L. Nieber (Department of Biosystems and Bioproducts Engineering, University of Minnesota)

Funded by the Minnesota Pollution Control Agency, Three Rivers Park District, and The Center for Urban and Regional Affairs

There are many aspects of urban runoff that create problems for receiving water bodies. First, the typical urban area is covered with impervious surfaces, such as roads, parking lots and roofs. Where most precipitation used to (without development) infiltrate into the soil and replenish groundwater tables, precipitation on impervious surfaces does not infiltrate, and instead causes more frequent flooding in our streams. In addition, roadways and parking lots are not that clean. Wear from tires and brake pads, lubricants leaked from cars, pathogens left by animals, dirt brought onto the road, and the nutrients and pollutants associated with that dirt are contained in urban runoff. One way to deal with this additional water is to have the soils that remain on lawns and parks infiltrate more water than in the past.

Figure 1: Urban runoff is actually dirty and full of pollutants. It is the main source of pollution to our urban streams and lakes (Photo courtesy of R. Bannerman).

Unfortunately, the converse is true. Most soils in residential developments have lower stormwater infiltration capacity than the soils they replace. This is due to reduced topsoil depth and increased subsoil compaction as land is reshaped and worked with heavy equipment during development. Loss of infiltration capacity further leads to increased stormwater runoff and associated downstream problems of pollutant transport into our streams and lakes, flooding and warming stream temperatures.

Figure 2: The effects of compaction on grass growth (a) shallow compaction (b) no compaction. The deep roots will keep the pores open and allow for infiltration. Reproduced by Batey and McKenzie (2006) from Soil Husbandry.

We investigated two methods of remediating soil compaction for grassed developed areas. Field studies were conducted to measure the practical application and performance of amending soil with tillage and compost. Three different sites (Maple Lakes Park, Lake Minnetonka Regional Park, and French Regional Park) were divided into three plots for comparison:

• Control: No remediation was performed
• Tilled: Tilled to 24” depth and spaded to a depth of 16” to smooth furrows. Grass was then reestablished on the surface by seeding.
• Compost: Tilled to 24” depth and spaded with 3” of compost incorporated to a depth of 16”. Grass was then reestablished on the surface by seeding.

Each plot was tested to assess compaction before and after soil remediation with the primary indicator of infiltration capacity: saturated hydraulic conductivity. Saturated hydraulic conductivity (Ksat) measures the permeability of the soil under saturated conditions, such as occurs during heavy rainfall. Ksat generally decreases as compaction increases.

We measured Ksat with a new infiltrometer that we developed for rapid infiltration measurements. Called the Modified Philip Dunne (MPD) infiltrometer, it is patterned after the Philip-Dunne permeameter, developed for soil permeability measurements in the Amazon region. The measurements are made when it is not raining. The infiltrometer is inserted into the soil to a depth of 2” and is filled with water to a given height. The water level and associated time is then recorded. The results are then entered into a spreadsheet program that computes the Ksat value for that location. The advantage of the MPD infiltrometer over other types of infiltrometers (e.g., the double ring infiltrometer) is that it requires a limited amount of water, so that many measurements can be made simultaneously. We mapped the infiltration of the three sites with three people simultaneously operating up to 20 infiltrometers.

Figure 3: Modified Philip-Dunne Infiltrometer (Photo courtesy of B. Erickson).

Figure 4: Greta Schmalle and Lanre Anakola setting up MPD infiltrometers to begin measurements (Photo courtesy of N. Olson).

Figure 5: High School Intern Lanre Adekola takes measurements with a Modified Philip-Dunne Infiltrometer. (Photo courtesy of N. Olson).

Comparison of each site before and after remediation revealed that the compost amended plots at all sites had the largest Ksat values and were up to 5.7 times greater than pre-remediation plots (see table). Tilled plots had Ksat values 0.5 to 2.3 times greater than pre-remediation plots, but were not statistically different from the control plot averages at Lake Minnetonka Regional Park and French Regional Park.

Figure 6: Maple Lakes Park before and after remediation. (Figure courtesy of N. Olson).

Table 1: The effectiveness of remediation techniques.

		Ratio of the Geometric Mean of Ksat
Study Site	Assessment Period	Till Plot/Control Plot	Compost Plot/Control Plot
French Regional Park	Year Two (Summer 2009)	0.6*	1.5*
	Year Three (Spring 2010)	0.5	1.2*
	Year Three (Summer 2010)	N/A	2.7
Lake Minnetonka Regional Park	Year Two (Summer 2009)	1.2*	3
Maple Lakes Park	Year Two (Summer 2009)	2.3	5.7
	Year Three (Spring 2010)	2.1	5.5

* indicates the means of the compared plots are not statistically different from one another

Remediation by tillage alone was ineffective in improving the saturated hydraulic conductivity, even though soil strength was reduced by the tillage. Spading in the compost after a soil has been tilled resulted in increased Ksat and reductions in bulk density, both of which improved the soil condition. This is likely due to the ability of the compost to increase the porosity and connected pathways in the soil. Tilled-in compost is therefore an acceptable means of increasing infiltration rates, and should be considered on all new developments.

So, tilling in compost will increase infiltration capacity to make our lawns look, to the rain, more like the soil of prairies and forests. How long will it last? It is known that compost will compress over time and lose its high porosity. However, the roots of plants on the surface will preferentially grow into the compost, and decaying roots create the soil environment conducive to producing high infiltration capacity. It is possible that, with the right plants on the surface (fescues instead of Kentucky blue grass), the high infiltration rates created by tilled compost will last virtually forever.

While this study focused on vegetated areas in developments, the same remediation ideas should be applicable to constructed areas along roadways. Construction activities can cause severe reductions in infiltration capacity of the grassed surfaces of roadside swales. Remediation efforts such as those reported in this study could help to increase the infiltration capacities of these roadside facilities.

 

We want to hear from you!!!

Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions: 

• How can turf management/soil remediation be incorporated into stormwater best management practices?
• Can soil remediation be used to disconnect more impervious surfaces?