By Shelene Codner

A two-year, $188,000 study conducted by Iowa State University's (ISU) Agricultural and Biosystems Engineering Department concluded that blanket compost applications in large-scale construction projects have the potential to reduce runoff, minimize erosion, and inhibit weed growth along the state's 100,000 mi. of roadway.

Funded by the Iowa Department of Transportation (IDOT) and the Iowa Department of Natural Resources (IDNR), with an additional $1,000 donation from Blairsburg, IA-based composting firm Chamness Technology Inc., the study took place on a representative 3:1 slope provided at an existing overpass near Story City, IA, 10 mi. north of ISU. ISU's project team examined the impact of applied compost blankets on rill and interrill erosion. The team was led by compost researchers Tom Glanville, Ph.D. and Tom Richard, Ph.D.; Research Assistant Russell Persyn; IDNR liaison Jeff Geerts; and IDOT liaisons Mark Masteller and Ole Skaar.

Establishing Test Plots

Site construction was completed in late 1999 preceding the onset of winter and subsequent spring rains. Because it was too late in the season to establish vegetation, the embankment was vulnerable to erosion caused by melting snow and early-spring rain. In the spring of 2000 and 2001, rill and interrill test plots were established for treatments using a randomized block design. Square plots were established for testing interrill erosion, and long rectangular plots were built for testing rill erosion. Project team members used a cultipacker to provide a firm seedbed for the erosion control vegetation. Following cultipacking, half of the plots were fertilized and seeded with a mixture of oats, rye, timothy, and clover, replicating current IDOT specifications. Half of the plots were not seeded to simulate runoff and erosion that might occur immediately following completion of a late-season construction project. After seeding and fertilizing, all of the plots were cultipacked a second time to provide good soil-to-seed contact.

Rill plots measuring 3 x 26 ft. and interill plots measuring 4 x 5 ft. were treated with three different composts at depths of 2, 4, and 6 in. A tractor equipped with a front-end loader and row-crop­-spaced tires was used to move the compost up the slope. The three types of compost used for this study represented a cross section of the 350,000 tons produced annually in Iowa. Benefactors were interested in receiving quantitative results to assist in developing marketing strategies for their respective products. Bluestem Solid Waste Agency of Cedar Rapids, IA, provided compost manufactured from bioindustrial byproducts, including paper-mill sludge and grain-processing wastes; Davenport Compost Facility of Davenport, IA, provided compost manufactured from municipal sewage biosolids and yardwaste; and Metro Waste Authority of Des Moines, IA, provided unscreened compost produced from municipal yardwaste.

Because compost often contains elevated concentrations of metals and nutrients that potentially could pollute runoff from compost-treated areas, project sponsors had a particular interest in obtaining results concerning the water-pollution potential of runoff from certain types of compost. A topsoil treatment at a depth of 6 in. and a compacted subsoil treatment were placed on the slope as a control to replicate conventional IDOT practices. Each treatment was replicated at least six times for both rill and interrill plots. In each year, three replications were used to evaluate erosion and water-quality data on bare plots. Seeded treatments were tested after vegetation was established - approximately six weeks after seeding.

Rainfall Simulation and Runoff

Test plots were subjected to high-intensity (4-in./hr.) rainfall applied with a Norton rainfall simulator, developed by the United States Department of Agriculture. Interrill plots received rain until runoff began. After runoff was initiated, two different samples were taken at set time intervals: a composite sample to measure water quality and an erosion sample to measure the amount of compost or soil leaving the plot. Rain also was applied to the rill plots until runoff was observed. After runoff began on the rill plots, a hose was placed at the top of the plot and a measured flow of water was added to the plot. The data collection procedure for these plots was to take an erosion sample and a flow-rate sample at set time intervals. Once the grass had been analyzed, the project team collected erosion and water-quality data similar to how it was collected at the bare plots.

In addition to these data, weeds and planted species were harvested from the test plots. Samples were dried and weighed to quantify the amounts of biomass produced by each type of compost and soil. Samples were stored in coolers after collection until they were returned to the laboratory where they were preserved at 24.8°F. Data collected during sampling events were entered, and visible observations were documented. Erosion samples were analyzed for dissolved solids and total suspended solids to determine the amount of material (compost, topsoil, and compacted subsoil) collected during a rainfall event. One week before analysis, samples were removed from the freezer and placed in the refrigerator. The day before analysis, samples were warmed to room temperature and then placed on a magnetic stirrer while three 20-ml duplicate samples were removed and placed in preweighed, plastic centrifugal test tubes.

Compost-treated areas produced significantly less runoff during high-intensity rainfall than conventionally treated roadside areas did. For the first hour of runoff, composts had less than 15% of the total erosion of the two soils. Total runoff on the highly absorptive composts was less than 60% of the total runoff of the two soils. As shown in Table 1, runoff from compost-treated areas during a 30-minute high-intensity rainstorm was less than 0.8% of the runoff from areas treated with topsoil and 0.5% or less of the runoff from areas treated with compacted subsoil.

Although the amount of runoff from the yardwaste compost (shown in Table 1) appears to be less than that from the other two composts, researchers believe these differences were not statistically significant. Due to the water-absorbing capacity of the compost, initiation of runoff from compost-treated areas was delayed significantly. While compacted subsoil and topsoil typically started producing runoff 5-8 minutes after rainfall began, areas treated with any of the three types of compost took an average of 30-60 minutes to begin producing runoff (Table 2). Because most naturally occurring high-intensity storm events last less than 30 minutes, compost treatments would be expected to reduce the total number of storms each year that produce runoff. Due in part to the lower volume of runoff produced by compost-treated areas, they also produced substantially less erosion than conventionally treated slopes.

On both bare and vegetated slopes, the highest interrill erosion from composted areas during the first 30 minutes of intense rainfall was 0.02% or less of the erosion from slopes receiving conventional treatments (Table 3).

As shown in Table 3, one of the most important benefits of blanket compost treatments is their potential to provide significant erosion protection in unvegetated conditions. In both vegetated and unvegetated conditions, the relatively coarse yardwaste compost produced significantly less interrill erosion than the more fine-textured and soil-like biosolids compost did. Tests showed rill erosion highest on topsoil-treated slopes. Slopes treated with yardwaste compost and compacted subsoil areas typically showed the lowest rill erosion. Rill erosion was typically greater on the biosolids and bioindustrial composts than on the yardwaste compost but well below what it was on the topsoil treatments. These results held true regardless of whether the roadside areas were vegetated or bare. Because rill erosion will not occur until rills are initiated by interrill erosion, compost-treated areas highly resistant to interrill erosion are expected to suffer relatively little rill erosion as long as they are protected from concentrated runoff discharged from adjoining areas.

Nutrient and Metal Concentrations in Runoff

Conventional roadside areas started producing runoff five to eight minutes after rainfall began, but areas blanketed with compost took 25-60 minutes to start producing runoff. Considering a 30-minute storm at the 4-in./hr. average intensity, composts had less than 1% of the total runoff and total erosion of the two soils. Therefore, under the same 30-minute storm, pollutants moving off-site did not pose an increased environmental risk. Furthermore, for all elements except soluble phosphorus on unvegetated plots, composts had less than 10% of the pollutants moving off-site compared to the two soils. This was true despite the fact that composts (especially biosolids and yardwaste) had higher concentrations of these elements in the raw material. Therefore the reduction in mass movement of the elements off-site resulted from the significantly lower total runoff and total erosion (including the delayed response by these materials). "This will be of particular interest to regulators who may be concerned that elevated concentrations of metals and nutrients in certain types of composts - particularly biosolids compost - might lead to higher amounts of these pollutants in runoff from compost-treated areas. That wasn't the case for our study," notes Glanville.

As shown in Table 4, the biosolids compost tested in this study contained significantly higher concentrations of eight metals and two nutrients than any of the other composts or soils did. Although the metal concentrations were higher in the biosolids compost than in the other materials, they were well below limits set by EPA for biosolids and would not pose an environmental threat.

Statistical analysis shows soils associated with the conventional treatments generally contained the lowest concentrations of nutrients and metals, with the exception of arsenic. Yardwaste and bioindustrial composts contained higher levels of some nutrients and metals than the conventional soils did but contained considerably lower levels than the biosolids compost did.

Despite the significantly higher concentrations of several metals and nutrients in the composts (particularly the biosolids compost), very few of these potential water pollutants were found in runoff from compost-treated areas. Zinc, phosphorus, and potassium were the only soluble pollutants present at detectable levels in the liquid portion of the runoff samples. With the exception of phosphorus in runoff from the biosolids compost, the total soluble mass of each of the three pollutants contained in runoff caused by a 30-minute storm was significantly lower in compost runoff than in runoff from conventionally treated test plots (Table 5). This is primarily the result of the significantly lower runoff produced by the compost blankets. Runoff from vegetated test plots contained lower total masses of soluble phosphorus and potassium than the runoff from unvegetated plots did. Again the total mass of pollutants was much lower in runoff from test plots treated with compost than from conventionally treated test plots. Only five metals and three nutrients were detected in the eroded solids contained in runoff from the test plots. As was the case for the soluble pollutants, the total mass of adsorbed pollutants carried by eroded particles in runoff caused by a 30-minute high-intensity storm was significantly lower for compost-treated areas than for test plots treated conventionally (Table 6). Similar trends among treatments were exhibited by runoff samples collected from vegetated test plots.

The excellent retention of both soluble and adsorbed nutrients and metals by the relatively pollutant-rich composts is the combined result of low runoff and low erosion exhibited by these materials. In most cases, metal and nutrient concentrations were significantly higher in the composts than in the subsoil or topsoil. Concentrations of eight metals and two nutrients were significantly higher in the biosolids compost than in any of the other composts or soils. Despite higher initial concentrations in the composts, the total mass of nutrients and metals was significantly less in runoff from composted areas than in runoff from subsoil- or topsoil-treated plots. Researchers note that statistically higher nutrient and metal concentrations in runoff from conventionally treated areas do not imply that runoff from subsoil or topsoil treatments poses an environmental hazard. Comparisons do show, however, that elevated chemical concentrations in composts do not necessarily lead to elevated chemical concentrations in runoff from composted areas.

Vegetative Cover

Growing vegetative cover is one of the most common and effective ways to reduce erosion on new roadway embankments. Vegetative cover reduces the erosive effects of raindrop impacts on bare soil and establishes a dense network of roots that help to hold soil in place during storm runoff. In some cases, however, the compacted subsoil used to construct roadway embankments does not promote good plant growth. For many years, adding a sufficiently thick blanket of imported topsoil has been the most typical solution to this problem, but recently highway designers also have attempted to improve the organic matter and structure of deficient subsoils by amending them with appropriate types of compost. These can be applied as blankets or by tilling them into the surface of the compacted subsoil. Compost applications tested in this project were applied as blankets. Blanket applications were selected so measurements of erosion and runoff would directly reflect the performance of the individual composts rather than the performance of an uncontrolled mixture of compost and subsoil. Furthermore, since blanket applications take less time and equipment to apply, their performance is of interest to roadway designers and to managers of other types of construction sites who are looking for ways to minimize costs associated with erosion control.

A key question regarding blanket applications of compost, however, concerns the ability of relatively coarse and mulchlike yardwaste composts to provide a suitable seedbed for good cover-crop emergence and growth. Elevated metal concentrations, particularly in biosolids composts, also occasionally raise concerns since some metals are potentially toxic to plants (note that none of the composts evaluated in this study contained metal concentrations above those permitted by EPA rules for biosolids).

As shown in Table 7, all compost-treated areas (differences were not statistically significant) produced as much planted cover-crop growth as a conventionally prepared roadside consisting of compacted subsoil or subsoil capped with 6 in. of imported topsoil (note that the compacted subsoil at the research site was of a quality that would not normally have been amended with topsoil or compost treatments). What was equally important was that the combined dry mass of weeds harvested from test plots at the ends of two growing seasons showed compost-treated plots produced 36% or less weed growth than conventionally prepared embankments did (Table 8). Reasons for reduced weed production on composted areas are believed to be twofold: Because composting processes often produce internal temperatures in excess of 140°F, the number of viable weed seeds is generally lower in compost than in many soils. Further, blanket applications of compost presented a barrier to the emergence and growth of native weed seeds present in the subsoil beneath the compost blankets.

Project results generally indicated that in all areas there were no significant differences between 2- and 4-in. applications in terms of runoff, erosion, and vegetation growth. Logistically there appears to be no reason to apply more than 2 in. of compost.

Conclusions

Although the three tested composts originated from varying feedstocks, all performed well in terms of runoff reduction, vegetation growth, and weed suppression. Based on total erosion and chemical pollutants contained in runoff during a 30-minute storm, the unscreened yardwaste compost provided the most favorable results. At the same time, this material was the least visibly appealing because it contained refuse materials picked up during yardwaste collection activities. A screened product might be a more desirable alternative for road designers and engineers. Some composts, particularly those derived from industrial organics, might contain elevated concentrations of heavy metals and nutrients. Because these chemicals are potentially harmful to streams, as well as to both terrestrial and aquatic life, it is important to know whether runoff from composts enriched with metals or nutrients is likely to contribute to increased amounts of these pollutants in nearby streams.

Conclusions of this study provide a useful stormwater and erosion management tool for highway designers, engineers, contractors, regulators, and others responsible for complicated construction sites requiring immediate erosion control intervention with minimal ecological impact. While compost can be used in routine revegetation efforts, it has particular benefits for projects completed too late in the growing season to establish vegetation; projects where periods of abnormally wet or dry weather delays establishment of vegetation, thereby interfering with erosion control; areas with poor-quality soils that do not support vigorous vegetation growth; and steep or wet locations that are difficult to reach with the heavy equipment needed for topsoil applications but that can be blanketed with compost using a compost blower truck. Increased use of compost in such examples as these inevitably could assist in creating a more sustainable, viable compost market; further assist IDNR in its pursuit to divert the more than 900,000 tons of organic material landfilled in Iowa each year; and provide an economical erosion control alternative.

Author Shelene Codner is director for the Butler County, IA, Solid Waste Commission.

EC - May/June 2004