Reducing the cost of stormwater infrastructure through better soil management
(View the SlideShare: Fix a good soil gone bad)
An estimated 80-90 percent of all rain events are one inch or less, and where native soils have been spared the ravages of human development and climate extremes, a thick topsoil layer — absorbent, nutrient-rich, and biologically active — dissipates that rainfall energy and holds water.
But in urban/suburban areas, where so much of the surface is impervious and the topsoil layer all but gone, a one-inch rain event becomes a major management issue. In the District of Columbia, a one-inch rain dumps more than a billion gallons of water on its 61.4 square miles. New York City, with its much larger footprint, contends with more than 5 billion gallons from that same rain, while a big stretch like metro Houston deals with more than 173 billion gallons — more than the entire state of New Jersey.
According to the U.S. Geological Survey (USGS), enough precipitation hits the lower 48 states each year to cover the entire land mass in 30 inches of water. A one-inch rainfall event can drop more than 27,000 gallons of water per acre or about 61.5 billion gallons of water across the continental U.S. About 3 percent of that moisture will splash down in an urban area where over 90 percent of the surface is impervious.
Pervious paving, green roofs and rain gardens are beginning to break up the great swath of solid surfaces in city centers. But as one moves farther away from downtown toward suburban lawns and sports fields and, eventually, farmland, it is the soil that plays the greater role in reducing the cost of stormwater infrastructure, defending rivers and streams against heavy stormwater flows and the subsequent runoff of sediment and pollutants.
A healthy soil with sufficient levels of soil organic matter (SOM) and robust microbial activity holds water and degrades pollutants. Nature replenishes organic matter by recycling dead plants, animals, and their wastes through the process of decay. But human land disturbance has scraped, eroded and depleted topsoil without restoration, breaking the natural soil cycle. In many parts of the world, including the U.S., nature can no longer keep up with replenishment.
A fertile, prairie grassland may contain organic matter in the ideal range of 5-6 percent. But elsewhere, organic matter can be at 1 percent or less, and most everywhere, topsoil is at a fraction of its historic depth. In fact, in developed areas, that which is commonly referred to as “topsoil” may actually be subsoil, near barren and incapable of supporting basic soil functions.
Fortunately, the natural soil cycle can be restored. When captured, composted and returned to the soil to rebuild organic matter content, the so-called “waste” generated by modern societies in the form of MSW organics can transform public parks, sports fields, suburban lawns and outlying farms into mechanisms for water retention, percolation and pollutant filtration/degradation.
Compost, with its high organic matter content, has been shown to have significant water-holding capacity, so much so, that there may be no runoff from low-to-moderate rain events where compost is used as a stormwater best management practice (BMP). Reducing the cost of stormwater infrastructure is as simple — and economical — as improving the soil.
Every 1 percent of soil organic matter equates to about 16,000 gallons of water-holding capacity per acre foot, and simple arithmetic indicates just 2 percent organic matter is more than enough to make stormwater a nonissue after a typical rain event.
This level of performance predicts wider adoption of compost-based strategies for stormwater management in the years ahead. The convergence of these three key elements required for compost infrastructure deployment indicates this new era may be just around the corner:
- The growing number of jurisdictions mandating soil amendment as a water management strategy, creating new, high-value markets for compost products.
- Expansion of organics recycling directives ranging from landfill bans to zero waste initiatives, making high quality compost feedstocks more readily available to compost manufacturers.
- A shift from public to private-sector composting for biosolids, food waste and other MSW organics, growing the industry with aggressive marketing, professional management, and the private financing to pay for build-out of the national composting infrastructure. The commercialization of composting promises the high product quality and emissions-secure facilities demanded by a general public that must live, play and work in close proximity to sites where these products are manufactured and used.
The age-old story of good soil gone bad
Where native soils offer a generous layer of organic matter and a healthy stand of vegetation, about 50 percent of rainwater returns to the atmosphere through evapo-transpiration, 35 percent percolates, and 15 percent drains to surface waters.
But after development, the loss of vegetation and topsoil combine with the addition of impervious surfaces to cause evapo-transpiration to drop to 15-30 percent and detention/infiltration to 15 percent, while runoff jumps to 55-70 percent.
One doesn’t need to be a soil scientist to know a patch of earth has gone belly-up. Weeds, compaction, exhausted nutrients, and lack of water retention are all signs of a dead or dying soil in desperate need of regeneration.
In the last 40 or so years, about one-third of the world’s arable acres have been lost to erosion or pollution.
No sibyl is required to predict where this trend is headed. Between soil depletion, topsoil loss, pollution, burgeoning populations, and vanishing arable acreage, humans are staring at a terrestrial event horizon of cataclysmic proportions.
But there is good news. Unlike natural or cosmic disasters, people “broke” the soil system, and people have the means and methods to fix it.
The Soil Fix
Soil repair doesn’t require rocket science, just a bit of logic. That which was removed from the soil needs to be returned to the soil. Topsoil lost to development and runoff can be replaced through a regular infusion of fresh nutrients, microbes, and organic matter.
Synthetic fertilizers add nutrients, but no microbes or organic matter. They seldom offer a wide range of macro and micro nutrients in one product. Manufacture may be based on limited-resource fossil fuels, while off-shore manufacturing requires even more fuel to get the product to market. When applied at rates exceeding plant uptake capacity (and they frequently are), use of these products also contributes to polluted runoff.
Peat moss is almost all organic matter and contains some nutrients, too. But it is generally considered too expensive to be used on a large-scale. Peat moss is acidic, requiring the addition of lime for many crops/plants. It’s also classified as a limited-resource, since it takes a long, long time for peat to form.
Neither synthetics nor peat replenish soil microbial populations essential to nutrient uptake, disease resistance, and pollutant degradation. Neither is considered “sustainable” by any definition.
Raw manures add nutrients and organic matter, but their organisms can do more harm than good. Raw manure can be loaded with parasites, pathogens, and drugs that take weeks or months for Mother Nature to neutralize in-situ. Manures also tend to be unpleasantly odiferous, meaning they can’t be used in close proximity to homes or businesses without controversy.
A residual waste produced during anaerobic digestion (digestate) and minimally treated biosolids, while improvements over raw manures, are plagued with some of the same issues. They are best used in rural settings and carry little to no market value unless composted.
Biochar is the charcoal residual from pyrolysis, an incineration-like waste-to-energy (WTE) technology that thermally-processes biomass without air or oxygen. While adding charcoal to soil is not exactly news, the use of this waste treatment residual is a relatively new soil amendment with some research gaps related to long-term use. Like compost, end value is influenced by feedstocks. It is also considered a long-term carbon sink (1,000+ years). Biochar, which is ~60 percent organic matter, is known to increase soil fertility, but it is not a complete soil amendment. The residual may contain some nutrients, depending on feedstocks and the specific production conditions that produced the charcoal. It can be used as an inoculant carrier, but is not biologically active on its own. It is often used in conjunction with a synthetic fertilizer or compost.
That means there’s really only one product that’s –
- high in organic matter,
- macro and micro nutrient-rich,
- moisture retentive,
- biologically active with beneficial microorganisms,
- locally sourced,
- easy to apply, and
- safe and pleasant to use anytime, anywhere, by anyone,
… and that’s a quality compost.
As a product, compost delivers big-time. Use it to raise soil organic matter and get significant improvement in stormwater retention/percolation and pollutant degradation. It even absorbs rainfall energy, so rain dislodges fewer soil particles to wash away as sediment, reducing erosion.
Compost’s organic matter improves soil texture and relieves compaction, allowing water, nutrients, air, and microbes to move freely throughout the compost-amended zone. It offers a high cation exchange capacity (CEC). Compost use results in a loose, friable (crumbly) soil that encourages good root development.
Beyond benefits to the application site and local receiving waters, amending soil with compost also offers far-reaching environmental benefits:
- Organics recycling. While compost could be made from virgin materials, it’s not. Compost is manufactured from waste material, the biodegradable fraction of municipal, industrial, and agricultural discard streams, making its feedstocks “sustainable.” Since organics typically make up 40-60 percent of the municipal solid waste stream, diverting organics can have a significant positive impact on landfill fill rates and eliminate the need for inefficient landfill gas recovery systems. Tipping fees at composting facilities tend to be lower, too, reducing disposal costs for waste generators. The removal of high-moisture organics from incineration/WTE streams (ever try to burn water?) can cut operating expenses for those waste management technologies, as well.
- Reductions in greenhouse gas (GHG) generation. Linked to organics recycling is the reduction in greenhouse gas generation when composting organics vs. landfilling. Composting releases CO2, which is a greenhouse gas. But as with human breathing, this source of carbon dioxide is considered to be carbon neutral, since its source is biogenic and its GHG impact is the same as natural degradation.
However, methane – generated during the anaerobic digestion of landfilled organics — is considered 20-25 times more damaging than CO2. Landfills are the third largest source of methane in the U.S., most of it released before a cell can capped and gas recovery system installed.
That a poorly-managed compost pile can go anaerobic, resulting in methane generation, is certainly true. But the active aeration, embedded sensors, and computerized controls of more modern composting facilities ensure aerobic conditions within the composting mass from beginning to end of process.
Composting organics in high-rate facilities instead of landfilling could eliminate about 18 percent of total methane emissions.
- Carbon sink. As atmospheric levels of carbon dioxide rise, so do global temperatures. Beyond the soil-related advantages of compost use is a compelling environmental benefit – carbon sequestration. A quality compost product will have an organic carbon content of at least 50 percent (by dry weight). Therefore, compost-amended soils that will remain undisturbed for long periods of time make the best carbon sinks. While general agriculture can use practices to improve carbon sequestration in fields used for annual crops, farmers make the most positive impact with compost-amended soils used for perennial crops, grasslands, and trees because these soils are not disturbed on a regular basis.
Compost incorporation during final grading and seeding for highway construction, utility right-of-way acreage, and public parks are examples of non-farm project sites where soils are not disturbed for years and years.
A one percent increase in topsoil organic matter stores about 60 tons of carbon per acre. In the U.S., the soil in just a 10-ft. right-of-way on both sides of its 4 million miles of roads could provide sequestration for 2.91 billion tons of carbon if amended to the recommended 5 percent organic matter content.
5,280 ft./mile x 4 million miles = 21.12 billion feet
10 ft. x 2 sides of the highway = 20 ft. x 21.12 billion feet = 422.4 billion square feet
422.4 billion sq. ft. divided by 43,560 sq. ft. per acre = ~9.7 million acres
60 tons of carbon stored per acre @ 1% SOM x 5 = 300 tons per acre at 5% SOM
300 tons x 9.7 million acres = 2.91 billion tons stored carbon
The U.S. EPA says a typical passenger vehicle emits 4.6 metric tons (5.07 U.S. tons) of carbon dioxide each year.
Hedges and Company projects a 2019 total of 281.3 million registered vehicles in the U.S., with an estimated 35 percent of those being passenger cars. By calculation, those approximately 98.46 million cars will emit about 500 million tons of CO2, which means a modest right-of-way along that 4-million-mile ribbon of highway could store 5-6 years’ worth of emissions from those vehicles while managing stormwater runoff and degrading pollutants … if soil organic matter content is upped by 5 percent.
Reducing the cost of stormwater infrastructure — using compost blankets, socks, and berms
There are several erosion control measures that rely on compost blended with wood chips to reduce runoff. These systems provide superior performance, melding with the soil below to eliminate infiltration gaps.
FILTER SOCKS – When filled with erosion control compost, these mesh tubes (also referred to as compost “logs”) become a versatile tool in the fight against sediment loss and surface water pollution. In a January 2011 technical paper, the National Resource Conservation Service lists the following uses for filter socks:
- perimeter sediment control
- as a check dam to reduce soil erosion in swales, ditches, channels, and gullies
- storm drain and curb storm inlet protection
- reduction of fecal coliform, E. coli., nitrogen, phosphorus, heavy metals, and petroleum hydro-carbons from stormwater
- reduction of suspended solids and turbidity in effluents
- slope interruption practice used to reduce sheet flow velocities and prevent rill and gully erosion
- energy dissipation of sheet and concentrated stormwater flow, thereby reducing soil erosion and habitat destruction
- use on paved, compacted, frozen, or tree-rooted areas where trenching is not possible or is undesirable
- treatment of polluted effluents, pump water, wash water, sediment dredge, lagoon water, pond water, manures, and slurries
- in-situ biofiltration and bioremediation of stormwater pollutants
- capture irrigation-induced sediment from flood and sprinkler irrigation systems
- use RUSLE 2 for design applications
- use in low impact development (LID), green infrastructure, and green building programs
- protection of sensitive wildlife habitat, wetlands, water bodies, and ecosystems
Whew! That’s quite a list. The itemization of compost filter sock advantages is lengthy, too, and includes obvious benefits like pollutant and sediment reductions. But the ability to use socks without disturbing root systems or on paved surfaces (like parking lots and streets) make filter socks a unique addition to the stormwater control arsenal.
They can be formed around culverts and other inlets to divert or filter flow, laid perpendicular to flow in drainage channels, or stacked and embedded to form protective buffers along streambanks.
For compost filter socks to be most effective, soils must be prepared correctly, socks must be filled with compost designed for erosion control, and socks must be installed and maintained according to specifications.
COMPOST BERMS – A compost berm is a control device used instead of silt fence to manage and filter stormwater. It is more effective than silt fence and offers many of the same filtration, degradation, and flow management advantages as filter socks. But berms are a more permanent installation, especially when vegetated. There’s a good photo of a compost berm at work in this EPA bulletin that also includes more technical information.
COMPOST BLANKETS – As the name implies, a ½-to-4-inch compost blanket is placed on top of the soil to absorb rainfall energy and hold water. Like berms and socks, blankets can be seeded and remain as a permanent installation after construction. They are often used on slopes with and without compost berms that buffer the blanket from a concentrated runoff flow. See this EPA bulletin for more information.
Reducing the cost of stormwater infrastructure — general soil amendment
Where blankets, socks and berms are unnecessary or inappropriate, a general program of soil conditioning to raise (and maintain) organic matter content to 5 percent will allow soil to retain the volume of a typical rainfall of 1 inch or less.
Compost should constitute 30 to 50 percent of the disturbed soil profile. If applying a couple of inches of compost, for example, till it into the top 4-6 inches of soil. For a high-grade compost, choose 30 percent. A low-grade product could push the volume required to 50 percent. When in doubt, opt for the lower concentration.
Don’t overdo it. Reducing the cost of stormwater infrastructure requires planning, not guessing. When organic matter content is too high, it can waterlog soil while delivering more nutrients than plants can use, polluting runoff. For stormwater managers, this defeats the purpose of compost use, so always start with soil testing, then amend accordingly.
Also remember compost is a concentrate. It should not be used as a substitute for topsoil. If topsoil is specified for the project, blend compost with native soil scraped/excavated from the site. It’s cheaper than hauling in dirt and results in an enriched topsoil product.
Dollars and sense
According to a Milwaukee study, soil amendment is among the most cost-effective options for stormwater management per gallon of storage. Except for native plantings at 19 cents per gallon, soil amendment at 28 cents per gallon beats everything else, including rain gardens ($1.59/gallon) and deep tunnel storage ($2.42/gallon).
Another study using yard waste compost at a construction site found the compost blanket performed much better (for about the same cost) as straw mat with silt fence.
Amending soil with compost costs about the same as tilling/scarifying sub-soils.
Overall, Low Impact Development (LID) strategies (which include compost use) have been shown to save 15 to 80 percent on total capital costs compared to conventional stormwater management systems. When developing a stormwater plan, begin with efficient, cost-effective, compost-based solutions, then fill in the gaps with other management strategies.
Stormwater management begins here
Compost quality will impact product performance and safety, including germination rate, pH, salts, odor potential, pathogen levels, etc.
Participants in the U.S. Composting Council’s Seal of Testing Assurance (STA) program regularly test product and make those results available on request. The report not only provides information about the product, but also gives recommended ranges for each quality indicator.
While most laboratories test compost as if it were soil, STA-certified laboratories have the equipment and expertise to test for qualities specific to compost. When writing specifications for stormwater projects, require product testing at an STA-certified lab.
Endnotes — Reducing the cost of stormwater infrastructure
 Generalized range for the U.S. based on data/metadata about storm events of 1 inch or less as reported by the EPA and other government agencies for various locales. “Typical” events for some regions will fall outside these norms.
 By calculation.
 http://www.ijsrp.org /research-paper-0516/ijsrp-p5381.pdf