Soil Amendments  - Watershed Benefits
 

Water Quality

Soil amendments increase the spacing between soil particles so that the soil can absorb and hold more moisture.  This in turn reduces runoff and the damaging effects of excessive runoff on local streams. The amendment of soils changes various other physical, chemical and biological characteristics so that the soils become more effective in maintaining water quality.¹

Southern Maryland Wood Treating Site:
On-Site Thermal Desorption of Contaminated Soils,
Final Grading and LeafGro Placement
(Source: US EPA)

Water Quality Benefits

Soil Amendments

Soil amendments help to  provide water quality benefits, not only by increasing the infiltration capacity of the soil, but also by:

·        Filtering and breaking down potential pollutants.

·        Immobilizing and degrading pollutants by holding potential pollutants in place so that soil microbes can decompose them.

·        Reducing the need for fertilizers, pesticides and irrigation by supplying more nutrients and a slow-release of them to plants.

·        Holding more rainwater on-site, decreasing runoff, and providing increased soil moisture and infiltration capacity.

·        Increasing soil stability, leading to less potential erosion.

·        Providing added protection to groundwater resources, especially from heavy metal contamination.

·        Reducing thermal pollution by maintaining runoff in the soil and on-site longer.  

A study on the effectiveness of compost amended soils for stormwater management benefits was undertaken in Richmond, Washington in 1995.²  The objective of the study was to examine the use of compost as an amendment to increase the water holding capacity, reduce peak flows, and to decrease phosphorus in both surface runoff and subsurface flows in the local Alderwood soil series.  Seven 8-foot by 32-foot beds plywood beds were constructed with mixtures of soil and compost.  Surface and subsurface flows were obtained over a three-month period from March to June, 1995 using a series of seven simulated rainfall events. 

The study achieved a number of promising results.  It was determined that amendments to previously compacted urban soils not only increased infiltration rates substantially, but also provided significant treatment for both surface runoff and local groundwater.  Relative to unamended soils, water conveyed through compost amended soils had

• 70% less total Phosphorous

• 58% less soluble reactive Phosphorous

• 7% less total nitrate.

In a study undertaken in Seattle on the use of soil amendments for stormwater management , compost was found to have a significant sorption and ion exchange capacity that was responsible for pollutant reductions in the infiltrating water.³

While the results of both tests are very promising, they did find that compost amendments may initially cause increased nutrient discharges into surface and subsurface waters until the material have become stabilized.  However, related tests in the Seattle area found much less pronounced degradation with aging of the compost amendments.  Both studies exhibit the positive impacts that soil amendments can make on the local water quality resources of an area.

Water Quantity

One of the primary objectives of LID site design is to minimize, detain, and retain post development runoff throughout a site so as to replicate the site's predevelopment hydrologic functions.⁴  By restoring or improving the physical and therefore hydrological characteristics of a soil, that soil can then best be utilized for stormwater management purposes.  Compared to compacted, unamended soils, amended soils provide greater infiltration and subsurface storage and thereby help to reduce a site's overall runoff volume, helping to maintain the predevelopment peak discharge rate and timing.  The volume of runoff that needs to be controlled in order to replicate natural watershed conditions changes with each site based on the development's impact on the site's curve number (CN), a measure of infiltration based upon soil type and landuse.  Soil amendments, with their added storage volumes, are just one LID tool that can be utilized to help reach this goal.

Water Quantity Benefits

Soil Amendments

Soil amendments provide stormwater management, quantity control, benefits by:

·        Holding more rainwater on-site, attenuating peak flows and decreasing runoff.

·        Helping to maintain base flow to local waterways, especially during dry periods.

·        Providing increased groundwater recharge through better infiltration and by maintaining the water on-site longer.

·        Improving soil structure and stability, while increasing infiltration capacity and available storage within the soil.

·        Reducing paving and compaction of highly permeable or problem soils through a site fingerprinting approach.

·        Increasing soil stability, leading to less runoff and erosion through improved cover conditions.

The Richmond, Washington 1995 stormwater management study also yielded a number of promising results in regards to the effectiveness of compost amended soils for stormwater quantity control.  It was determined that soil amendments made on previously compacted urban soils significantly increased infiltration rates.  The following points summarize some of the findings and conclusions from the study in regards to the control of stormwater.

• Within the test plots the water holding capacity of the soil was doubled with a 2:1 compost to soil amendment.

• Total storage for compost amended soils increased by about 65% from those of unamended soil values.

• Rainwater runoff rates were moderated with the compost amended soils. The amended soils showed a greater lag time to peak flow at the initiation of a rainfall event and attributed to an overall greater baseflow.

In short, the results of the study exhibited that compost amended soils consistently had longer lag times to response, longer times to peak flow, higher base flow, higher total storage, and smaller total runoff than unamended soils. ²

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