Using Soil Compost Amendments to Improve Stormwater Management
Among the proposed BMPs in the Virginia BMP Clearinghouse is Number 4: Soil Compost Amendment. The intent of the measure is to deeply till compacted soils and restore their porosity by amending them with compost. Intuitively, this is a great idea. In my own experience on a small scale, a tilled topsoil layer, rich in organics, can accept a large amount of water. With my rain barrels at home, I can empty 40 gallons of water in 15 minutes into a landscape area less than 9 square feet. Also, I think most of us have seen that construction, particularly in tight sites, essentially destroys the existing soil structure that made it permeable. I think that this measure will see a lot of use when the new regulations go into effect, as it can be credited with as much as a 50% reduction in runoff volume and phosphorus.
I have had success using this standard on a project in Richmond, Virginia – the Northside Family Learning Center. In this case, the existing soils were heavy compacted clays under a parking lot. Instead of bringing in topsoil, which is a valuable resource, the contractor tilled compost into the clay layer. From the photo, you can see that the results were very good, even without an irrigation system.
Despite this success, I have found that the measure has some drawbacks in its current form. In order to achieve the maximum numerical benefit allowed under the standard, it is necessary to till the existing soils to a depth of as much as 24”. The standard recommends a subsoiler to achieve this (see photo). I have discussed this with several members of the construction community and there is virtually no experience using subsoilers. From my cursory research, I have found that subsoilers are primarily used in agriculture, with limited use in Virginia. Contractors that I have discussed this with have advised that, for small areas particularly,it would be priced similar to excavating to a depth of 24”, mixing the excavated soils with compost, and replacing the mixture. Obviously this is very expensive.
This gets to the larger issue of how the construction community will adapt to the new BMPs in the proposed regulations. It is inevitable that there will be a long period of time where prices of the new types of BMPs will have an associate premium attached to them until contractors have determined the best strategies to construct these items. I am very interested to hear more from the construction community on suggestions for amending soil with compost.
For more about soil compost amendments and other tools available to help manage stormwater, visit the Virginia BMP Clearinghouse at http://vwrrc.vt.edu/swc/NonProprietaryBMPs.html