Effectiveness of Compost and Mulch Practices for Erosion Control in a Reclaimed Rock Quarry
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Background
Stormwater runoff from areas of bare soil can carry large amounts of suspended solids, and associated pollutants, into streams and lakes. Quarries include a variety of disturbed lands that often lack vegetation, sometimes completely. Similarly, areas disturbed by construction can generate large amounts of sediment during storms, leading to highly visible impacts on nearby surface waters.
To minimize the discharge of sediment and pollutants into stormwater from construction activities, the U.S. Environmental Protection Agency (EPA) requires disturbed areas to be stabilized under the Construction General Permit (CGP)
. Stabilization can be achieved through planting vegetation, however this is often expensive and time consuming. Alternatively, applying mulch or compost to disturbed lands is less expensive and has been shown to substantially reduce erosion and to enhance vegetation establishment.
The Texas Department of Transportation (TxDOT) has been using two specifications of compost on construction projects since 1998. Both specifications contain equal volumes of wood chips and compost, but differ in the organic content of the compost – one requiring between 25-65% organic matter and the other allowing organic matter down to 10%.
Back to topProject Goals
The aim of this project, led by Texas Institute for Applied Environmental Research (TIAER), was to compare the effectiveness of the two types of manure compost products specified by TxDOT to a commonly used wood-based hydromulch in establishing vegetation, and reducing runoff volume and pollutants. TIAER also compared the amounts of nutrients that each of the treatments lost to runoff. A quarry-reclamation project site in Parker County was selected for field trials.
TIAER established quarry plots treated with low organic matter (OM) compost, high OM compost, hydromulch, and with no treatment as a control. They conducted stormwater monitoring to compare runoff volumes, and the amounts of total suspended solids (TSS), nitrate, Total Kjeldahl Nitrogen (TKN), dissolved phosphorus, and total phosphorus in runoff between the plots. TIAER also measured nutrient concentrations in the soil, and conducted visual assessments of vegetation growth.
Results
Runoff volume was greatest in untreated plots and lowest in the compost plots. Low OM and high OM plots did not differ in runoff volume. Compost plots established vegetation the fastest (~4 months to achieve 100% coverage) and untreated plots established vegetation the slowest (~15 months to achieve 100% coverage). The hydromulch plots took about nine months to reach 100% vegetative coverage.
The greatest differences in runoff nutrient concentrations between the treatments occurred early in the study period. Concentrations of TSS and TKN decreased over time in all plots as vegetative cover increased.
During the first few months of the study, TSS concentrations in runoff were highest in the untreated plots and lowest in both the compost plots. By the end of the study, TSS concentrations in all treatments were similarly low.
Early in the study period runoff TKN concentrations were highest in the untreated plots and lowest in the hyrdomulch pots. High OM compost plots had higher TKN concentrations than low OM compost plots, likely due to the higher TKN concentrations in the compost itself. However, by the end of the study the low OM compost plots had slightly higher TKN than the high OM plots.
Unlike the other runoff nutrient concentrations, nitrate increased in all treatments over the study period, with the compost treatments showing the highest runoff concentration by the end. Again, this is likely due to the compost itself having a higher nitrate concentration than the mulch or control soil.
Runoff concentrations of dissolved phosphorus were highest in compost plots, with the high OM compost being higher than the low OM compost. Dissolved phosphorus concentrations in runoff from compost plots decreased in the middle of the study, but then began to increase again toward the end. Concentrations in runoff from the hydromulch decreased in the first couple months and then remained stable throughout the study period. Runoff concentration of dissolved phosphorus remained low throughout the entire study period in the untreated plots.
TIAER used runoff volume and nutrient concentration data to estimate the mass (i.e. load) of nutrients and sediment exported to stormwater per treatment plot. The high OM compost plots had the lowest sediment loss, exporting 15 times less sediment than the hydromulch plots and 60 times less sediment than untreated plots. Although the compost mixes had inherently higher nutrient concentrations, they exported less total nitrogen and phosphorus than the hydromulch and untreated plots. However, loadings of dissolved phosphorus from compost treatments were higher than those of the hydromulch or untreated plots.
Conclusions
TIAER found that compost was most effective at reducing sediment loads exported from a quarry site by establishing vegetative cover (which retains more rainfall and protects soil) more quickly than the hydromulch and no treatment. The compost treatments also exported substantially less total phosphorus and nitrogen than the hydromulch and no treatment plots.
Although compost is effective in quickly establishing vegetation and reduces total phosphorus loads, it does export relatively large loads of dissolved phosphorus, particularly the high OM compost. Because dissolved phosphorus is more readily available for algal growth, users should evaluate the receiving water body's existing nutrient loads and its sensitivity before using compost. However, the impacts will also vary depending on the area and slope of the land involved and the distance from water bodies. Runoff volume, and therefore nutrient and sediment loads, decreases as slope decreases. Because of these results, TIAER recommends using a compost with lower phosphorus content.
For More Information
See the related Composted Manure Incentive Project
To find out more about the NPS Program, call 512-239-6682 or e-mail us at nps@tceq.texas.gov.