The Center recently completed a 15-month monitoring project in Talbot County, Maryland to evaluate the cost-effectiveness of bag filters as a nutrient management strategy. The study was intended to address water quality issues in the Tred Avon watershed, whose upper reaches are highly impacted by urban stormwater runoff. The study design involved estimating baseline gross solids patterns, type and loads in the watershed and operation and maintenance costs of the bag filters. The findings of this project are summarized in Gross Solids Characterization Study in the Tred Avon Watershed Talbot County, MD (Stack et al., 2013).
Gross solids are an emerging pollutant of concern that is comprised of trash and litter, coarse sediment, and organic debris (primarily leaf litter). Unlike other pollutants, gross solids have not been well characterized or quantified in terms of their source areas and associated pollutants, in part due to the limited ability of automated samplers to collect particles sizes larger than 75 μm. Research studies indicate that the levels of metals, nutrients and toxic pollutants associated with these particles are potentially significant (ASCE, 2010; Rushton 2006). Nutrient loadings from leaf litter may be detrimental to urban impacted streams such as the Tred Avon, which already have elevated nutrients and reduced biological processing as a result of high storm flows that readily transport organic debris downstream instead of allowing for decomposition and uptake by stream microorganisms (Walsh et al 2005, Meyer et al. 2005, Wallace et al. 2008).
The Center collaborated with the Talbot County Department of Public Works and the Town of Easton to reduce gross solids in the Tred Avon watershed and to collect data to better define pollutant loads associated with leaf litter and removal efficiencies associated with bag filters. Four stormwater outfalls in the watershed were fitted with Kristar Enterprises Nettech© bag filters. The gross solids captured in these bag filters were removed, drained, and the wet material weighed. A subset of samples were dried and analyzed for total nitrogen, total phosphorus total solids dry, total volatile solids, and biological oxygen demand.
The majority of gross solids collected through this study consisted of leafy organic material, which is consistent with findings from other studies. On average, the percent contribution by weight[i] was 93% leafy material (and sediment), 3% woody debris and 4% litter. According to Dr. Neely Law, who was the Center lead on the study, these findings were not unexpected, but the quantities of leaf litter collected were surprising because the study area canopy cover was less than twenty-five percent. The bag filters picked up more leaves than expected and less trash. According to Neely: “the study confirmed that there is a lot of leaf litter from catchments that are going through the storm drains and reaching the waterways. Natural recycling of nutrients that you see on a forest floor with the leaves isn’t happening in the waterways. The question is, what nutrients are breaking down into the waterways? The next step in the research is to determine what happens with these nutrients.” Neely stated that these results point to the evaluation of better management of trees and other vegetation in urban areas. “This study is not trying to discourage the use of trees and vegetation; we just need to appreciate the impacts and effects of vegetation on impacted and currently degraded urban streams.”
The study of the Tred Avon watershed also determined that bag filter technology is highly cost-effective compared to other stormwater controls for nutrient removal. Calculated as an annualized cost per pound of pollutant removed per impervious acre treated, the cost effectiveness metric used in the study took into account capital costs and operation and maintenance. For nitrogen removal, bag filters were found to be 2.2 times more cost-effective than street sweeping, 4.5 to 8.6 times more cost-effective than bioretention, and 9.9 times more cost effective than wet ponds.
This project concluded that the bag filters are a cost-effective source control practice to remove nutrients associated with gross solids from streams. Additional research is needed to statistically quantify the impact of leaf litter on urban stream nutrient loadings and determine if there is a seasonal effect on composition of gross solids collected by the bag filters and associated nutrient loadings. This would develop a more comprehensive urban nutrient mass balance for watersheds and identify the most cost-effective management practices.
Environmental Water Resources Institute of the American Society of Civil Engineers (ASCE). 2010. Guidelines for Monitoring Stormwater Gross Solids. Urban Water Resources Research Council Gross Solids Technical Committee. ASCE
Meyer, J. M. J. Paul, and W.K. Taulbee. 2005. Stream ecosystem function in urbanizing landscapes. J. N. Am. Benthol. Soc., 24(3): 602-612.
Rushton, B. 2006. Broadway Outfall Stormwater Retrofit Project. Southwest Florida Water Management District, Brooksville, FL.
Stack, B., Law, N., and S. Drescher2013. Gross Solids Characterization Study in the Tred Avon Watershed Talbot County, MD. Prepared for Talbot County Department of Public Works. Center for Watershed Protection. Ellicott City, MD.
Wallace, T.A., G. G. Ganf, and J. D. Brookes. 2008. A comparison of phosphorus and DOC leachates from different types of leaf litter in an urban environment. Freshwater Biology, 53: 1902-1913.
Walsh, C. J., A. H. Roy, J. W. Feminella, P. D. Cottingham, P. M. Groffman and R. P. Morgan II. 2005. The urban stream syndrome: current knowledge and the search for a cure. J.N. Am. Benthol. Soc. 24(3): 706-723
[i] Field or ‘as-is’ dry weight from the drying tables and no oven-dried from the lab