Most traditional water sampling methods are designed to capture dissolved pollutants and fine particles, but they miss larger debris such as leaves, twigs, and litter that accumulate in catch basins. By overlooking gross solids, municipalities may underestimate the total pollutant load entering surface waters and miss opportunities for effective pollution reduction. Researchers Sarah E. Waickowski, Consuelo Arellano, Andrew R. Anderson, Shawn G. Kennedy, and William F. Hunt wanted to address this critical gap in stormwater pollution research. While urbanization is a known contributor to surface water degradation, most stormwater control measures are evaluated using automated samplers that cannot effectively capture particles larger than 250 μm. As a result, nonpoint source pollution from particulate-bound gross solids, especially in gray infrastructure such as catch basins, is often overlooked.
Their paper, “Gross Solids Collected from Urban Catch Basins in North Carolina: A Potential Pollutant Accounting and Capture Opportunity,” aimed to quantify the nutrient content and pollutant loads of gross solids collected from urban catch basins in North Carolina, establishing a baseline for understanding their environmental impact. This research found that although total nitrogen and total phosphorus made up a small percentage of the dry mass of gross solids, the cumulative nutrient loads could be significant due to the large volume of material collected.
This finding can help municipalities prioritize stormwater maintenance efforts, suggesting that focusing on areas with dense tree cover and using metrics such as sweepable curb length can maximize pollution reduction. Learn more about this study and its efforts to improve urban stormwater management in the Journal of Sustainable Water in the Built Environment at https://ascelibrary.org/doi/10.1061/JSWBAY.SWENG-603. The abstract is below.
Abstract
Gross solids are particles at least 5 mm in diameter and characterized as organic debris, litter, or coarse sediments. These large particles are conveyed to water bodies via storm drainage networks and contribute to nonpoint source pollution. Previous research in North Carolina, US, has characterized gross solids pollution along highways, but there are a lack of data quantifying the nutrient concentrations and loads of organic debris passing through urban catch basins. Sixteen catch basins located across the state were monitored for approximately 1 year to measure gross solid nutrient loads. Within four municipalities, four land uses were examined: 1) high-density residential, 2) low-density residential built before 1990 or 3) after 2000, and 4) downtown ultra-urban districts. Catch basins were retrofitted with custom-made traps to collect gross solids. Monthly total nitrogen (TN) loads ranged from 3.36×10−4 kg / ha to 7.32 kg / ha, while monthly total phosphorus (TP) loads varied between 1.68×10−5 kg / ha and 7.28×10−2 kg / ha. Statistical analyses determined nutrient loads were affected by location, land use, and season. Significant differences (𝑝-value < 0.05) were attributed to variations in rainfall, tree abundance, and pollination. Because TN and TP accounted for 0.40% to 3.43% and 0.03% to 0.25% of the dry leaf litter, respectively, this study illustrates that jurisdictions can reduce nonpoint source pollution through typical municipal activities (cleaning collected debris from catch basins).
See how improved monitoring of larger debris enhances water pollution reduction in the ASCE Library: https://ascelibrary.org/doi/10.1061/JSWBAY.SWENG-603.
