Nitrogen Treatment by a Dry Detention Basin with Stormwater Wetland Characteristics

Abstract

Dry detention basins (DB) are commonly used to reduce the rate of runoff in urban areas and may provide open space for recreation between storms. However, most are not effective at nitrogen removal in comparison to other measures, such as constructed wetlands. The study goal was to assess the nitrogen treatment efficiency of a DB that exhibited some wetland characteristics, including saturated soil near the inlet and wetland vegetation that covered 40% of the surface area. Influent and effluent samples were collected during multiple stages of eight storm events for nitrogen concentration analyses. High-frequency water stage, pH, dissolved oxygen (DO), and temperature loggers were deployed at the inlet and outlet prior to anticipated rain. As stormwater passed through the DB, the event mean concentrations (EMCs) and masses of TN declined by 20.7% and 52.3%, respectively, while the DO and pH dropped by 62% and 20.5%, respectively. Load reductions of TN exceeding 93% were observed during two small storms with rain depths of less than 0.16 cm and when the outflow volumes were reduced by greater than 82%. Temperature was significantly correlated (p < 0.001; r = 0.964) with volume reductions (via infiltration and evapotranspiration), and, thus, the treatment was better during warmer periods. The DB was effective at removing inorganic nitrogen, likely via nitrification, denitrification, and immobilization, but frequently exported higher EMCs of organic nitrogen. Overall, the DB exceeded the 10% TN removal expectation for dry basins. The findings from this study suggest that the TN treatment efficiency of DBs may be improved by incorporating wetland characteristics.

Assessment and Mitigation of Fecal Bacteria Exports from a Coastal North Carolina Watershed

“…The observed lower discharge during the warmer months was not a result of lower precipitation totals because the mean rainfall received during the warm months (June-August) was 15 cm, while during the cold months (January-March), it was 11 cm. A similar finding was observed in another study conducted in Eastern North Carolina, where it was concluded that evaporation and transpiration rates were greater during warm periods relative to colder periods and that resulted in less discharge during warmer months [12]. The mean pH of samples collected from the eight locations was slightly alkaline and during baseflow, ranged from 7.1 at site WO-2 to 7.4 at site WO-7, while during stormflow, the range was 7.1 (WO-3) to 7.7 (WO-1).…”

“…Stormwater control measures including bioretention, constructed wetlands, and/or detention basins are now required in many larger cities to slow the delivery of runoff and pollutants to streams [10][11][12]. However, many communities were developed prior to the required implementation of the control measures and thus the streams draining these watersheds may exhibit erosion, frequent flooding, and other characteristics symptomatic of "urban stream syndrome" [13].…”

Evaluation of Phosphate and E. coli Attenuation in a Natural Wetland Receiving Drainage from an Urbanized Catchment

“…Rainfall was evenly distributed throughout the seasons (Supplemental), and differences in monthly precipitation were not significantly different (p = 0.464) when comparing warm (mean 11.8 cm/month) and cool (mean 10.0 cm/month) periods. Evapotranspiration rates are typically higher during the "growing season" spanning the spring and summer months (April-September) due to longer daylight hours, warmer temperatures, and active plant uptake and transpiration of water, thus resulting in lower discharge [36]. Differences in evapotranspiration may explain the higher inflow and outflows during cooler periods as the mean temperatures during fall and winter months (8.9 • C) were significantly (p < 0.001) lower relative to summer and spring (22.2 • C), thus resulting in less evaporation (Supplementary Materials).…”

“…Like with the removal of phosphorus, the hydraulic retention time has been reported as a significant factor regarding bacteria removal [23]. Wetlands may be constructed in upland areas to mimic processes in natural wetland and to remove pollutants including nutrients and E. coli from wastewater [24,25,31,33], urban stormwater [24,[34][35][36], and agricultural drainage [30,37].…”

Special Issue: Stormwater/Drainage Systems and Wastewater Management

“…Pochwat and Pizzo [8] revealed that the use of a detention tank may significantly reduce the amount of sewage outfall from a drainage system, and that the method of connecting the device to a network has an influence on its efficiency. Humphrey et al [9] evaluated the nitrogen treatment efficiency of a detention basin (DB) that exhibited some wetland characteristics. The findings from their study suggest that the TN treatment efficiency of DBs may be improved by incorporating wetland characteristics.…”