Adapting the Relaxed Tanks-in-Series Model for Stormwater Wetland Water Quality Performance

Laura Merriman,Michael Burchell,Jon Hathaway,William Hunt

doi:10.3390/w9090691

Laura Merriman, Michael Burchell + Show 2 more

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https://doi.org/10.3390/w9090691

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Abstract

Across the globe, water quality standards have been implemented to protect receiving waters from stormwater pollution, motivating regulators (and consequently designers) to develop tools to predict the performance of stormwater control measures such as constructed stormwater wetlands (CSWs). The goal of this study was to determine how well the relaxed tanks-in-series (P-k-C*) model described the performance of CSWs in North Carolina. Storm events monitored at 10 CSWs in North Carolina were used for calibrating the model, and statistical evaluations concluded the model could adequately predict the performance for all pollutants except organic nitrogen. Nash–Sutcliff calibration/validation values were determined to be 0.72/0.78, 0.78/0.74, 0.91/0.87, 0.72/0.62, 0.88/0.73, and 0.91/0.63 for total nitrogen, total ammoniacal nitrogen, oxidized nitrogen, organic nitrogen, total phosphorus, and total suspended solids, respectively. Sensitivity analysis revealed only one calibration parameter with strong sensitivity, the Arrhenius coefficient (temperature dependent model coefficient). With this model, CSWs can be optimized to treat watershed-specific influent concentrations to meet effluent targets. In general, the current design technique used in North Carolina and many other locations (a first flush volume detention method) oversizes CSWs for water quality vis-à-vis the method herein, suggesting improved designs for water quality may be possible through scientifically-informed methods.

Highlights

Constructed stormwater wetlands (CSWs) have become one of the more commonly utilized stormwater control measures (SCMs) [1,2,3,4]
The apparent number of Tanks in Series (TIS) (P) ranged from 1.4 to 7.6 for each pollutant and CSW (Table 6). This range fell well within the scope of tanks in series values (1–10) derived from conservative tracer experiments compiled by Kadlec and Wallace (2008) [3] and simulations conducted by Persson et al
Design of CSWs could follow this same reasoning by optimizing designs to treat watershed-specific influent concentrations to target effluent thresholds based on receiving waterbodies

Summary

Introduction

Constructed stormwater wetlands (CSWs) have become one of the more commonly utilized stormwater control measures (SCMs) [1,2,3,4]. Most regulatory entities set design standards to capture and detain the water quality volume of runoff, which is released over two to five days and subjected to treatment during the inter-event period [5,6] If these design requirements are met, pollutant removal credits for total nitrogen (TN), total phosphorus (TP), and total suspended solids (TSS) of 40%, 40%, and 85%, respectively, are granted in North Carolina (NC), with similar removal credit systems being in place in other municipalities in the United States [7].

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