Low-impact development facilities for stormwater runoff treatment: Field monitoring and assessment in Xi’an area, China

Field studies are valuable because of the insufficient number of field performance data in low-impact development (LID)-related research. Four typical LID facilities in northwest China were built and monitored. These facilities differ in their impervious/pervious bottom, inflow underlying surface, media types, and combinations. The water volume reduction of impervious rain garden filled with native loess decreases by approximately 20% when the total daily rainfall levels gradually increase from light to heavy. Several technical measures, such as media modification, setting internal storage area, reducing drainage area, and increasing media thickness, have been taken to improve the runoff regulation effect. The volume reduction rate in light and medium-sized rainfall (daily rainfall totals < 25 mm) is above 68.2% for two improved facilities, namely, mixed media rain garden and layered media bioswale. For impervious native loess rain garden, mixed media rain garden, and layered media bioswale, the median values of inflow total suspended solid (TSS) event mean concentrations (EMC) are reduced from 43.5 to 68.5 mg/L to 22.4–24.3 mg/L. In accordance with the Class IV limits of Surface Water Environmental Quality Standards (total nitrogen (TN) = 1.5 mg/L; total phosphorus (TP) = 0.3 mg/L), the probabilities of the effluent TN concentration exceeding the IV limit are 32.7%, 42.3%, and 19.8% for the three facilities. The exceedance probabilities for TP are 18.5%, 6.5%, and 8.9%. The weighted results indicate that the annual load reduction rate is more stable than the fluctuating concentration removal, which is above 60%. An approach to estimate the total scale of LID construction was established on the basis of rainfall level proportion, land use types, and monitored data. When native loess and mixed media rain garden are applied in building areas and layered media bioswale is applied in road area, the LID implementation cost is approximately 36.0 billion CNY until 2030 for Xi'an City.