IMPACTS OF DYNAMIC ENVIRONMENTAL FACTORS ON POTENTIALLY TOXIC ELEMENTS (PTEs) FATE AND TRANSPORT DURING STORMWATER MANAGEMENT

Abstract

Stormwater management practices (SMPs) are engineered landscapes that treat runoff by increasing infiltration and retention, reducing the peak flow, and improving water quality. In this study, the mutual impacts of Kd and SMP loading ratio (ratio of contributing to the infiltration area) on SMP performance were assessed by developing a 1-D transport model. After simulation of 20 years of SMP service under baseline salinity conditions, Cl- was the only PTE that showed high mobility and quickly reached the groundwater table, while the other PTEs (e.g., metals) were effectively retarded in the top ~ 60 cm of the fill media. However, experimental batch and column studies showed that in higher salinity conditions, PTE Kd values reduced significantly, resulting in higher PTE mobility. Cation exchange followed by complexation with ligands were the main drivers for lower PTE Kd under high salinity conditions. Such reduced retardation can be alarming because it could readily lead to the overestimation of fill media capability in water quality enhancement. Moreover, columns experiments revealed that cycles of baseline and high salinity conditions cause substantial episodic PTE leachate from the fill media, even for PTEs like Cu and Pb that were not abundant in the fill media. The previously captured PTEs during baseline salinity were desorbed during the periods with high salinity, resulting in leaching. Dynamic transport modeling that included cycles of high and baseline salinity simulated the PTE transport more accurately because it prevented the overestimation of fill media capacity in the high salinity seasons, and it also modeled the substantial episodic leaching. PTE retardation was mainly attributed to the adsorption of metals by the top layers that caused surficial accumulations. Field samples were collected along the gradient of a SMP to study the dynamics of horizontal PTE transport. When compared with coarser particles, the fine particles (d < 10 µm) had two to three times greater PTE Kd values owing to their higher specific surface area and organic content. Fine particles, as vectors for metal transport, were readily resuspended due to runoff flowing over the SMP bed, while resuspension of coarse particles only occurred during the storms with an average intensity of greater than 4.5 mm/hour in the monitored SMP. SMPs may increase the particle and metal loads due to resuspension, imposing a high risk of point source contamination for the downstream water bodies. Also, increasing salinity to environmentally relevant levels resulted in higher stability of fine particles, yet another negative impact of high salinity on SMP performance. In-situ fill media remediation techniques like flushing with cysteine showed promising evidence of cleansing the fill media top layers, which would lower the likelihood of resuspension and redistribution of surficial accumulated PTEs. Well-designed and maintained SMPs may notably reduce the likelihood of surficial resuspension/redistribution via prioritizing infiltration over other forms of the discharge from the SMP as well as decreasing the stream power by adopting measures like appropriate vegetative cover or installing forebays, gabions, and weirs.