Article pubs.acs.org/est
Fate of Naphthalene in Laboratory-Scale Bioretention Cells: Implications for Sustainable Stormwater Management Gregory H. LeFevre, Paige J. Novak, and Raymond M. Hozalski* Department of Civil Engineering, University of Minnesota, 500 Pillsbury Drive SE, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: Bioretention cells are increasingly popular in low-impact development as a means to sustainably mitigate the environmental problems associated with stormwater runoff. Yet, much remains to be known regarding the removal and ultimate fate of pollutants such as petroleum hydrocarbons in bioretention cells. In this work, laboratory-scale bioretention cells were constructed inside sealed glass columns. The columns were periodically spiked with 14C-naphthalene over a 5-month period and the fate of this representative hydrocarbon and the influence of vegetation on naphthalene fate was studied. Three column setups were used: one planted with a legume (Purple Prairie Clover, Dalea purpureum), one planted with grass (Blue-Joint Grass, Calamagrostis canadensis), and one unplanted (i.e., control). Overall naphthalene removal efficiency was 93% for the planted columns and 78% for the control column. Adsorption to soil was the dominant naphthalene removal mechanism (56−73% of added naphthalene), although mineralization (12−18%) and plant uptake (2−23%) were also important. Volatilization was negligible (50%) sorbed to the soil was present in the upper portion of the columns (top 15 cm for clover and unplanted, top 30 cm for grass) (Figure S3). The concentration of naphthalene in the soil samples with depth, as measured by GC-FID, positively correlated with the 14C concentration in the same samples (Rho = 0.42, p = 0.0247, n = 29). Furthermore, the ratio of 14 C/12C naphthalene added to the columns in the simulated rain events was not significantly different from that extracted from the soil (p = 0.61), suggesting that the 14C extracted from the soil was primarily undegraded naphthalene. The presence of naphthalene biodegradation products in the soil samples, however, cannot be entirely ruled out. Biodegradation of naphthalene, as indicated by 14CO2 evolution, was a substantial naphthalene loss mechanism, representing 12−18% of the total 14C recovered from each column. The extent of naphthalene mineralization was not significantly different among the three columns (p > 0.5). Vegetative uptake accounted for 23% of the total 14C for the grass column and 2.5% for the clover column. Of the 14C incorporated into plant tissue, 88 and 92% (clover and grass, respectively) was present in the above-ground biomass with the remainder in the roots (Table S1). Overall, leaching of 14C was minor for the vegetated columns (7%) but not for the unplanted column (22%). Most of the 14C leached from the control column (91%) was collected immediately following the first naphthalene dose (Figure S1). This “first flush” pattern was also observed in the grass and clover columns, but to a lesser extent (39% and 37%, respectively). Thus, following the initial flushing of 14C from the columns, little additional mass (
