Phytoremediation of slightly brackish, polycyclic aromatic hydrocarbon-contaminated groundwater from 250 ft below land surface: A pilot-scale study using salt-tolerant, endophyte-enhanced hybrid poplar trees at a Superfund site in the Central Valley of California, April-November 2019.

Abstract

Slightly brackish groundwater contaminated by polycyclic aromatic hydrocarbons (PAHs) at a Superfund site in the Central Valley of California was pumped from 250 feet below land surface to a water storage tank using solar power and then gravity-fed into 18, 330-gallon intermediate bulk containers (totes) as follows: (1) Five totes contained planting medium with three salt-tolerant hybrid poplar trees per tote (n = 15); (2) Seven totes contained planting medium with three salt-tolerant hybrid poplar trees per tote and inoculated with the naturally occurring, PAH-degrading endophyte Pseudomonas putida PD1 (n = 21); (3) Three totes contained planting medium only (n = 0); (4) One tote contained groundwater with three PD1-inoculated trees (n = 3) and one tote contained groundwater with three regular trees (n = 3); and (5) One tote contained groundwater only (n = 0). All trees grew well during the 7-month growing season in spite of the area's hot, dry air temperature, little precipitation, tote-influent chloride concentrations of 290 mg/L, and tote-influent naphthalene concentrations that ranged from 650 to 5,100 mg/L. PD1-inoculated trees initially had 56% larger tree area (tree height × tree width) than regular trees and up to 69% larger tree area by the end of the growing season, indicating some conferred phytoprotection to the PAH contamination. All trees had similar trunk caliper (diameter) and leaf chlorophyll content by the end of the growing season. Total naphthalene removal ranged from 88% to 100% across all totes. The lowest naphthalene removal of 88% was observed in a tote that contained only planting medium and indicates substantial adsorption of naphthalene onto the high organic content of the planting medium. Contaminant removal due to uptake by the hybrid poplar trees was confirmed by the detection of naphthalene in in vivo passive samplers placed in tree trunks. Benzene, toluene, ethylbenzene, total xylenes, 2-methylnaphthalene, 1,2,4-trimethylbenzene, and isopropylbenzene were also detected. These results from the pilot-scale study indicate that a full-scale application of using salt-tolerant hybrid poplar trees at this site could effectively decrease naphthalene concentrations in groundwater pumped from the deep aquifer. These initial results provide hope for similar application at other contaminated sites characterized by groundwater at considerable depths, especially at Superfund sites where costly pump-and-treat systems have been used long-term to treat low levels of groundwater contamination.