Natural attenuation/phytoremediation in the vadose zone of a former industrial sludge basin

The natural attenuation of polyaromatic hydrocarbons (PAHs) in the vadose z one of a naturally revegetated former industrial sludge basin (0.45 ha) was examined. This was accomplished by comparing the concentration of 16 PAH c ontaminants present in sludge collected below the root zone of plants with contaminants present at 3 shallower depths within the root zone. Chemical a nalysis of 240 samples from 60 cores showed the average concentration of to tal and individual PAHs in the 0-30 cm, 30-60 cm, and bottom of the root zo ne strata were approximately 10, 20, and 50%, respectively, of the 16, 800 ppm average total PAH concentration in deep non-rooted sludge. Statisticall y significant differences in average PAH concentrations were observed betwe en each strata studied and the non-rooted sludge except for the concentrati ons of acenaphthene and chrysene present at the bottom of the root zone in comparison to sludge values. The rooting depth of the vegetation growing in the basin was dependent on both vegetation type and plant age. Average roo ting depths for trees, forbs (herbaceous non-grasses), and grasses were 90, 60, and 50 cm, respectively. The deepest root systems observed (100-120 cm ) were associated with the oldest (12-14 year-old) mulberry trees. Examinat ion of root systems and PAH concentrations at numerous locations and depths within the basin indicated that plant roots and their microbially active r hizospheres fostered PAH disappearance; including water insoluble, low vola tility compounds, i.e. benzo(a)pyrene and benzo(ghi)perylene. The reduced c oncentration of PAHs in the upper strata of this revegetated former sludge basin indicated that natural attenuation had occurred. This observation sup ports the concept that through appropriate planting and management practice s (phytoremediation) it will be possible to accelerate, maximize, and susta in natural processes, whereby even the most recalcitrant PAH contaminants ( i.e. benzo(a)pyrene) can be remediated over time.