Metal Uptake and Accumulation by Water Lettuce (Pistia stratiotes) from Eutrophic Stormwater.

Water lettuce (Pistia stratiotes) was planted in East and West stormwater retention ponds to remove nutrients, mainly N and P, and metals from the waters. Water samples and plant samples were collected weekly and monthly, respectively from the control (without plant) and/or remediation plot (with water lettuce) for metal concentration analysis. Aluminum, Ca, Fe, K, Mg, and Na were the main metals detected in the waters with mean concentrations of 0.26, 22.42, 0.15, 3.87, 2.82, and 16.25 mg/L in the West Pond, respectively and 0.23, 43.80, 0.33, 7.83, 15.01, and 51.18 mg/L in the East Pond, respectively. Cadmium, Co, Cr, Cu, Mn, Ni, Pb, and Zn were of very low concentrations or undetectable. Aluminum and Fe were significantly reduced (by more than 20%) in the remediation plots compared to the control plots. Potassium was reduced by more than 10% in the remediation plots. Calcium, Mg, and Na were also reduced by the water lettuce although to a less extent. Metals were concentrated and accumulated in the roots of water lettuce even when their concentrations in the water were under detection limit. These metals can be grouped according to their removal rate (RR, which is the ratio of metal concentration in and around root to that in the surrounding water), with Group 1 having a RR less than or about 1000, Group 2 with a RR between 1000 and 10000, and Group 3 with a RR higher than 10000. Group 1 includes Ca, Mg, and Na, Group 2 is composed of K, Co, Cu, Ni, Zn, Al, Cd, and Pb, and Group 3 consists of Fe, Mn, and Cr. All the metals except for Ca had a shoot/root ratio in concentration less than 1. Calcium had a shoot/root ratio > 3.
Key words: Metal removal, stormwater remediation, water lettuce.