| Water quality impairment by nutrient and metal enrichment from agricultural activities has been a concern worldwide. Phytoremediation technology using aquatic plants was evaluated for its efficacy in removing N, P, and metals from stormwater in detention ponds. Water lettuce (Pistia stratiotes) plants were grown in treatment plots in two stormwater detention ponds and water quality in both ponds was monitored. To better utilize water lettuce and investigate the possibility of a water lettuce-common salvinia (Salvinia minima) polyculture system, water lettuce and common salvinia were tested for their N and P requirements for normal growth with hydroponic studies conducted in a greenhouse. Water lettuce was also evaluated for its growth performance in water with different pH and salinity levels.;Water quality in both ponds was improved by phytoremediation with water lettuce, as evidenced by decreased turbidity, total solids, and nutrient concentrations. Turbidity was decreased by more than 65%. Total solids decreased by about 20%. Ammonium-N and NO3-N concentrations in the treatments plots were 31-72% lower than those in the control plots (without plants), and total Kjeldhal N was decreased by more than 20%. Reductions in PO4-P, total dissolved P, and total P concentrations in water were approximately 18-58% compared to the control plots. Annual removal of N and P from the water was 190 and 25 kg ha-1, respectively in the East Pond, and 329 and 34 kg ha-1, respectively in the West Pond by harvesting plant biomass.;Compared to the control plots, Al, Fe, and Mn concentrations were reduced by an average of 20%, and K by 10% in the treatment plots. Calcium, Mg, and Na concentrations were also reduced by 5-10%. Metals were substantially accumulated in the roots of water lettuce. A larger proportion of Ca, Cd, Co, Fe, K, Mg, Mn, and Zn was attached to external root surfaces by adsorption or surface deposition while more Al, Cr, Cu, Ni, and Pb were absorbed and accumulated into the root.;The critical N concentrations required for water lettuce and common salvinia to have net growth in biomass were 1.25 and 2.5 mg L-1, respectively, and the critical P concentrations were 0.1 and 1 mg L-1, respectively. Higher N and P requirements make common salvinia less desirable for a polyculture system with water lettuce.;Water lettuce could tolerate the salinity level (<1766 muS cm -1) of freshwater but its biomass could be reduced by up to 30% by high salinity (1766 muS cm-1). This plant could not survive in brackish water with salinity >6937 muS cm-1. We can also expect optimum performance from this plant in neutral and slightly alkaline water. |
