Hydrilla-based Phytoremediation Water Quality Management Strategy to Reduce Lead Concentrations in an Urban Lake, Central Florida

Urban lakes are common throughout the United States and are susceptible to anthropogenic inputs stemming from point and nonpoint source pollutions. In this study, effectiveness of an in situ water quality management strategy was investigated by employing phytoremediation and subsequent water quality modelling methodology for an urban lake, Lake Wire, in the downtown area of the central Florida city of Lakeland. Lake Wire is an oligotrophic natural depression with a surface area of 22 acres, acts as a flux sink for a contributing 140-acres watershed.  Lake has an average depth of 12 feet with a maximum depth of 21 feet, and with an average dissolved oxygen concentration of 4 mg/L and pH of 8. Surrounding basin had historically been used for light industrial and commercial purposes with the former Florida Tile plant. Florida Department of Environmental Protection (FDEP) water quality assessment, mid 1990’s to early 2000’s, indicated that soluble lead concentrations in the lake is above the FDEP Class III Surface Water Standard (SWS). Sediment samples also indicated high levels of lead. To reduce the lead concentrations, 9,780 cubic yards of impacted sediments was removed in 2003.  Removal of the impacted sediments was intended to lower the lead concentration in Lake Wire to meet the Class III SWS of 0.0019 mg/L over time. However post-audit water quality samplings in late 2003 to 2013 taken at five locations and at three depths – shallow, mid, and deep — indicated that while lead concentrations were reduced significantly, lead concentrations in the lake remained above the Class III SWS.

Monitoring data also shows a seasonal pattern in lead concentrations. During the winter months, lead concentrations were the highest whereas the lowest concentrations were observed in the summer, which coincided with the growth of hydrilla, an aquatic plant, which grows in dense mats reaching 20 feet in length, and which covers 2/3 of Lake Wire. Analysis of the water quality data and plant tissue samples registered that hydrilla plant tissues do adsorb lead and effectively remove the lead from the water column however when hydrilla begins to senesce in the winter, the lead in the plant tissues is released back in the water column.

Reflecting seasonal lead- hydrilla correlationship, phytoremediation strategy for lead concentration reduction was implemented by harvesting of hydrilla during the mid-Fall during 2013 to 2015.