Description
The main objective of this study was to examine the effectiveness of heavy metal removal by two pilot constructed wetlands in Imperial Valley, California. The Brawley Wetlands treats the New River, which originates in Mexico and collects effluent sewage treatment plants and maquiladoras in Mexico, crosses the U.S. Mexico border into the U.S. and then collects agricultural drainage from the Imperial Valley before discharging into the Salton Sea. The New River is polluted by nutrients, pesticides, heavy metals, pathogens, and suspended sediments. It is listed by the California State Water Resources Control Board Region 7 on the 303(d) List of Water Quality Limited Segments because it has constituents that exceed water quality objectives. The other constructed wetlands tested, the Imperial wetlands treat solely agricultural drainage from Imperial Valley. Due to the tremendous loss of wetland habitat in CA, the Salton Sea is an especially important stop on the Pacific flyway for many migratory bird species. The Salton Sea National Wildlife Refuge has over 380 species of birds, among the highest totals of all national refuges (Salton Sea Authority, 1997). This includes six species that are known to live around the sea that are protected by the Federal Endangered Species Act (ESA) and other species protected by the Migratory Bird Treaty. Both Imperial and Brawley wetland sites were constructed in the year 2000 to help improve the quality of water in the New River. Essentially, the heavy metals have only become a focus of public interest since analytical techniques have made it possible to detect them even in very small traces. Between April 2009 and December 2011 influent and effluent samples were taken at each wetland and analyzed for heavy metals (Cr, Ni, Cu, Zn, Pb, Se, As, and Cd) by an inductively coupled plasma mass spectrometry (ICP-MS) analyses. Overall removal efficiencies for the period 2009-2011 for the Imperial Wetlands were -7% for As, -35% for Cr, -39% for Cu, -16% for Pb, -3% for Ni and -35% for Se. For two metals, Cd and Zn, there were net increases in the level in the outflow (as compared to the inflow) of +43% and +23% respectively (Table 3). For the Brawley Wetlands, the only metals that showed a net overall removal were As (-17%), Ni (-5%), and Se (-27%). Most of the other metals showed small increases in the outflow concentration (as compared to the inflow) of +1% for Cr, +7% for Cu, +4% for Zn and +14% for Pb. However, cadmium at the Brawley site increased by +72% in the outflow as compared to the inflow. Statistical analysis using a Wilcoxon signed rank test, showed that among all the metals tested, only Se was found to be significantly (p<.05) reduced by constructed wetland treatment at the Imperial and Brawley sites.
