TRACE METAL AND PHOSPHATE PARTITIONING BETWEEN STORMWATER RUNOFF, GROUNDWATER AND SEDIMENTS ADJACENT TO LAKE HAVASU, ARIZONA

Abstract

A baseline study was established to characterize heavy metal, arsenic, selenium, and nutrient levels in stormwater runoff from Lake Havasu City, Arizona. It also lead to investigating the fate of those constituents in lake water, shallow groundwater and surficial sediments at the mouths of five washes draining from the city into Lake Havasu. Composite core and pit sediment samples collected at wash mouths before and during historic low lake water levels, reveal a fairly consistent metal and total phosphate presence at most wash sites. Two sites have generally higher concentrations where clay is a more significant component in the sediment. Regression correlations support that nickel has a strong association with chromium and cobalt in all sample types as well as with barium in runoff and groundwater. Copper and cobalt also show strong affinities with several other transition metals in groundwater and sediments. Reducing conditions prevail within 1-2 inches of the sediment surface at all locations, yet metal concentrations within 1.5 feet of the surface display a spatial change from increasing with depth at northern wash sites to a more variable pattern at southern sites. Phosphate concentration with depth is more variable and may contradict the local metal trends. Metal ion and complex adsorption to mineral and organic surfaces is expected though total organic carbon levels, reflected by the presence of freshly buried vegetation, are highly variable. Black color of sediment cores converts to yellow-brown overnight indicating chemical changes and the subordinate effect of organic matter in the sediment. Wash mouth sediments appear to be a repository for most metals and for total phosphate, though more work is needed to determine the mechanisms involved associating stormwater runoff with chemical and sediment deposition.