Metal Transport in the Mixing Zone of an Estuarine River to the Northern Gulf of Mexico

Abstract

To better understand the pollution potential of metals in estuaries heavily concentrated with petrochemical industries, we measured concentrations of total recoverable metals in the lower Calcasieu River in southwestern Louisiana that flows into the northern Gulf of Mexico. Water samples were collected at six sites along the last 88 km reach of the river monthly between May 2013 and November 2015, during which salinity ranged from 0.02 to 29.5 ppt from upstream to downstream. The samples were analyzed for a series of total recoverable metals, including aluminum (Al), arsenic (As), boron (B), cadmium (Cd), calcium (Ca), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), lithium (Li), magnesium (Mg), manganese (Mn), nickel (Ni), silicon (Si), titanium (Ti), vanadium (V), and zinc (Zn), of which only Al, As, B, Ca, Fe, Li, Mg, Mn, Si, Ti, and Zn had a detection rate higher than 30% over the 31-month study period. In the freshwater portion of the river, Si, Ca, Al, Fe, and Mg showed the highest concentration (8, 5, 4, 2, and 2 mg/L, respectively), while Li, As, Zn, Ti, and B had the lowest concentration (6, 16, 27, 34 and 50 µg/L, respectively). The concentrations of Al and Si declined by more than 30% from the freshwater to the river mouth, but the concentrations of Li and B increased by 61 and 66 times, respectively. None of these metals’ concentrations were found to exceed US EPA standards, but the rapid increase in Li and B concentrations may indicate a potential anthropogenic influence. On average, the Calcasieu River discharged a total of 35,484 tons of the elements each year (or 8059 kg/km2/yr), of which Si, Al, Ca, Fe, and Mg contributed 98%. Three major components of factors were extracted explaining 34, 20, and 13% (cumulative 67%) of the total variation in the metal concentrations. Salinity and pH were the major (>0.3) parameters in component 1 explaining the variability of B and Li; TSS was the major (>0.3) parameter in component 2 explaining the variation of Al, Fe, and Ti concentration; and temperature and DO% were the major (>0.3) parameters in component 3 explaining the variation of Mn concentration. Further studies on riverbed sediment metals and their effect on metal concentrations in surface water can help understand the metal sources and their potential effects on coastal aquatic ecosystems.