TRACE METAL SPECIATION IN TEXAS RIVERS

Using rigorous "clean chemistry" procedures, filtered and unfiltered samples were taken at several locations on each of the nine major rivers in the eastern 2/3 of Texas. With few exceptions, total dissolved trace metal concentrations were similar to those in the Mississippi and other rivers which show little impact from human activity. Phosphate, DOC, pH and major ions were determined and a dual-column ion exchange technique was used to concentrate dissolved trace metals and fractionate them based on chemical reactivity. An operationally defined labile metal fraction was obtained from a Chelex-100 column yield and an organic fraction from an AG MP-1 anionic exchange column. Total dissolved metals were determined on a separate sample after acidification and UV irradiation. An inert fraction was calculated from the difference between total dissolved metal concentration and the sum of the two column yields.

The Sabine and Neches Rivers flow through forests and wetlands in east Texas and have high dissolved organic carbon (DOC) concentrations compared to other Texas Rivers. In their waters, a large fraction of metals such as Cu and Fe were organically complexed. However, for some metals organic complexation was inhibited by the low pH from high organic acid concentrations. In that case, total dissolved Fe, Mn and Al concentrations were strongly dependant on pH values, and a large portion of Fe and Al were associated with organic complexes or colloidal material.

High concentrations of ethylediaminetetraacetic acid (EDTA) and phosphate, which form strong complexes with some metals, were found in high population density sections of the San Antonio and Trinity Rivers. Distribution coefficients between particulate and dissolved metals and EDTA (or phosphate) concentrations showed an inverse relationship, suggesting that EDTA and phosphate are capable of extracting metals from suspended particulate matter (SPM) and bottom sediments. This process elevated total dissolved Cd, Co, Cu, Ni, Pb and Zn concentrations in these river sections. The EDTA and phosphate complexes are not retained by either Chelex-100 or AG MP-1 resin and are thus classified as an inert fraction. The inert fraction was small in most samples but was as high as 95% of total dissolved metal concentrations in a few San Antonio River samples.