Geochemistry of St John and influence on marine systems

Ramos-Perez, Carlos; Gines-Sanchez, Cincy

This study was conducted to determine if three geochemical anomalies in St. John, USVI contribute significant amounts of heavy metals to near shore marine ecosystems. Stream water, sea water, marine sediments, and marine organisms from five watersheds were analyzed for Fe, Mg, Mn, cu, Cr, Ni, and Zn in order to determine the transport of the metals. Metal concentrations in stream water were typical of small, unpolluted streams, and only iron exceeded the U.S. Environmental Protection Agency (EPA) water quality standard. Magnesium was transported primarily in the dissolved phase, and iron and manganese were transported in the particulate phase. The streams did not appear to have consistently lower or higher concentrations of the metals, but Coral Bay had markedly higher iron and copper than the other streams. Annual metal export was estimated for Fish Bay Gut. Mg and Fe were exported in quantities greater than 100 kg; the other metals only rarely exceeded this quantity. There was great variability in annual export rates depending primarily on the·magnitude and frequency of rain events. Sea water samples also had low metal concentrations, but iron and manganese exceeded U.s. EPA water quality standards. There was considerable variability in metal concentrations among the sampling sites. Marine sediments also had low concentrations and high variability among the samples. Intensive sampling conducted in Fish Bay showed that Mn, Ni, and Fe were horizontally zonated with respect to the discharge point of Fish Bay Gut. The high concentrations found in sediments make this sample type the easiest compartment for heavy metal monitoring. Snails, crabs, and sea urchins from Fish Bay were analyzed. Snails and crabs appear to be good indicator organisms because the levels of heavy metal present in their tissue is well above the detection limit of the standard analytical techniques. The geochemical anomalies do not pose a serious threat to the marine ecosystems of St. John island.