CHARACTERIZATION OF CEMENTS IN MOKI MARBLES AS A POSSIBLE SOURCE OF ARSENIC IN GROUNDWATER, WASHINGTRON COUNTY, UTAH

The EPA recently lowered the acceptable levels of arsenic in drinking water. To comply with this new regulation the Washington County Water Conservancy District tested their wells and found several wells with high arsenic levels. One theory for these high arsenic levels is that the arsenic is leaching from the iron concretions (moki marbles) in the Navajo Sandstone which supports one of their aquifers. To test this theory, fifty moki marble surface samples were collected from four areas near St. George, Utah. Random samples of moki marbles were ground down and analyzed using x-ray diffraction. A subset was then made into thin sections, where minerals and textures were identified using reflected light. These iron concretions are cemented with hematite and goethite, which are both known to adsorb arsenic. The hematite exists as particles disseminated in other cements (quartz, opal, and calcite), as hematite coated grains, and as pore fillings. The sandstone in our thin sections consisted of mature quarts arenite with minor constituents of feldspar, microcline, tourmaline, and opal. Based on our thin sections there is also evidence of pressure solution and quartz overgrowth. The ultimate purpose of this study is to find where the arsenic resides within the moki marbles. Microprobe Analysis is pending.