Water-rock interactions and seawater-freshwater mixing effects in the coastal dunes aquifer, Coos Bay, Oregon

Abstract

Groundwater from the seawater-freshwater interface in sandy sediments was sampled using a unique profile well designed in which separate PVC tube screens were installed at various depths. All major ions (except chloride) showed non-conservative behavior in the mixing zone; SO 4 −, Mg 2+ and K + are depleted and HCO 3 − is enriched, relative to Cl −. Na + is enriched in solutions containing less than 20% seawater and depleted in solutions containing higher percentages of seawater. The behavior of Ca 2+ is the mirror image ofthat of Na +. The chemical data on the stratification of groundwater above and within the interface zone were obtained using a unique profile well design in which separate PVC tube screens were installed at various depths, thus making it possible to extract water samples at close vertical intervals. The chemical changes in the groundwater reflect the following processes: 1. (a) Ca 2+-Na + and Ca 2+-Mg 2+ base-exchange reactions; 2. (b) oxidation of organic matter in the sediment; 3. (c) reduction of sulfate, probably due to bacterial processes; 4. (d) reaction of HCO 3 − and S 2− with Fe 2+ to produce mineral coatings and cements of pyrite and siderite; 5. (e) K-feldspar formation producing lower of K + concentrations. The authigenic phases were identified using scanning electron microscopy. Thermodynamic calculations suggest that all solutions are saturated with respect to quartz and chalcedony and many are saturated with respect to calcite, dolomite, siderite and hematite. Kaolinite is the stable clay mineral in the top of the freshwater column, whereas smectite is the stable clay mineral in the lower part of the freshwater column and in the seawater-freshwater mixing zone.