Field and laboratory experiments on high dissolution rates of limestone in stream flow

Field and laboratory experiments were performed to examine dissolution rates of limestone in stream flow. Field experiments were conducted in three stream sites (A–C) with different lithological or hydrological settings around a limestone plateau in the Abukuma Mts., Japan. Sites A and B are allogenic streams, which flow from non-limestone sources into dolines, and site C has a karst spring source. Tablets made of limestone from the same plateau with a diameter of 3.5cm and a thickness of 1cm were placed in the streams for 3years (2008–2011) where alkalinity, pH and major cation concentrations were measured periodically. The saturation indices of calcite (SIc) of stream water were −2.8±0.4 at site A, −2.5±0.4 at site B and −0.5±0.4 at site C. Annual weight loss ratios for tablets were extremely high at site A (0.11–0.14mgcm−2d−1), high at site B (0.05mgcm−2d−1), and low at site C (0.005mgcm−2d−1). The contrasting rates of weight loss are mainly explained by chemical conditions of stream water. In addition, laboratory experiments for dissolution of limestone tablets using a flow-through apparatus revealed that flow conditions around the limestone tablet is another important factor for dissolution in the stream environment. These results revealed that limestone dissolves at a rapid rate where water unsaturated to calcite continuously flows, such as in an allogenic stream.