At Kujukuri Beach, Chiba, Japan, the Holocene series unconformably overlies the Pleistocene Kazusa Group, which was deposited mainly in the deep sea and contains natural gas (methane) dissolved in water. These conditions allow gas to emerge from the ground as it moves upward through faults and sandy layers. In 2007, some tidal pools at Kujukuri Beach became cloudy white, and water clouding still persists in areas where the tidal pools overlap with methane gas emissions on the beach at low tide. Water clouding occurs when yellow groundwater in a strongly reducing state is produced because underground methane gas is discharged through the beach surface into tidal pools, and comes into contact with the atmosphere or seawater. A survey conducted in April 2009 confirmed that tidal pools change from being transparent to yellowish white, then become white, yellowish white, yellow, and yellowish white, and again transparent. Accordingly, in this study, this phenomenon is analyzed and elucidated using groundwater level measurements. The rate of discharge of yellow groundwater declined as tide level decreased, and the water turned white at the lowest tide. As the tide level increased, the water again turned yellow as yellow groundwater emerged from the bottoms of the tidal pools. Seawater flowed into the tidal pools as the tide level further increased, causing seawater in the pools to become yellowish white and transparent.
