Continuous monitoring of groundwater radon for evaluating chemical and structural properties and fluid flow variations of shallow aquifer systems

We have carried out continuous monitoring of radon concentration in groundwater at eight wells in Nishinomiya City, Hyogo Prefecture, Japan. The shallowest major aquifer, which we call the first aquifer, extends widely at a depth about 3-5 meters in the region. We pay our major attention to the first aquifer, because it provides us with abundant high quality water, which has long been utilized for brewing Sake. We made the radon monitoring at 5 wells tapping the first aquifer. The first aquifer can be characterized by very high radon concentration and its large temporal variation. Time-averaged values of radon concentration at 4 shallow wells were about 70-80 Bq/l, which is almost the highest value ever reported for natural water throughout the Japanese Islands. These high radon concentrations can be attributed to uranium-rich sediments in the aquifer, which had been brought from the Rokko Mountains region on the north. Large temporal variations of radon concentration observed at the shallow wells probably reflect the heterogeneous distribution of the grain size of sediments, which makes the groundwater flow unstable. We examined radon concentrations in deeper aquifers at three wells with depths of 8-17 m for comparison. Absolute values of radon concentrations and their temporal variations at the deeper wells are smaller than those in the first aquifer, which suggests the small contributions of uranium-rich sands to the sediments compared with the first aquifer. The radon concentration at a well with a depth of 16 m showed periodical variations responding to the ocean tide, which demonstrates that very small signals of crustal deformations can be detected by the continuous monitoring of groundwater radon.