Geochemical Processes Regulating Groundwater Quality in Coastal Aquifer in Odawara Region, Kanagawa Prefecture, Japan

Abstract

Groundwater in the coastal aquifer in the Odawara region, the southern region of the Ashigara plain, is considerably used for industrial, agricultural and municipal water supplies. Since 1980s, many studies focused on the problems of water level decline and groundwater salinization in this region. However, for considering a better management of this groundwater resource, more precise understanding of the geochemical processes occurring in this aquifer and changes in the underground environment due to salinization is necessary. For this purpose, field surveys were carried out in winter (January and February, 2009) and in summer (July and August, 2009). More than 100 groundwater samples were collected from the unconfined and confined aquifers in the region and analyzed for their chemical and stable isotopic compositions. A series of comprehensive interpretations on water quality data using Piper diagram and Stiff diagram together with stable isotopic data have been carried out to gain insight into the spatial and temporal variations of the quality and the geochemical evolution of the groundwater. Several geochemical processes were identified as principal factors controlling the groundwater quality in this region. Silicate weathering which is the result of water rock interaction is an important process dominating unconfined and confined aquifers, over a wide area. The other geochemical processes such as ion exchange reactions and salinization are also occurring in this region. The latter process, salinization, is limited in area but an important phenomenon and seems to be caused by salt water up-coning in the confined aquifer due to the over-extraction of groundwater since 1970s. This phenomenon was detected typically at particularly deep wells in the industrial area near the coast where the pumping rate is high. These phenomena, which occurred in the study region, were summarized in a schematic conceptual model as a helpful tool for the management of the groundwater resources.