Groundwater deterioration caused by induced recharge : Field survey and verification of the deterioration mechanism by stochastic numerical simulation

Our field survey showed that the quality of shallow groundwater around the Katsura River in the Kyoto Basin was strongly affected by the infiltration of river water. Furthermore, that the deterioration of the groundwater in the southern area to the west of the Katsura River may be related to the increase in groundwater extraction. To clarify the mechanism of groundwater deterioration, we have developed a stochastic method to simulate groundwater flow. The results showed that there was a large reduction in the groundwater level where groundwater extraction was intense and recharge flowed from the Katsura River to the high extraction areas in the southern region. Another simulation showed that if the groundwater extraction was 10% of the present removal rate, there would be little recharge from the Katsura River into the groundwater and the quality of the groundwater would be improved. Thus, we conclude that the cause of groundwater deterioration is probably due to the induced recharge of deteriorated river water from the Katsura River.