EVALUATION OF NUTRIENT LOADING AMOUNT DUE TO SUBMARINE GROUNDWATER DISCHARGE －A CASE STUDY OF IMAZU TIDAL FLAT, FUKUOKA－

Abstract

 Located in the western part of Fukuoka prefecture, the Imazu tidal flat’s main inflowing river is the Zuibaiji River. The interior of the tidal flats is an ecologically precious environment, it house’s many threatened species such as horseshoe crabs and black-faced spoonbill. Moreover, Imazu Bay’s closure indicator is high & affects the closure property of the sea leading to a high likelihood of eutrophication occurrence. In order to preserve ecologically precious environment and prevent eutrophication, this study aims to estimate the nutrient loading amount flowing out through groundwater and stream from the northeastern part of Imazu tidal flat. In this study, the nutrient loading is calculated by multiplying the groundwater discharge or the streamflow by the nutrient salt concentration and, groundwater discharge and streamflow were calculated by analyzing the water balance in the study area. After analysis, the distribution range of groundwater flowing toward Imazu tidal flat were calculated . In addition, ground and stream water were sampled from six observation points in the area and their nutrient concentrations were analyzed. Since agriculture is conducted near some of the observation points, high nutrient concentration, and groundwater contamination by fertilizers was predicted in three points. In two observation wells, water level was continuously measured in addition to taking water quality samples from all six observation points. As a result of comparing the water level of the observation well with the tide level, groundwater level seems not to be affected by the tide level. In addition, nutrient loading amount calculated in the study area and measured in the sample’s analysis is considerably small compared with the amount of nutrient loading flowing from the Zuibaiji River.