Katsuaki Komai

INFLUENCE OF COASTAL GROUNDWATER ON BRACKISH WATER ENVIRONMENT IN A TIDAL ESTUARY

INTRODUCTION Understanding the hydrological dynamics of brackish water bodies is considered important for the effective conservation and management of the natural environment of tidal estuaries. Especially, fresh water inflows and residence times have a marked effect on the salinity within an estuary. Residence time, which refers to the mean time required to transport dissolved or suspended matter out of a water body, can be one of the limiting factors affecting phytoplankton abundance within a system (Monbet, 1992). Howarth et al. (2000) reported that a decrease in river inflows results in increased residence times and primary production in estuaries. Moreover, since the settlement and deposition of suspended matter is dependent on residence time, short residence times are considered to improve the water quality of estuarine water. A box model using salt as a conservative tracer is often applied to estimate both fresh water inflows and residence time within a system, both of which are considered important indicators of the physical condition of the estuarine environment. However, while the box model is simple in principle, the cases to which it can be applied are relatively limited, particularly in tidal estuaries because of its non-steady flow. Tidal flats along the periphery of tidal estuaries are important habitats for a variety of coastal organisms, particularly as nurseries for fish. The tidal flats of Ota River Estuary support numerous benthic fauna (16 Eucrustacea spp., 14 Pelecypoda spp. and Gastropoda spp., and 6 Polychaeta spp.) (Hibino et al., 2006). The salinity and dissolved oxygen concentration of the pore water in these areas and the particle size of the bed material has been shown to restrict the habitat preference of these species (Lalli and Persons, 1993). Submarine groundwater discharge is also important for material circulation in coastal region (Simmons, 1992; Moore, 1996; Burnett, 2003). Nonetheless, the groundwater environment of tidal flats is relatively poorly understood.

Investigation of Water Exchange Through Riverbeds in Tidal Rivers

The longitudinal profile of salinity near the riverbed and the measuring method of water exchange through the riverbed in a tidal river were investigated. Observation of the Ota River Floodway showed that high salinity water masses occurred sporadically on the riverbed from the middle to the upstream reaches. The groundwater discharge through the riverbed was successfully estimated and was verified by the observed salinity of groundwater below the riverbed. The calculations provided evidence that water exchanges through the riverbed ranged from 10−3 cm/s. Water exchange was significant at the 30 cm below the riverbed surface.

EFFECT OF THE INFLOWING RIVER WATER THROUGH THE SOIL ON THE GROUND WATER IN TIDAL FLAT

Salinity distribution was observed under the ground surface of the tidal flat formed at middle part of Ohta River Flood Control Channel, and the mechanism of salinity retention under the ground was considered. The measurement method of salinity under the ground was developed and the detail ground water quality profiles were measured. Furthermore, to clarify the supply mechanism of river water into the tideland ground, the characteristics of salinity variation and current field were discussed by using the continuous measurements of salinity and flow under the ground. It can be understood that intrusion into the ground of river water is limited under the condition that the ground water level has remained at the ground surface level, and the tideland environment depends on the ground water quality.