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| NIRE Annual Report |
| 1998 |
Discharge Process of Nutrients from Sandy Beach through Groundwater to Coastal WatersInter-Spheric Environment DivisionEnvironmental Assessment Department |
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Objectives
Shoreline environments are the areas where much amounts of groundwater having rich nutrients are discharged from the hinterland. In some cases, shoreline environments are the sources of nutrients to coastal waters. The observations have been made in the sandy beaches faced to Tokyo Bay as a typical shoreline environment.
In the sandy beach, the fresh-groundwater flows and mixes with the seawater intruding from seashore. We found many strange phenomena different from an estuary.
Results
The seasonal observations were made for river qualities at the mouth of river and groundwater qualities at the starting point (D in Fig.1) of sandy beach of Futtsu in a small watershed faced to Tokyo Bay. The ratios of discharge of NO3-N to coastal waters are calculated as 55~99% from the river and 1~45% from the sandy beach having 3.4 km in length through groundwater, and the ratios of discharge of PO4-P are calculated as 90~100% from the river and 0~10% from the sandy beach.
Fig.2 shows the disitibutions of qualities of seawater and groundwater in the sandy beaches at two different observation lines (I,II) of Futtsu and one observation line of Miura. The concentrations of dissolved NO3-N, PO4-P and COD (DCOD) et al of the groundwater are gradually decreased to seaward. But, they have the maximum concentratins at a very low chlorinity area where fresh-groundwater and seawater intruding from the shoreline mix. After that, these concentrations are decreased mainly by tidal mixing in the intertidal zone. These phenomena are universally found at another fields also.
The reasons of maximum concentration are clearly not revealed at present time. Maximum points of NO3-N and DCOD are nearly the location (x=-25m) where suspended matters in coastal waters are washed ashore.
