We have developed a multi-sensor, salinity/temperature measurement system based on the XCTD
(Expendable Conductivity, Temperature & Depth Profiling System) and applied it to an observation
well for a groundwater survey in a salt intrusion area. Five CT sensors made by modification of
XCTD probes were connected by a cable, and the distances between sensors were adjustable. The
cable was 50m long and connected at its end on the ground to a measurement control and data
logging system consisting of a notebook computer and peripheral components. In addition, a
pressure sensor for measuring fluctuation of water level in the well was included in the system.
Observation experiment using this system was carried out in the observation well of the 50m
depth in the embankment (about 8.7km upstream from the river mouth) of the River Gonokawa
located in the western part of Shimane Prefecture, Japan. We obtained conductivity, salinity,
temperature and water-level data every 5 minutes for about two months.
By the long-term observation of salinity and water temperature by the CT sensors, it was
clarified that the fresh-salt water interface in the groundwater was fluctuating with time.
Furthermore, there was a correlation between fluctuation of the ground water level and
water-level/flow rate of River Gonokawa.
Saline wedge invasion in the Gonokawa River was observed over lengthy periods in summer
and winter in 1998. Here we discuss the observed hydrological data from an hydraulic analysis
viewpoint. Saline wedge behavior was captured precisely using newly developed observation
systems which measured salinity, temperature and identified the interface between fresh and salt
water by sonic echo. According to these observations, the length of the saline wedge decreased
when the river discharge increased. In other words, the saline wedge moved downstream as the
fresh water velocity increased. This showed that the river comprised a two-layer laminar flow
with a viscous interface. Wecalculated the length of the saline wedge using specific parameters
including depth to the interface, temperature and salinity. The results accorded well with the
observed data, and a formula to evaluate the interfacial resistance coefficient incorporating the
Keulegan number was proposed. We have also measured the invasion speed of the saline wedge.
Simulations were carried out to estimate the invasionspeed, but the results did not agree with the
observed phenomena, because the simulation model assumed behavior as a perfect fluid.
Seasonal variation of carbon of Bacteria, Synechococcus, ANF,HNF,Phyioplankton, and Zooplankton
were investigated in Ise Bay, in 1995. In addition, estimated production rates of Bacteria,
Synechococcus, phyioplankton, and grazing rates of HNF, zooplankton. Using these results, we
approximate production rates of each food chain. In this place, we categorized lOum or less
phytoplankton Bacteria Synechococcusand ANF as a producer of Microbial food chain, and cat
egorized other producers as a producer of grazing food chain. Then, we compared and examined
that interrelationship between each microorganism and environment factor.
As a result of this research, water temperature and grazing pressure by HNF were important as
change factor of Bacteria, and grazing pressure of HNF and Oikopleura were important as
Synechococcus. The microbial food chain/grazing food chain production ratio was 0.66 on the
whole, grazing food chain was more important, but microbial food chain was more important in
September and December. On the other hand, dominant species of the zooplankton were Penilia
avimstris (CLADOCERA), and Bivalve larva and Oithonadavisae (COPEPODA) when produc
tion of Bacteria, Synechococcus, etc. was the highest in summer. P. avirostris and Bivalve larva
are herbivorous pico- nano, nanoplankton feeder, and O.davisae is omnivorous nano-microplankton
feeder. The grazing rates of these dominant species occupied 86% (in August, and the whole
67%) to the grazing rates of the whole zooplankton, and the seasonal variation was similar to
change of production rates of Microbial food chain. These results suggest production of a Micro
bial food chain links to production of zooplankton. On the other hand, zooplankton population
and that species composition were seasonal changing so that food can be used effectively.