The purpose of this paper is to point out the effectiveness of strainer well observation to obtain information on water quality of different groundwater units. A 30 m deep well drilled through Tertiary sedimentary rock and Cretaceous granite in Kanamaru area was observed to draw water quality profiles under various hydrological conditions. In dry season stepwise change of well-water quality, that is, increase of pH and electric conductivity, and decrease of dissolved oxygen and Eh with increasing depth, is observed. On the other hand, in wet season especially in snow melting season all of well-water column has characteristics of near surface water down to the bottom.
The difference of vertical distribution of well-water quality depending on seasons can be explained as follows. In dry season all of the groundwater units have similar hydraulic heads bringing little intra-well vertical flow, which makes similar water quality distribution in intra-well water column to that in surrounding bedrocks. In wet season hydraulic head of the shallower unit exceeds that in the deeper ones bringing downwards intra-well flow, which makes most well-water column replacing to the near surface water.
It is generally thought to be very difficult to obtain reliable information of each water quality for several different groundwater units from a simple strainer well. The reason is that different hydraulic heads possibly kept by each groundwater unit cause vertical infra-well flow that brings severe disturbance of water quality distribution in well-water column.
As long as obtaining water quality profiles logged under several different hydrological conditions combined with reasonable hydrogeological model, it can be possible to get much more reliable information on water quality of independent groundwater units.
