The spline technique, a kind of interpolation, is introduced to estimate the horizontal distribution of water quality in a lake. Before application, the termination condition, a parameter of SOR method and a spline parameter are discussed, and their optimum conditions or values are selected from the viewpoint of the accuracy and the rapidity of the interpolation calculation. The spline thecnique applied to the field data in the Southern Basin of Lake Biwa produces a good result in the estimation of the two-dimensional distribution of water quality and its seasonal variation. The effect of each monitoring station on a curved surface interpolated by this technique is evaluated. The rational allocation method of the monitoring stations on the basis of this evaluation is proposed.

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Occurrence and behavior of trihalomethanes (THMs) and total organic halides (TOX) precursors along the Tsurumi River were investigated.
From the results of behavior of THMs and TOX precursors obtained by measuring THMs and TOX formation potentials after chlorination of the Tsurumi River, it is represented that amounts of THMs and TOX formation potentials are different at each sampling stations, especially high THMs and TOX formation potentials at the down stream near the estuary are caused by higher formation of bromide compounds because of the presence of bromide ion derived from sea water.
It is recognized that TOX formation potential per unit amount of TOC is higher in the down and upper streams than that in the middle stream, meanwhile component ratio of THMs formation potential against TOX formation potential is lower in the middle and upper streams than that in the down stream.
Differences of THMs formation potential patterns when varing pH and chlorine concentration on chlorination and IR spectra patterns of the residues demonstrated that chemical forms of THMs and TOX precursors are different at each sampling stations in this river.

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