Characteristics and the seasonal changes in water quality of small rivers in a rural agricultural catchment area

Monitoring of water quality on dry weather days was carried out throughout one year to learn the characteristics and the seasonal changes in water quality of small rivers in a rural agricultural catchment area. The river is about 2.5 km long (catchment area; 292 ha) and finally reaches Lake Kasumigaura through the Kawamata River, and then the Koise River. For comparison, monitoring of the quality of mountain stream water, ponding water of a paddy field, irrigation water piped to the monitored river from the Lake Kasumigaura and domestic waste water flowing into the river from an agricultural community were also carried out. The following water characteristics were analyzed: flow rate of the river, SS and pH, EC, IC, DOG, COD, OD260, Si, NH sup(4) sup(+), Na**+, K**+, Mg sup(2+), Ca sup(2+), NO sub(3) sup(-), SO sub(4) sup(2-), Cl**-, Br**- and P0 sub(4-)P for the filtrate of under 1.0 micro m. 1) The quality of river water in the non-irrigation season was supposed to be influenced by both the inflow of domestic wastewater and the cultivated upland field, and EC and concentration of SS and almost all of the components were increased by flow down from the mountain stream to the middle reaches. However, the changes in water quality of each sampling point were not very large except for NO sub(3) sup(-) and PO sub(4-)P. 2) During the irrigation season, the water quality of each sampling point was influenced fairly strongly by inflow of Lake Kasumigaura water resulting in increased concentration of DOG, COD, Cl**- and Br**-, and in greatly decreased concentration of Si. 3) A higher concentration of DOG, COD and SO sub(4) sup(2-) was observed in ponding water, particularly in soil puddling and rice transplanting time. Concentration of HCO sub(3) sup(-) was decreased soon after irrigation started, then at soil puddling and rice transplanting time, probably by lowering of the water pH. 4) Correlation among the 19 analyzed components was calculated and tabulated as a correlation coefficient matrix. 5) Cation and anion composition were calculated by the equivalent concentration for the non-irrigation season. By using the composition, the characteristics of the each water from different sampling points and the changes in the composition by flow down to the lower reaches were discussed. 6) Daily and annual out-flow loadings at the lower reaches of river water for SS and other components were calculated.