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Effects of discharge level on the load of dissolved and particulate components of stream nitrogen and phosphorus from a small afforested watershed of Japanese cypress (Chamaecyparis obtusa) Abstract The behavior of dissolved and particulate components of stream nitrogen (N) and phosphorus (P) were measured for 2 years in a small mountainous watershed covered primarily with a plantation forest of Japanese cypress (Chamaecyparis obtusa). The load of dissolved N and P bore a consistent relationship to discharge while the load of particulate N and P varied by up to two orders of magnitude at a given discharge level. Most N was exported in a dissolved form (DN) while most P was exported in a particulate form (PP), which bears similarity to loads from agricultural watersheds. Owing to the different behaviors of DN and PP, changes in the total nitrogen (TN) load were primarily attributed to variations in discharge unlike changes in the total phosphorus (TP) load. High fl ow conditions, resulting from heavy rainfall, displayed PP release significantly larger than expected. The TP load in high fl ow conditions was severely underestimated using a regression equation expressed as a function of discharge, which was based on the weekly sampling data biased toward low fl ow conditions. In addition, the TN load during peak discharges in heavy rain events was underestimated by the regression equation because of unpredictable increases in the particulate component. Our study shows that the particulate component ratio determines whether discharge can explain changes in load regardless of chemical species. The results suggest that plantation forests in rainy regions can be a diffuse source of particulate nutrients depending on soil surface conditions.

Runoff characteristics of particulate pesticides from paddy fields have been intensively observed in the Koise River in Japan. The 8 pesticides that are applied to paddy fields were analyzed in both particulate and dissolved forms. The concentrations and the detection frequencies of particulate pesticides were lower than those of dissolved pesticides. The particulate pesticide concentrations in the river water were evaluated based on the soil sorption coefficient, particulate organic carbon concentration, and dissolved pesticide concentrations. The particulate pesticide concentrations in the river were higher than evaluated concentrations because the paddy soil contained more pesticides than did suspended solids in the river water discharged during rain events, and because the desorption rates of pesticides were slow. In observations made during rains, the particulate pesticide concentrations increased with the increases in both the discharge rate and the concentrations of suspended solids. The particulate loading was slight compared with dissolved loading, but particulate pesticides may be influenced by enclosed areas such as a lake or estuary because under such conditions particulate matter settles vertically and the pesticide decomposition rate in sediment is slow compared with that in water.

Patterns of runoff behaviours in concentration and loading of pesticides from paddy fields have been intensively observed in the catchment area of Rikimaru Reservoir, mainly during the regular application period of fungicide during the summer. The runoff characteristics of three pesticides (pyroquilone, isoprothiolane and flutolanil) from paddy fields to the drainage river differed from one another due to their water solubilities. The linear correlation between the water solubilities and the amounts of pesticide runoff loadings from paddy fields to the drainage river and reservoir during dry weather days was confirmed as good on log-log paper. To reduce the risk to raw water for drinking in the reservoir the method of irrigation water management for paddy fields should be improved.

Based on the horizontal and vertical distributions of pesticides in the river and the reservoir, the fates of pesticides in these systems were investigated in this study. Four pesticides, i.e., MEP, thiobencarb, BPMC and isoprothiolane had the highest concentrations among the eighteen pesticides. These pesticides in suspended components except isoprothiolane were detected not only during stormy weather days but also during dry weather days. The percent detection of pesticides in suspended component was 59.8-61.9% in MEP, 34.6-38.9% in thiobencarb and 17.0–19.0% in BPMC. However, isoprothiolane in the suspended component was undetectable. These pesticides in the suspended component have existed as a part of the suspended solids in the river, then disappeared by sedimentation during the flow of the river from the upper stream to the backwater in the reservoir. On the other hand, the pesticides in the dissolved components that flowed into the reservoir were uptaken or adsorbed by phytoplankton. Moreover, the concentrations of the less absorptive pesticides like isoprothiolane have decreased by dilution and diffusion in the reservoir.

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The concentrations of phosphorus determined by spectrophotometric analysis (soluble reactive phosphorus; SRP) and ion chromatography (orthophosphate only) were compared in fresh water from inflowing rivers of Lake Biwa (Shiga, Japan) and its outflow (Seta River). The high-volume sample injection and high-capacity analytical column used for ion chromatography enabled detection of orthophosphate ions at levels below 10 nmol/l. In river waters sampled from inflows to Lake Biwa, the ratios of orthophosphate to SRP ranged from 0.06 to 0.79, and varied considerably between rivers. This result shows that the fractional determination of dissolved phosphorus in natural waters is important for understanding its bioavailability.

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