Decrease in herbicide concentrations and affected factors in lagoons located around Lake Biwa.
The contamination levels and changes in the concentrations in four lagoons around Lake Biwa of paddy-use herbicide were studied. Four lagoons, Sone-numa (52 days of HRT (hydraulic residence time) estimated from the lagoon volume and the average discharge at the outlet, 21 ha area), Yanagihira-ko (40 days, 5.0 ha), Noda-numa (11 days, 6.0 ha), and Iba-naiko (2 days, 55.5 ha), were selected as monitoring sites. Intensive water sampling was carried out once a week from May to June at the outlet of each lagoon. Although twelve of the monitored herbicides were detected, the maximum concentrations did not exceed the guidelines for water-supply law in Japan. The relation between half-lives in herbicide concentrations and characteristics of a lagoon such as HRT and chlorophyll-a concentrations were examined. The shorter half-lives of herbicide concentrations in lagoons with shorter HRT means that replacement by influent water effectively decreased the pesticide concentrations. Shorter half-lives in lagoons with high chlorophyll-a concentrations between the lagoons with similar HRT suggest that biological degradation during the residence time worked more efficiently in the lagoon with high chlorophyll-a concentrations.
Sudo
M
M
School of Environmental Science, The University of Shiga Prefecture, 2500 Hassaka Hikone, 522-8533, Japan. sudo@ses.usp.ac.jp
Nishino
M
M
Okubo
T
T
D002734:Chlorophyll / Q000032:analysis; D000077194:Chlorophyll A; D017753:Ecosystem; D004784:Environmental Monitoring; D006540:Herbicides / Q000032:analysis*; D007564:Japan; D010573:Pesticide Residues / Q000032:analysis; D010575:Pesticides; D012621:Seasons; D013997:Time Factors; D014867:Water; D014872:Water Movements; D014873:Water Pollutants; D014874:Water Pollutants, Chemical / Q000032:analysis*
D006540:Herbicides; D010573:Pesticide Residues; D010575:Pesticides; D014873:Water Pollutants; D014874:Water Pollutants, Chemical; D014867:Water; D002734:Chlorophyll; D000077194:Chlorophyll A