The mobility of the herbicide diuron and its main soil metabolites (DCPMU, DCPU, mCPU, DCA, mCPMU, and mCPDMU) were studied under non-steady state flow conditions in two undisturbed sandy soils exposed to northern European climatic conditions. Leachate was collected from two lysimeters of each soil in which black currant bushes were planted during a 26-month period following the application of diuron in April, 1993. The herbicide was applied as [phenyl(U)-14C]diuron at two rates, i.e. 2.0 and 4.0 kg active ingredient (ai)/ha, representing single (1x) and double (2x) the maximum use rate of the compound for weed control in Scandinavian bush plantations. Potassium bromide was applied over the soils when diuron was applied to provide information on the drainage characteristics during the experimental period. All lysimeters received supplemental irrigation to simulate reasonable worst-case conditions with regard to pesticide leaching in Scandinavia.
When applied at the normal rate (1x), only five leachate samples (out of 59 analyzed) contained diuron concentrations exceeding 0.1 ug/L, and none exceeding 1 ug/L. Application at the doubled rate (2x) resulted in ca. 30% of the analyzed samples exceeding 0.1 ug/L. The highest concentrations were associated with flow-events of relatively short duration during winter. During spring each year, when the largest amounts of leachate discharged, diuron concentrations were in most cases considerably below 0.1 ug/L. This suggests that, after the soils were completely thawed in spring, the amount of diuron, readily available for leaching, were extremely small. It also implies that leaching of diuron in the true sense of the word is very limited, and freeze-thaw periods during winter accelerate leaching. A similar leaching pattern was observed for DCPMU and DCPU (the only major metabolites found at detectable concentrations). A plausible explanation would be that on freezing, the pesticide redistributes to the smaller unfrozen pores. During subsequent thawing periods, the percolating water, which is moved more rapidly through the larger pores, transports only what is not protected in the small pores and depletes the soil solution of diuron. Later on during spring, when the bulk of water from melted snow percolates through the profile, leachate will be free from pesticide because it is completely sorbed and protected in the small pores. There were no major differences in leaching pattern between the two soils, but relatively large differences occurred between the two application rates. The total leaching loads of diuron over the 26-month period reached 0.24 g ai/ha in 1x treated lysimeters and 1.07 g ai/ha at the 2x rate. These amounts represent 0.012 and 0.027 % of the applied active substance, respectively. Leaching of DCPMU and DCPU was also very small. The dominating constituents of the recovered radioactivity in leachate were CO3= and other more polar constituents than diuron or its major metabolites. Leaching of bromide through the soils indicated that approximately two pore volumes of water had leached through the profiles after 26 months. In contrast to diuron, the bromide concentrations decreased drastically on a few occasions during winter. This clearly indicates that the two solutes were partly located in different pore systems, with different exposure to leaching during critical winter flow-events.
In conclusion, the results obtained in this study showed that most of the diuron leaching occurred in partly frozen soil during winter. Still, the leached amounts were very small, indicating that there is little risk of diuron contaminating groundwater at currently labeled rates.
Lars Bergstrom Swedish University of Agricultural Sciences Department of Soil Sciences P.O.B. 7072, S-75007 Uppsala, Sweden Phone: +46-18-672463 Fax: +46-18-673430 E-mail: Lars.Bergstrom@mv.slu.se