Environmental behavior of bentazon herbicide.
Bentazon is a postemergence herbicide used in early spring to early summer in many crops, usually at application rates of 1.0 kg a.i./ha. Its selectivity is based on the ability of the crop plants to metabolize bentazon quickly to 6-OH- and 8-OH-bentazon and conjugate these with sugars, while weeds do not, so that photosynthesis is disrupted and the weeds die. Also, there is a further degradation to small fragments, which subsequently are incorporated into natural plant products. Residues in the raw agricultural commodities range from 0.1 to approximately 1.0 mg/kg (straw and other "leftover" plant parts). In the upper soil layer, bentazon is quickly degraded, microbially and aerobically, via the intermediary and instable products 6-OH-, 8-OH-bentazon, and AIBA. These are immediately bound biotically and abiotically to the mostly nonbioavailable soil organic matter fraction. Additionally, a considerable part (24-50%) is mineralized to CO2. Half-lives in field soils ranged from 3 to 21 d with an average of 12 d. Abiotic degradation processes predominantly involve photolyses on plant and soil surfaces and in surface water. The physical-chemical properties and soil column laboratory studies with bentazon would seem, at first glance, to predict a leaching potential. However, several field lysimeter studies unambiguously proved that it does not leach under field situations. Annual averages in the leachates were always < 0.1 microgram/L, also after the second year. Reasons for the favorable field behavior, in contrast to the laboratory studies, are discussed. Reports on bentazon findings in groundwater and drinking water were classified as resulting predominantly from former filtrate along Rhine banks. Since 1988, the bentazon levels in the Rhine River ranged below or near 0.1 microgram/L. Very few isolated point-source contaminations, arising from accidents and other reasons, are marked by sporadic findings of concentrations > 1 microgram/L. Nonvalidated findings at various locations are reported that lie below or near the determination limit. In such situations, it is highly recommended to identify bentazon with a "full" MS-spectrum. The ecotoxicological effects of bentazon lead to "no classification necessary." The small octanol/water partition coefficient precludes bioaccumulation. Bentazon is rapidly excreted by warm-blooded animals without any uptake of residues in edible tissues. Based on its toxicological properties, bentazon was classified as noncarcinogenic ("Group E") by the EPA. The ADI is set at 0.1 mg/kg body weight/d. The WHO drinking water guideline value, based on the toxicological profile of bentazon, was recently raised to 30 micrograms/L.(ABSTRACT TRUNCATED AT 400 WORDS)
Huber
R
R
BASF Aktiengesellschaft, Agricultural Research Station, Limburgerhof, Germany.
Otto
S
S
D000818:Animals; D001581:Benzothiadiazines / Q000737:chemistry / Q000378:metabolism* / Q000493:pharmacokinetics / Q000633:toxicity; D004787:Environmental Pollution; D006207:Half-Life; D006540:Herbicides / Q000737:chemistry / Q000378:metabolism* / Q000493:pharmacokinetics / Q000633:toxicity; D007928:Lethal Dose 50; D010573:Pesticide Residues / Q000378:metabolism; D010944:Plants / Q000378:metabolism; D012989:Soil Pollutants / Q000378:metabolism; D014877:Water Pollution, Chemical
D001581:Benzothiadiazines; D006540:Herbicides; D010573:Pesticide Residues; D012989:Soil Pollutants; C002003:bentazone