|Reducing Pesticide Residues
in Drainage Waters - 84
DEGRADATION AND MOVEMENT OF PESTICIDE RESIDUES
The rice industry, over the last 10 years, has supported research on the degradation and movement of pesticides used to control weeds, diseases and invertebrate pests. The objectives of these studies are to discover the environmental factors that govern the movement and fate of pesticides, determine their relative importance, and apply this information to improved management of pesticides. This information is essential for meeting regulatory requirements, increasing pesticide efficiency, and decreasing side-effects. Each pesticide responds differently to environmental forces that cause its movement and degradation. In some cases, the degradation products may cause environmental problems.
MCPA studies show that sunlight is the principal degradation force and that this herbicide is broken down rapidly in field water and spray. Reports that the ozone in the lower atmosphere might be important in degradation of pesticides have been proven to be untrue. Very low levels of MCPA were detectable on orchard foliage immediately following spraying of adjacent rice fields, but there has been no permanent injury to the orchard crops.
Analytical methods with sensitivity, of 1 ppb and greater than 98 percent accuracy have been perfected for the detection and measurement of Ordram in water. Field dissipation rates were essentially the same as those obtained from volatilization in a microcosm (model ricefield). In both the laboratory chamber and monitored fields, the peak concentration of volatilized Ordram was delayed about 12 hours after treatment, and the half-life varied from 54 to 144 hours depending on temperature and wind. The rapid volatilization of Ordram has led to slow release formulations that reduce its dissipation into the atmosphere.
Bolero was found to have a slightly slower volatilization rate than Ordram, reaching its peak at 44 hours and a half-life of 173 hours during a cool spring period. In a previous year, the half-life was only 108 hours (4.5 days). Sacramento, which draws its water from the Sacramento River, has been concerned with an off-flavor believed to have been caused by Bolero, but no herbicides or degradation products have been detected in treated water, and the source of the off-flavor still is unknown.
Studies show that Duter does not degrade rapidly enough to avoid toxicity to fish. Therefore, it has not been approved for use in rice fields.
One of the most active and promising research activities involves finding procedures to intentionally degrade herbicides as water moves out of treated rice fields. Several chemicals have been found that can oxidize Ordram and Bolero in a matter of minutes. For example, zinc oxide catalyzes the sunlight oxidation of Ordram and Bolero to give a half-life of only 30 minutes. The degradation products have been identified and are not persistent. Practical procedures and costs for using additives to enhance natural pesticide oxidation are being investigated.
1984 RICE HERBICIDE PROGRAM1
The 1984 rice herbicide program was developed to reduce the movement of the herbicides Ordram and Bolero from rice fields into drains and the Sacramento River. The plan was designed by the Department of Food and Agriculture in cooperation with the Water Resources Control Board, and the Departments of Health Services and Fish and Game.
Additional restrictions were placed on the use of Ordram and Bolero, the amount of Bolero sold by the manufacturer was reduced and a conditional registration of one year was made for Bolero. Ordram and Bolero were classified as restricted materials. Permits were required by rice growers from the county agricultural commissioner to use these herbicides. Growers also were required to notify the commissioners before and after each application so that compliance with use restrictions could be checked. Growers were required to hold water treated with Ordram for eight days and water treated with Bolero for six days. The water either had to be contained within the rice field, on other land controlled by the grower, or in water recycling systems approved by the county agricultural commissioner.
Action levels of residues found in drains and rivers were set at 90 ppb for Ordram and 24 ppb for Bolero. Levels for tap water were set at 20 ppb for Ordram and 10 ppb for Bolero. A monitoring program was established to determine if the action levels were being exceeded. Water samples were taken at 26 locations and at various times by the Departments of Food and Agriculture and Fish and Game, the State Water Control Board, the Central Valley Regional Water Quality Control Board and the City of Sacramento.
Total rice acreage in the valley was estimated at 510,400. Ordram application permits were issued for 362,310 acres and Bolero permits for 82,390 acres. Except within a special study site, residues of Bolero never exceeded the Department of Fish and Game action levels. Levels of Ordram were significantly below the levels in previous years. Figure 18 compares the levels of Ordram in 1982 and 1984. The amount of Ordram used in these two years was very similar. No significant fish kills due to residues of rice herbicides were reported and neither Ordram nor Bolero was detected in tap water. Sacramento treated its water with potassium permanganate for approximately one month, and fewer complaints of "off taste" were received than in years when no treatment was made.
There were 4,599 applications by rice growers for use of Ordram and Bolero in the Sacramento valley and only 15 incidents of violations reported, most of which were single field incidents of an accidental nature.
Use of water recycling systems have probably contributed significantly to the improvements in water quality in drains and rivers in 1984. By the end of the season, 35 systems were in use covering 43,837 acres or 8.8 percent of the rice acreage in the Sacramento Valley.
Results of the 1984 Rice Herbicide Program are very encouraging, particularly with reductions in Ordram residues. Grower cooperation has been very good.
1This information was provided from a report prepared by the State Department of Food and Agriculture, Division of Environmental Monitoring and Pest Management.