This summary is based on a draft report titled "1987 Program to Prevent Off-Site Movement of Molinate and Thiobencarb from California Rice Fields," dated October 9, 1986, and prepared by the Department of Food and Agriculture.
The principal herbicides in the 1986 management program were molinate (Ordram) and thiobencarb (Bolero). Maximum concentration of Ordram recommended by the Department of Fish and Game in the Sacramento River, its tributaries and associated drains was 90 ppb. Maximum Bolero concentration was set at 24 ppb. These standards were based on chronic toxicity studies with aquatic species.
The Department of Health Services recommended primary action levels of 20 ppb for Ordram and 10 ppb for Bolero to protect public health. These action levels were defined as the sum of the concentration of the parent compound and the oxidation products resulting from water treatment that may be found in the drinking water. In 1985 the department stated that the concentration of the parent compounds at the raw water intake could be used as a substitute for this sum. Howev- early June. er, it stated that concentrations of Bolero greater than 1 ppb at the intake may result in taste problems in drinking water after the water is chlorinated. Treatment with potassium permanganate may be required when concentrations exceed this level.
Regulations established for 1986 for management of these two herbicides were considered adequate to meet these standards. Ordram was applied on 272,822 acres and Bolero on 81,121 acres. The Ordram-treated acreage was 7 percent below that of 1985 and 25 percent below 1984. Bolero applications were limited not only by the amount sold (enough to treat 80,000 acres) but as to location where it could be used.
Dilutions of rice herbicides discharged into the Sacramento River were low
in May and June because river flows were quite low. Even with this
limitation, the 1986 program for controlling movement of Ordram and Bolero
was very successful. Peak concentrations of both herbicides were the lowest
since 1980. Ordram concentrations for 1982, 1984, 1985 and 1986 are shown in
Figure 1. No fish kills that could be attributed to herbicides were observed
in the agricultural drains. Reclamation District 108 used 965 acres for
ponding rice field discharge and avoided any discharge into the river during
the critical period of late May and early June.
 |
| Figure 1. Ordram concentrations in the Colusa Basin Drain. |
Concentrations of Ordram in the Sacramento River were lower overall than in 1985, but on May 26 at the intake for Sacramento city water, a concentration of 13.6 ppb was detected. Concentrations of Bolero were much lower at the intake than those observed in 1985. The 1986 secondary action level of 1 ppb recommended to prevent taste complaints was exceeded on only one day (May 26). In 1985, the 1 ppb level was exceeded for 19 consecutive days.
Sampling of river and drain systems for pesticide residues in 1986 provided information on the sources of peak residue levels. This information was helpful in designing the 1987 program by the Department of Food and Agriculture.