Presence of Soil-Applied Herbicides
in Three Rights-of -Way Infiltration Basins
in San Joaquin County, California
EH 94-01

ABSTRACT

Rights-of-way infiltration basins are natural or excavated depressions in the ground used to collect highway storm water runoff in order to provide adequate roadway drainage. Water collected in the basins is disposed by infiltration into the soil and by evaporation. Storm runoff which has moved across roadsides recently treated with soil-applied herbicides may transport herbicide residues to basins, where there is the potential for herbicide transport to ground water by soil infiltration of accumulated water. This study was conducted to determine the presence of soil-applied herbicide residues in water and soil samples at three Caltrans infiltration basins in San Joaquin County. Water samples were collected from an inflow canal delivering runoff to a basin, from basin storage, and from basin drywells. Soil at each basin was sampled at 1-foot increments. Maximum herbicide concentrations detected in water sampled from the inflow canal were diuron at 1.42 ppb, oryzalin at 27.40 ppb, oxyfluorfen at 5.62 ppb, and simazine at 83.47 ppb. Maximum concentrations detected in water sampled from basin storage were bromacil at 25.62 ppb, diuron at 36.32 ppb, oryzalin at 12.50 ppb, oxyfluorfen at 3.47 ppb, and simazine at 78.80 ppb. In water sampled from basin drywells, maximum concentrations were diuron at 0.46 ppb, oxyfluorfen at 0.28 ppb, and simazine at 17.88 ppb. Simazine and diuron had not been applied to the highway right-of-way in the Peltier basin drainage area and probably originated from agricultural land. The only herbicide detected in basin soil was simazine at 36.0 ppb. Detection of herbicide residues in water sampled from storm runoff flowing into a basin, and in water sampled from basins and basin drywells, indicates a potential for the herbicides to be transported to ground water by soil infiltration. Detection of herbicides other than those used exclusively for rights-of-way weed control suggests a contribution of herbicide residues from adjacent properties.


EXECUTIVE SUMMARY

PURPOSE

This study was conducted by the Environmental Monitoring and Pest Management Branch of the Department of Pesticide Regulation (DPR) in cooperation with the California Department of Transportation (Caltrans) to investigate the presence of herbicide residues in rights-of-way infiltration basins. The objective was to determine the presence of soil-applied herbicide residues in: 1) water flowing into basins, 2) water collected from basins, and 3) soil sampled from basins.

BACKGROUND

The disposal of highway storm water runoff is a growing problem within densely populated areas of California. Urbanization of rural areas along major freeways and interchange systems has increased runoff and eliminated natural drainage courses. Collection of runoff water in infiltration basins has proved to be a very effective and practical approach. Infiltration basins are natural or excavated depressions in the ground that collect highway storm water runoff to provide adequate roadway drainage. The large storage capacity of basins is essential where high-intensity rainfall and large volumes of runoff occur, especially in areas of flat terrain and limited natural water channels. Water collected in the basins is disposed by infiltration into the soil and by evaporation.

Soil application of preemergent herbicides during the fall and winter is the most widely-used practice for rights-of-way vegetation management in California. Preemergent herbicides are usually applied to rights-of-way soon after the beginning of the fall rainy season in order to avoid application to dry, dusty soil surfaces and to allow rainfall to move the herbicide into the upper layers of soil before weed seed germination. Storm runoff which has moved across roadsides recently treated with soil-applied herbicides may transport herbicide residues to infiltration basins, where there is the potential for transport to ground water by soil infiltration.

This study was conducted from January through May, 1992 at three Caltrans infiltration basins in San Joaquin County. The selection of pesticides to be tested for was based on herbicides applied to the highway right-of-way and to neighboring properties. Water samples from the Peltier basin along Highway 99 near Lodi were analyzed for diuron, oryzalin, oxyfluorfen, and simazine; and soil samples were analyzed for diuron,oxyfluorfen and simazine.

Residues in this basin may have been transported from applications to ROW or adjacent properties. Water and soil samples from two basins along Highway 120 near Manteca were analyzed for bromacil, diuron, and simazine, which could have only moved from rights-of-way applications.

STUDY METHODS

Water samples were collected immediately following substantial rainstorms from an inflow canal delivering water to a basin, from the basins, and from basin drywells. Soil core samples were taken at 1-foot increments from each basin, and from a Caltrans roadside treatment strip.

RESULTS

Maximum herbicide concentrations detected in water sampled from the inflow canal were diuron at 1.42 ppb, oryzalin at 27.40 ppb, oxyfluorfen at 5.62 ppb, and simazine at 83.47 ppb. Maximum concentrations detected in water sampled from basin storage were bromacil at 25.62 ppb, diuron at 36.32 ppb, oryzalin at 12.50 ppb, oxyfluorfen at 3.47 ppb, and simazine at 78.80 ppb. In water sampled from basin drywells, maximum concentrations were diuron at 0.46 ppb, oxyfluorfen at 0.28 ppb, and simazine at 17.88 ppb. Simazine and diuron had not been applied to the highway right-of-way in the Peltier basin drainage area and probably originated from adjacent agricultural land. The only herbicide detected in basin soil was simazine at 36.0 ppb.

CONCLUSIONS

Detection of herbicide residues in water sampled from storm runoff flowing into a basin, and in water sampled from basins and basin drywells indicates a potential for the herbicides to be transported to ground water by soil infiltration. Detection of herbicides other than those used exclusively for rights-of-way weed control suggests a contribution of herbicide residues from adjacent properties.

Caltrans has adopted an entire set of measures in its new Integrated Vegetation Management (IVM) Program to protect surface waters and ground water from herbicide contamination. A Memorandum of Understanding between DPR and Caltrans was established February, 1993 for minimizing potential environmental contamination from pesticide movement in rights-of-way surface water runoff and in rights-of-way drainage systems. Cooperative efforts will include: environmental monitoring to validate the effectiveness of proposed IVM mitigation measures; technical and policy consultations concerning pesticide use and contamination prevention; educational outreach to pesticide applicators and farming operations in rights-of-way drainage areas; and cross-training, and information exchange with other rights-of-way maintenance organizations.