To date, staff of DPR's Environmental Monitoring program have identified two pathways of pesticide movement to ground water: (1) leaching in coarse-textured soils; and (2) runoff in certain soils with hardpan layers or fine-texture.

1. Leaching in coarse-textured soils

Most scientific literature has focused on describing how pesticide residues and other agrichemicals, such as nitrates, move with water as it percolates downward and eventually recharges ground water. Many prospective well sampling studies confirm the ease of residue movement in coarse soils where percolation rates are high and a large amount of water is available for downward movement.

There are two potential sources of water to move pesticide residues downward in soil: rainfall and irrigation. In California, DPR scientists conducted two studies to determine the effect of rainfall on leaching of pesticides. They found that rainfall is insufficient in relatively low rainfall areas to drive residues deep into the soil profile of coarse soils. The results of these studies, conducted in Fresno and Riverside, are available in two reports at the following links:

2. Runoff in hardpan and some clay soils

DPR scientists took deep soil samples in hardpan areas treated with pesticides found in ground water. In contrast to coarse soil areas, DPR found very little pesticide residue, indicating that leaching was not the primary pathway. The results are reported in the following Proceedings, available from John Troiano at jtroiano@cdpr.ca.gov.

In a cooperative study involving DPR, pesticide registrants, and the Tulare County Agricultural Commissioner staff, rain runoff entering drainage wells was sampled for herbicides detected in ground water in hardpan soil conditions. High levels were detected up to 2.5 months following application, indicating that runoff, not leaching, was the principal pathway of movement to ground water in hardpan areas. These results are documented in the following EM report:

Mitigation measures

1. Coarse-textured soils

In the irrigation studies reported above, DPR documented that efficient irrigation (133% of crop need) significantly decreased the downward movement of pesticides in coarse soils, compared to over-irrigation (185% of crop need), and increased the degradation of herbicide residues. This means that fewer residues were available for continued downward movement.

So in coarse soil areas where leaching is designated as the predominant pathway to ground water, one of the management practices DPR recommends is to limit the amount of irrigation water to not more than 133% of crop need at each irrigation. This level would provide water for crop needs while also maintaining residues in the upper layers of soil and allow for degradation. This management practice should be followed for a 6-month period following pesticide application. The basis for this 6-month period is detailed in the following report, which is a modeling study of the effect of long-term irrigation management on residue movement to wells situated in a coarse soil:

• EH 00-01 Spurlock, F. 2000. Effect of Irrigation Scheduling on Movement of Pesticides to Ground Water in Coarse Soils. (PDF, 524 kb)

2. Hardpan and some clay soils

The low infiltration rates of these soils made them prone to producing large amounts of runoff water from winter rainfall events. DPR tested a practice to move these residues off the soil surface so that they would be less subject to movement in runoff water. Incorporation of pesticides, especially pre-emergence herbicides, by mechanical or irrigation methods, and not by rainfall, was found to greatly reduce the amount and concentration of a pre-emergent herbicide moving offsite in simulated rain runoff water. These results are detailed in the following journal article:

• Troiano, J. and C. Garretson. 1998. Movement of Simazine in Runoff Water from Citrus Orchard Row Middles as Affected by Mechanical Incorporation. J. Environ. Qual. 27: 488-494. (PDF, 102 kb). Reprinted with the permission of the American Agronomy Society.