The concerns associated with contamination of surface water by pesticides are somewhat different than those associated with ground water contamination. Transport of pesticides to surface water is a concern with regard to the effect it may have on wildlife. Both aquatic organisms and land-based organisms depend on streams, creeks, ponds and even ditches for habitat and food. The degree of toxicity presented by a pesticide is variable depending on the organism affected. For example, a pesticide with low mammalian toxicity may be extremely toxic to fish.
Water flow is an important transport mechanism for pesticides.
When water is added to the soil through precipitation or irrigation, the portion that doesn't evaporate may either infiltrate into the soil or runs off the soil surface. The fraction of water that infiltrates compared to the fraction that runs off depends largely on the intensity of precipitation and the infiltration capacity of the soil. Water that infiltrates into the soil is either stored within the soil profile or percolates downward toward ground water, depending on the soil water conditions. When soil conditions are dry, the added water will increase soil water storage. If the moisture-holding capacity of the soil is exceeded, the excess water percolates downward through the soil to ground water.