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This paper presents a mathematical approach for estimating the size of a grass filter area for removing pesticide residues from agricultural drainage waters. The method util-izes a water table management model, DRAINMOD, for simulating drainage waters from agricultural land and then a solute transport model, PRZM2, for simulating pesticide concentrations in drain effluent discharging from grass filters. DRAINMOD was used to estimate daily drain outflows that occurred in a 100 ha subsurface drained field in the Ot-tawa-St. Lawrence lowlands by running the model for a one-in-twenty year annual rain-fall period. Atrazine (AZ), metolachlor (MT) and metribuzin (MZ) are the most common herbicides that are found in drainage waters. The simulated drain outflows were as-sumed to contain 50 µg/l of AZ, MT and MZ residues, and simulations were carried out with PRZM2 to determine the required size of grass filter area to make drainage waters safer for aquatic life and a marine habitat. It was found that no more than 6% of the farm area could be used to reduce the concentrations in drainage waters from 50 µg/l to less than 1 µg/l for the three herbicides.