There's a price to pay for our love of bread and wine in the chemicals commonly used to grow wheat and grapes. Most of that bill is paid in farm soils, neighbouring veld and rivers downstream. But traces can also travel beyond the farm fence.

This is the fifth in a six-part series. Read Part One, Part Two, Part Three and Part Four.

Plant ecologist Eugene Moll describes the Western Cape as a land capable of forgetting itself. Plough renosterveld for long enough, he says, and its seed bank expires. The wildflowers, bulbs and grasses no longer return by themselves.

Farm chemicals are a less-visible part of that story, but their effects can travel. Sprays can wash into streams, seep into groundwater or drift through the air. At high levels they can threaten the insects and other small creatures that keep rivers alive.

Repeated use may also disturb the microscopic fungi and bacteria that recycle nutrients in soil. The damage is easy to miss because the water may still look clear and the crop may still look green.

Western Cape studies have found dozens of farm chemicals and their breakdown products in agricultural rivers. Most were at low levels, but several crossed the limits scientists use to warn of danger to aquatic life.

That doesn't make farmers villains. Wheat and grape growers face dry summers, stubborn weeds, fungal disease and crop-eating insects. A bad outbreak can erase a season's...