At picnics and World Cup watch parties across the United States this summer, people have been reaching for bowls of chips and guacamole. Most of that guacamole is made from avocados grown in Mexico – on orchards that, in what might come as a surprise, contribute less water pollution today than they did a decade ago.

There’s a story behind how these creamy fruits started to clean up their act. It begins with a big avocado deal between the U.S. and Mexico in 2016. And it provides evidence of how well-designed trade partnerships can do much more than promote trade.

Those benefits are worth remembering as officials from the U.S., Mexico and Canada negotiate the future of their free trade agreement.

How a trade deal cut Mexico’s farm pollution

Growing avocados on a commercial scale requires a lot of nitrogen fertilizers and other agrochemicals. Rain often washes those chemicals and fertilizers into nearby rivers and streams. Nitrogen downstream feeds algal blooms that starve rivers of oxygen and kill fish, and it can contaminate the drinking water that local communities rely on.

Economists have long argued that trade can exacerbate these environmental consequences: Wealthy countries with strict rules for their own growers buy the dirtiest goods from poorer countries with weaker regulations, relocating the pollution. Based on this “pollution haven” theory, America’s growing........

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