Nassau County has a dirty little secret on its North Shore.

We all know Long Island has a problem with nitrogen in its waters. It’s commonly taken to be mostly a Suffolk concern, because that’s where most of the talk and all of the action toward addressing it is taking place. Nassau correctly notes that more than 90 percent of the county is hooked up to sewers, which drastically reduce nitrogen, and that its really horrendous issue — the interminable poisoning of the western bays — is finally being rectified by upgrades to its Bay Park sewage treatment plant on the South Shore.

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But a new report makes clear it’s time to look to the other shoreline, where most of Nassau’s unsewered properties lie. And those North Shore septic systems and cesspools, many quite old, are the biggest contributors of nitrogen in all but one of the 13 watersheds from the Queens line into western Suffolk studied by The Nature Conservancy. They contribute 47 percent of the nitrogen in inner Hempstead Harbor, 62 percent in Cold Spring Harbor and 63 percent in Mill Neck Bay. That must be addressed; the Environmental Protection Agency has said current efforts won’t be enough to reduce nitrogen in Long Island Sound.

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New York City must finish upgrading plants that discharge into the Sound. And inefficient septic systems and cesspools must be replaced on Nassau’s North Shore as well as in Suffolk. Fortunately, Suffolk has been working on this. The county, most of which is unsewered, is testing advanced on-site systems that greatly reduce nitrogen, and is exploring ways to make switching to them affordable for homeowners. When Suffolk finds a solution that works, Nassau should adopt it. On nitrogen pollution, all of Long Island is on the hook. — The editorial board