Try phrasing your new rule. "People should stop being mean" will fail as a law as there are no criteria for defining who mean people are. "No one should dump sulphur into rivers" will likely work. We know what sulphur is and we know what rivers are. The only vague term is 'dumping' and that is defined in several places under existing law.
Which raises another issue. Is your rule already adequately covered by an existing law? If so, then there really isn't any need for a new one.
You then need to attach a penalty to your law. If there are no consequences for breaking your law, then it will be broken. Generally pollution statutes have multiple levels of offense, depending upon the quantity of substance used. A teaspoon of sulphur is quite a different matter from a ton. Usually minor breaches are punished by a fine, while major breaches may be punishable by prison.
The next issue is who enforces the law. The EPA is the obvious enforcement agency, but since you want to protect rivers, the Coast Guard might also be a good candidate.
Finally, what unintended consequences could there be? With sulphur dumping, probably little to none. The law already covers similar issues without any real difficulty. However, imagine that the law is to be enforced by the person you trust the least in American politics. If you think of yourself as a progressive, how might Sarah Palin or Mike Huckabee enforce your rule? If you are a conservative, Dennis Kucinich might be your bete noir.
Anyone can always enforce a law arbitrarily, so think of ways that your villain might enforce the law fairly, as written, but not within its spirit.
How might Pat Robertson see hate crime legislation, for example?
Next time you push for a law, ask yourself if you are potentially creating a new weapon to be used against you. Because if there is one certainty in politics, it's that the wheel turns and the outsiders eventually become insiders.