Nitrate levels in Iowa were sometimes dangerously high when white settlers first arrived in the early 1800s. (I have no doubt before then too, but I don't have information on what the native tribes or fur trappers dealt with). Nitrates are a natural process in soil, that is what makes Iowa so great for corn farming. Yes farmers are adding more, but even without farmers there would be a problem.

Farmers actually care about this - any nitrate that runs off is nitrate that they paid for and didn't use (or nitrate that they have to pay to replace). That doesn't mean they know how to solve the problem