Yes but that's a lot of limestone and doesn't explain why everyone on that aquifer has about the same level of nitrate (which is about the same as but higher than the Potomac's). One USGS study I quickly found shows that the deeper the well the less nitrate makes it through - unsurprisingly enough. I'd buy the argument that some nitrate would make it through 300' of stone but I'm betting that in this case it is nitrate bearing rock that is mostly responsible. Surface water - yes, fertilizer doubtless has something to do with that but with deep wells it has to be less so. It is interesting here that I measure the nitrate in the river at less than that in the wells. These are spot measurements of course and should not be regarded as anything more than that.