You are assuming farms are the only or current main source of nitrates just because there is a correlation. Overtreated lawns and golf courses could have taken up the slack of farming usage for one. The other sectors need to be factored in for nitrate content.
Parent didn’t say it was the only source just said that rates correlated.
The fluctuation year to year may be due better growing seasons so farmers use more, but potentially so do golf courses and lawns. The usage of the others may definitely be drivers as well but agricultural use is correlated
