I asked him the same question. I don't think he knows specifically. I can't imagine seeing surface fertilizer impacting a subsurface well. If I had to guess it's over 100' down. He's surrounded by about 20 houses and he's in a rural area. Just to his west is the vast expanse of the Hockomock Swamp.
Those things a re a workhorse. I bought my HRX in 2004. I just recently had to replace the carburetor on it. The drive cables have also had to be replaced but that's about it. Pretty much a 1st pull start mower to this day.