In a recent discussion with a farmer, the (at least in northwestern Germany) "popular" topic of nitrate infiltration into groundwater came up. The general view is that lots of fields receive more nitrate fertilizer than the plants can use, this then percolates through the ground and eventually shows up in the groundwater that is also used as drinking water. So far so plausible. Raised nitrate content in groundwater is indeed often observed.
One should probably add that in this area, a lot of agriculture is raising of hogs and cattle, consequently much of the fertilizer is manure that has a (compared to commercial fertilizer) low NPK content and is thus not worth to transport over far distances. So there's an incentive to dispose on ones own fields, nearby.
Now, this farmer claimed that the nitrate content we see now is decades old, that the bylaws and controls as well as agricultural practices in place now work. Is this true? Does nitrate penetrate ground so slowly? The only mechanism here I could think of that nitrate is adsorbed and desorbed on minerals in the ground, sort of like in chromotography column.
If neccessary, I can provide more figures for nitrate content of specific wells, groundwater table etc - but I don't think it's neccessary since I want a qualitative answer wether or not such a slow infiltration is plausible at all.