Half life [1] in biological systems isn't quite the same as radioactive decay. Essentially what is being relayed is that after a couple of months generally they are, by measuring the product directly or the secondary and tertiary by products, deducing a time when only half of the active product is left.
Farming systems that use herbicides, are going to cycle though every few years of using Glyphosate, so there's little point IMO of creating a special study that looks into Glyphosate residues over more than a few years where they are monitoring the degree of fractional amount that could remain - there are too many variations of soil types, rainfall and temperature to be greatly useful. Any results of how long it might persist would more likely come from real life examples - areas where its use has been phased out, but the difficulty most (if not all) would not have an accurate starting point of just how much Glyphosate was applied in total, time period and weather conditions to extrapolate carefully.
Detection is different to a noteworthy amount of product that is bound to clay or other particles which has prohibited biological degradation. Many who conduct studies in complex systems are usually focused on just a small number of instances they feel might encompass most of the issue they believe to be at heart ... to the point of dismissing local / on the ground knowledge.
Yes the study used fish which can be useful ... however they tried to simulate using the raw product. It would have been better had they had some water from a month after a paddock was sprayed and used surface or underground flow. What it actually means, for example to those like myself who use it regularly, is there is certainly evidence we have to be doubly careful to ensure no or as little as possible over spray of waterways occurs which contain aquatic life.
A better study might be what results might be when groundwater that has a noteworthy amount, is then heated with bound Glyphosate and it's by products, to simulate cooking and boiling of the water.
>"Nothing to do with soil persistance."
Do you think that if soil was measured for a study it was watered only with distilled water? The purpose of mentioning was to suggest that there's an uncertainty as to what may have been measured.
[1] https://en.wikipedia.org/wiki/Half-life
Edit: as not was