Food service dipper wells are basically constantly running sinks into which you put utensils at some restaurants to clean them, that I have heard use millions of gallons per year. I found a few articles surveying average flow rates for them, but not much inquiry into what the actual flow rate required for proper sanitation.
AFAIK, Dipper wells are generally used for things like ice cream scoops and as far as I can tell are meant to wash off food residue to prevent germs from growing, and not to clean off an already filthy surface. They usually run about 0.5 to 1 gallon per minute.
I think germs multiply at one doubling every 20 minutes at most? Does this mean that all that is needed is to half the number of germs every twenty minutes or less so they can't multiply, because dangerous levels of germs should never be present on a utensil anyway?
In that case, we could assume an ideal "perfectly efficient" dipper well would always spill out the very oldest water as new water came in, such that if 1 gallon was added to a 1 gallon well none of the original water would remain.
From there it seems likely that every well has an "efficiency" from which it should be possible to calculate the required flow rate if we know the maximum acceptable "half life" for water contaminates.
Since water generally spills out the top, and comes in from a faucet(which might be laminar and go straight to the bottom, and in fact one would probably want it to be laminar and thus quiet), the efficiency might actually be pretty high, and it could be that the standard 0.5gpm is more than what is needed to ensure a margin of safety. A 1 gallon dipped well might only need something like 0.05gpm. It seems like with all the drought business going on someone should have done a study or something because turning down the wells could be an easy step but I'm not finding anything.
If the theory is true, you could easily measure the dipper well efficiency by adding a known amount of salt, measuring electrical conductivity, and measuring how long it takes to reach half the original calculation using a resistance vs salinity chart.
Or, maybe germs don't grow well in dilluted ice cream and all we need to do is change the water a few times a day, and maybe cool it with a small peltier element?