Using UV necessitate a good water quality: Max. level: Fe = .3 ppm, Mn=.05 ppm, hardness=7 gpg, low turbidity, low color and high UV transmittance (more than 75%). For many sites you may need a 5 micron filter followed by a water softener and finally the UV.
A system already exists with a tank to reduce nitrates, a tank with calcite and a water softener. The UV would be at the end before it goes to the house. The system has had chlorine added previously but the bacteria comes back. The well is only 20 feet and there are cows all around so it will probably always be a problem. The system had chlorine tablets added to the well itself and the 5000 gallon tank plus a cup of chlorine bleach added to the tank. The level of bacteria was off the scale (before the chlorine was added). Supposedly the test stops at 200ppm and it was over that.
I agree it is unacceptable which is why a solution is being sought. A typical well in the area is 1 to 2 gpm. This one along with the neighbor's is about 13 gpm. To get a well where bacteria would not be a risk would need to be over 100feet. There probably isn't water here even at that depth.
Also, no one actually drinks the water, though it is used in other ways that could contaminate people and animals with the bacteria which is why it needs to be fixed.
Unfortunately the cows are on neighboring property so moving them isn't possible. This property does have horses so it is possible to keep them away, though there is very little manure around the well. It probably doesn't take much. If the neighbor will test their well and it is positive then it is pretty sure that it is the neighbor's cows causing the problem.
Fifty feet is the recommended buffer distance for cattle.
You should follow the guidelines spelled out in the surface water treatment rule.
Your best option is to install a filter and chlorinate. Since it is necessary to maintain a disinfectant residual to prevent biological regrowth, chlorination is the preferred disinfectant.
