It is tricky to remove ...

It is tricky to remove nitrates as they are so stable, and I expect that management of water quality in a borehole will bring with it many other challenges!  Depending on the amount of water needed daily, you could consider looking into an algal turf scrubber where water is abstracted and passed through conditions optimised for algal growth (i.e. lots of sunlight) to remove nutrients and various pollutants.  Algal productivity essentially strips nitrates from the water, and you simply have to harvest the algae at times to maintain healthy continuous growth.  There is quite a lot of literature on this method, but I have only used this approach for my marine fish tank at home.  It is scalable though.  However, the best (and most direct strategy) perhaps would be to identify the direction of groundwater flow to the borehole, and trace this back to the source of the nitrates (overuse of artificial fertilisers?  septic tanks? agricultural practices?).  You will then need to work with the various stakeholders to put in place some measures to reduce input of nitrates to the groundwater.  Good luck!