Surely there must be some ...

Surely there must be some info with regards to local hydrogeology that you can consult: geological investigations, borehole records, hydrogeological reports, hydrogeological maps? I d consult national and regional archives from your country's department or body responsible for geological research etc.

250 m is quite close to a river and if the river is groundwater fed, then there will be an impact on the river almost certainly. If your river has sand banks then it may be running on top of sandstone. However, the river may be running on top of an aquitard (for example clays), so it could be partially or totally confined, with low baseflow index (indicating no to little inputs from aquifer).

Normally you'd test the connectivity between your borehole and river properly, before you decide where to abstract - I'm not sure if your borehole is a test one but it sounds like it's not. This study may help you decide how to proceed best. Whatever you do, you will have to gather data over several months to understand what is going on, as interactions between groundwater and rivers can be complex.

It is crucial you understand what if any impact your abstraction has on the river, as a large decline in flows can cause ecological damage, especially during very low flows and can affect downstream abstractors whose livelihoods may depend on the water available to them. You may breach some environmental legislation too, which can lead to fines etc.