Select points downstream and within a certain radius

Hi,

my problem is the selection of points which are influenced by an area that is upstreams to these points.

I have a map with some areas, measurement points and a gorundwater flow direction. Now I want to select all points that are downstreams to those areas AND lie within a certain radius to them.

Right now I have no idea how to do this and would appreciate, if one of you could help me. Thanks!