Hi Anyone know what a hydrogeological

If the municipality does not allow for a road, in a particular place, for geological reasons, it means that the shift of land needed to create the site where they transit the road, seriously damages the static nature of the territories. Put simply, could create mudslides, the size of which would be detrimental to long range. In this case it is advisable, before submitting the application to the municipality, contact a geologist, possibly in the area, which of course, better than others, is able to assess the feasibility of the project, any costs to be incurred for the realization and maybe submit, with appropriate design, the application to the competent offices of the municipality. Ugo , by www.lifeinitaly.it 

A vincolo idrogeologico can be various things - but usually as Ugo suggests its a result of instabiliy in the area which could be affected or worsened by what you propose to do.   The geologist who would have examined your application will have looked a thte idrogeological maps of the area, which classufy the land into variuos zones, the highest risk being those at risk of frana.  It may be that your land has a lower risk , for example that where you want to pur the road is already a runoff area for heavy rainwater, and you could change the nature of the runoff which would affect other owners.   Therefore your permission or lack of them will be dependent on how and if you construct your access road.