Hiya Bingley - may be able to help here....Well, I'll have a go "off the top of my head" and, if nobody subsequently objects, we'll take it as gospel!
As G'Ted has said, I believe "hydrogeology" is moreso to do with subterranean water, whilst "hydrology" relates to the characteristics of water on the earth's surface.
To grossly simplify the situation, hydrogeologists would either be looking upon subterranean water ("groundwater") as a resource or a nuisance. I'd like the think that the former aspect of water is obvious - all to do with aquifers, the search for them, the estimation of the resource, the recharging dynamics of the aquifer and the exploitation of the resource. As a quick aside, Perth is coming off the driest summer in 30 years; our dams are all but empty, we are only allowed to water our lawns on a roster - and 60% of the supply is being pulled from underground.
Water as a nuisance refers to the fact that an excess of water in the ground reduces the strength of the ground dramatically. Frinstance, the nature, size and quantity of supporting structures (piles etc) for tall buildings is a product not only of the nature of the substrate, but also its water content. Open pit mines and road cuts are the same - the more water present, the shallower the walls must be - and thus (in the case of mines anyway), the greater the expense attributable to waste removal. Whilst vast sums of money are spent dewatering sites ahead of mining, this is less than would need to be spent in earthmoving costs to strip away the waste if the "wet" substrate was being mined. In underground mines slope stability is less of a factor (the openings are much smaller), but water is still treated as a threat as it can flood the operation.
As I understand it, hydrology is moreso to do with how the water behaves on the surface. There is a focus on runoff, flood mitigation, river control etc. What the Dutch have done and the Brits with the Thames are classic examples of the Hydrologists' art.
How'd I go?
stales
