History 184 Geology 88 Environmental aspects 82 Groundwater 80 Law and legislation 65 Geology, Stratigraphic 60 Water quality 49 Groundwater flow 48 Law 43 National parks and reserves 42 Water 42 Pollution 41 Aquifers 40 Politics and government 37 Indians of North America 36 Antiquities 35 General 35 Management 35 Water resources development 34 Earthquakes 33 Water-supply 33 Mathematical models 32 Geology, Structural 27 HISTORY 27 Mines and mineral resources 27 Hydrogeology 26 Indigenous peoples 26 Research 26 Historic sites 25 Public buildings 25