Subject
Geology
3,075
Groundwater
1,593
Water quality
1,510
Mines and mineral resources
1,245
Geology, Stratigraphic
1,198
Water-supply
1,081
History
761
Outdoor recreation
746
National parks and reserves
570
Stream measurements
565
Paleontology
529
Aquifers
518
Hydrogeology
501
Officials and employees
498
Forest management
465
Water Pollution
442
Hydrology
436
Trails
415
Groundwater Pollution
414
Groundwater flow
412
Forest reserves
391
United States
389
Wildlife refuges
356
Groundwater Quality
345
Historic sites
328
Floods
322
Public lands
318
Coal
311
Streamflow
286
Management
271
Water resources development
258
Land use
257
Wilderness areas
249
Recreation areas
240
Conservation of natural resources
234
Motor vehicles Recreational use
230
Petrology
226
Ore deposits
220
Uranium ores
213
Recruiting
212
Wildlife conservation
212
Petroleum
211
United States Foreign relations
198
Earthquakes
195
Geochemistry
188
Water quality management
180
Manganese ores
178
Roads
178
Drug abuse Prevention
176
Information services
168
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