California Department of Water Resources (DWR) Groundwater Information Center

USGS answers the basic questions: "What is ground water?", How does water get into the ground?", "What is an aquifer?", "Who uses groundwater?, "How do you get water out of the ground?", and "Can we run out of ground water?"

USGS's "Ground Water and the Rural Homeowner" a good primer for people with wells.

Sustainability of Ground-Water Resources, U.S. Geological Survey Circular 1186

Ground-Water-Level Monitoring and the Importance of Long-Term   Water Level Data,  U.S. Geological Survey Circular 1217
Ground Water and Surface Water - A Single Resource,  U.S. Geological Survey Circular 1139
Basic Ground-Water Hydrology, U.S. Geological Survey Water   Supply Paper 2220 
U.S. Geological Survey's Ground Water Resources Program, 2001, U.S. Geological Survey Fact Sheet 056-01
Arsenic in Ground Water of the United States, U.S. Geological Survey
Ground Water Studies, U.S. Geological Survey Fact Sheet     058-95  
 DWR Water Facts: Groundwater
   DWR Water Facts: Groundwater in Fractured Hard Rock
   Land Subsidence in the United States (USGS Circular 1182)
Introduction to Groundwater
Groundwater Levels
 Conjunctive Use
 Land Subsidence