Openings in consolidated rock or unconsolidated sediment hold groundwater. These openings can be classified as primary or secondary. Primary openings are present when a rock forms. Pores between sediment and vesicles in lava rock are examples of primary porosity. Secondary openings form afterward. Fractures and dissolution structures are common secondary openings. The interconnected passageways available for flow are usually narrow and irregular. Consequently, most groundwater flows exceedingly slow, often less than 10 m/yr. Newell and others (1989) compiled groundwater velocity data from 290 sites in various hydrogeologic settings in the U.S. Velocity ranged from approximately 0.001 to 30 m/d, with a median of 0.07 m/d. Occasionally, groundwater flows rapidly through underground caves, large fractures, or gravel deposits (Lohman, 1979).