Sometimes called groundwater hydrology or geohydrology, this branch of hydrology is concerned with the relationship of subsurface water and geologic materials. Of primary interest is the saturated zone of subsurface water, called groundwater, which occurs in rock formations and in unconsolidated materials such as sands and gravels. Groundwater is studied in terms of its occurrence, amount, flow, and quality. Historically, much of the work in hydrogeology centered on finding sources of groundwater to supply water for drinking, irrigation, and municipal uses. More recently, groundwater contamination by pesticides, chemical fertilizers, toxic wastes, and petroleum and chemical spills have become new areas of concern for hydrogeologists.