The central part of La Plata County, Colo., has undergone rapid population growth in recent years. This growth has resulted in an increased demand for information for additional domestic, industrial, and municipal water supplies. A knowledge of the occurrence of ground water will permit a more efficient allocation of the resource. Aquifers in central La Plata County include: alluvium, Animas Formation of Quaternary and Tertiary age, Fruitland Formation, Pictured Cliffs Sandstone, three formations of the Mesa Verde Group, the Mancos Shale, Dakota Sandstone, Morrison Formation of Cretaceous and Jurassic age, and undifferentiated formations. Well yields generally are low, usually less than 25 gallons per minute. However, higher yields, 25 to 50 gallons per minute may be found locally in aquifers in the alluvium and the Animas Formation. The quality of water from the aquifers is dependent on rock type. Most of the water is a calcium bicarbonate type. However, aquifers that are predominantly fine-grained or contain interbeds of shale may contain sodium bicarbonate type water. The dissolution of minerals in the coal beds, which are present in the Mesa Verde Group and the Dakota Sandstone, can contribute high concentrations of iron, sulfate, and chloride to ground water. (Woodard-USGS)