The U.S. Geological Survey, in cooperation with the city of Pella, Iowa, conducted a study of the ground-water resources in selected parts of Jasper, Mahaska, and Marion Counties near Pella, Iowa, during 2000–01. The purpose of the study was to describe the ground-water resources in the Pella, Iowa, area.
Following a review of available hydrologic and geologic information, the study focused on investigating the alluvial deposits along the South Skunk River within a 10-mile radius of the city of Pella. Thickness, lithologic, and water-quality data for the alluvial deposits in the South Skunk River Valley were collected at selected sites from August 2000 through July 2001.
The South Skunk River alluvial aquifer near Pella consists of stratified deposits of sand and gravel of glacial and fluvial origin. The upper 15 to 20 feet of the alluvial deposits are interbedded with flood-plain clay and silt lenses and are overlain by a developed soil horizon. The alluvial aquifer is underlain by a thin (less than 50 feet thick) layer of glacial drift. Glacial drift, as it is defined in the study area, is a heterogeneous, unsorted, unstratified, unconsolidated, relatively impermeable deposit consisting of clay, sand, and gravel. The bedrock surface beneath the glacial drift consists primarily of shale and limestone of Pennsylvanian or Mississippian age. The alluvial sand and gravel deposits along the South Skunk River near Pella range from less than 30 to more than 80 feet thick. Saturated thickness of the alluvial aquifer ranges from 15 to about 70 feet.
Water-quality samples were collected from four observation wells installed in the South Skunk River alluvial aquifer in the study area. Results of the water-quality sampling show that the chemical quality of the ground water is generally similar at all of the sampling sites. All ground-water samples were low in dissolved oxygen, which resulted in high concentrations of iron and manganese and reduced forms of nitrogen.
