HYDROGEOLOGY AND GROUNDWATER-LEVEL TRENDS IN SOUTHWESTERN MISSOURI

Groundwater use in southwestern Missouri has increased greatly in recent years, placing ever increasing demands on the Ozark aquifer, the regions only high-yielding aquifer. It supplies much of the municipal, industrial, and agricultural water needs in the region. Averaging about 1,200 feet in thickness, the Ozark aquifer can locally yield more than 2,000 gallons per minute, but more typically yields 600 to 800 gallons per minute to fully penetrating wells. It is a confined aquifer in most of the region where it is overlain by low-permeability Devonian and Mississippian-age carbonate and shale units that form the Ozark confining unit. The Springfield Plateau aquifer overlies the Ozark confining unit, and consists of as much as 450 feet of Mississippian-age limestone and cherty limestone units that, combined, generally yield 20 gallons per minute or less. Typically unconfined, only in the northwestern part of the region where it is overlain by Pennsylvanian-age limestone, sandstone, and shale units is the Springfield Plateau aquifer confined.

A groundwater-level observation well network established in Missouri in the late 1950s has been expanded greatly in recent years. Prior to 1999, water levels were monitored in only 9 wells in southwestern Missouri. Today, there are 38 observation wells in the region, 31 of which are completed in the Ozark aquifer. They show that groundwater-levels in the Ozark aquifer can fluctuate 150 feet or more annually in high-use areas. Much of the drawdown that occurs when demands are high recovers when seasonal usage decreases. Long-term groundwater-level declines of 400 feet or more have been measured in a few areas where groundwater use greatly exceeds recharge.

A comparison of two published U. S. Geological Survey Ozark aquifer potentiometric maps, one depicting pre-development water levels and the other showing 2006-2007 conditions, indicates that declines of 100 feet or less have occurred in about 59 per cent of the region, principally in the eastern part. Declines of approximately 100 feet to 200 feet have occurred in about 37 percent of the region, principally in the western part. In about 4 percent of the area declines have exceeded 200 feet. Declines of 300 feet or more are found only at major pumping centers such as the Joplin-Carthage area, Noel, and Springfield.