Big Spring, located in the Ozarks region (Carter County, Missouri), is one of the largest karstic springs in the United States and the world in terms of flow. With an average flow of 12 m3/s and an average daily discharge of 1.106 m3, this spring is a first magnitude spring which rises at the base of a dolomite cliff at the west side of the Current Valley, Ozarks.
The characterization of porous aquifers is based on knowledge of internal parameters such as permeability, transmissivity and geometry, and their boundary conditions. Unfortunately, the mechanisms of genesis and development of karstic systems (karstification) lead to a highly discontinuous and heterogeneous medium, not only in its spatial structure but also in its temporal functioning. Various degrees of karstified carbonate rocks consist of highly transmissive conduits through poorly transmissive blocks. Moreover, climate and tectonic movements change base level conditions, i.e. spring location, creating multilevel conduit karst systems.
We analyzed the daily discharge records collected by the USGS from 1921 through 1996 using the approach developed by Mangin (1975) and modified by Bakalowicz (2005). We characterized the infiltration (slow, fast or delayed) and estimated the dynamic volume, i.e. the phreatic storage easily renewable during yearly recharge. This approach is only based on the hydrogeological functioning of the karst aquifer. Four recession curves (1944, 1953, 1969 and 1972) were considered for Big Spring karstic system during long term summer recessions. The analysis of these curves indicated the presence of an unusual important dynamic volume varying between 570 106 m3 and 1.109 m3, a predominance of slow infiltration, and residence times varying between 1.5 to 2.5 years.
The analysis of the sorted discharge curves prepared for the analyzed period showed a systematic break at a discharge of 10 m3/s, indicating that Big Spring is in hydraulic contact with another aquifer (probably alluvial) and is discharging significantly into this adjacent aquifer, below this discharge value.
When compared to karstic springs around the world, it shows that Big Spring is characterized by a deep phreatic zone, largely karstified below the present day spring level, during previous karstification phases. It belongs to the ”non-functional karstic systems” which possesses a large storage capacity due to a complex drainage structure partly or totally flooded with significant residence times. However, the paleo-conduit systems existing in the phreatic zone remain partly functional.
Proven methods exist to analyze karstic systems that are different from those of porous media, allowing the exploitation, management and protection of karst water resources. The management of Big Spring karstic system should be based on the analysis of aquifer functioning and groundwater monitoring in the spring vicinity. This type of approach should be generalized to karst springs throughout continental U.S.