SPRINGS: WINDOWS INTO DECADAL-SCALE KARST AQUIFER PROCESSES (Invited Presentation)

Springs have long been used to evaluate sources of recharge, structure of flow paths, biogeochemical and inorganic reactions, and many other physicochemical characteristics of karst aquifers. These evaluations commonly use discharge hydrographs and chemographs to observe short-term changes (e.g., minutes to weeks) in aquifer processes. Long term (e.g., decadal) changes can also be evaluated from variations of spring discharge and chemical and isotopic compositions of authigenic mineral deposits. In one example, the residence time (e.g., “age”) of water in the upper Floridan aquifer has increased by around 20 years based on changes in CFC concentrations in Ichetucknee springs water (north-central Florida) over the past 17 years. The increased ages indicate most sampled water had recharged the aquifer between 1973 and 1980. During these years, rainfall increased due to variations in the El Niño Southern Oscillation and the Multi-decadal Atlantic Oscillation and may have enhanced recharge. Alternatively, increasing age may reflect increased withdrawal of young water over the past several decades. Another example of decadal-scale changes comes from authigenic Fe,Mn-oxide deposits, which may reflect intrusions of flooding stream water into springs. Water intruded into and subsequently discharged from Madison Blue and Peacock spring vents (northern Florida) have variable Fe and Mn concentrations that reflect formation and dissolution of solid Fe,Mn-oxide phases as intrusions alter redox conditions. Three Fe,Mn-oxide samples from Madison Blue and Peacock spring conduits have Sr and Pb isotope ratios intermediate between groundwater and intruded water compositions, indicating they formed at least in part during intrusion events. These Fe,Mn-oxides contain anthropogenic Pb isotopes derived from leaded gasoline, which first appeared in the region around 1930. These isotopes indicate the Fe,Mn-oxides formed during intrusions over at least the past several decades. These two examples show that decadal changes can be important to karst water quantity (recharge) and compositions (Pb sources) and that decadal scale processes can be evaluated based on physical and chemical dynamics in spring water flow and related deposits.