Complimentary use of dating and hydrochemical tools to assess mixing
processes involving centenarian groundwater in a geologically complex
alpine karst aquifer
Abstract
Environmental dating tracers (3H,
3He, 4He, CFC-12, CFC-11,
SF6) and the natural response of spring (hydrochemistry,
water temperature, and hydrodynamics) were jointly used to asses mixing
processes and to characterize groundwater flow in a relatively small
carbonate aquifer with complex geology in South Spain. Results evidence
a marked karst behavior of some temporary outlets, while some perennial
springs show buffer and delayed responses to recharge events. There is
also a general geochemical evolution pattern, from higher to lower
altitudes, in which mineralization and the relation Mg/Ca rises,
evidencing longer water-rock interaction. The large SF6 concentrations
in groundwater suggest terrigenic production, while CFC-11 values are
affected by sorption or degradation. The groundwater age in the
perennial springs deduced from CFC-12 and
3H/3He point out to mean residence
times of several decades, although the difference between both methods
and the large amount of radiogenic 4He in the samples indicates a
contribution of old groundwater (free of 3H and
CFC-12). Lumped Parameter Models and Shape-Free Models were created
based on 3H, tritiogenic 3He,
CFC-12, and radiogenic 4He data in order to interpret
the age distribution of the samples. The resulting groundwater-age
distributions evidence the existence of two mixing components, with an
old fraction ranging between 160 and 220 years. Some dating parameters
derived from the mixing models and their correlation to physicochemical
parameters permits to explain the hydrogeochemical processes occurring
within the system. All these results prove that large time residence
times are possible in small alpine systems with a clear karst behavior
when the geological setting is complex, and they highlight the
importance of applying different approaches, including groundwater
dating techniques, to completely understand the groundwater flow regime
within this type of media.