Origin of water masses in Floridan Aquifer System revealed by 81Kr
Introduction
High population density in coastal regions causes elevated water demand and increased levels of surface water pollution, resulting in severe dependence on and frequent overexploitation of coastal aquifers. Although understanding of the coastal aquifer system in its entirety is ideal for an efficient management of this valuable water resource, most studies only concern shallow portions directly relevant to exploitation. The primary reason for this is their complexity: Coastal aquifers embody geochemical boundaries where saline- and freshwater masses of contrasting chemical compositions meet and cause chemical disequilibrium. In addition to the hydraulic gradient and conductivity, the groundwater flow is driven by the variable density of fluids and by water-rock interactions causing dissolution and/or cementation. Furthermore, sea level fluctuation directly impacts the hydraulic gradient and thus the flow properties of the aquifer. At the same time, these geochemical and hydrological interactions make coastal aquifers potential messengers of the changes in hydrological cycles, the long-term geochemical flux of elements across the land-ocean boundaries, and fossil geochemical signatures.
As a typical example of coastal aquifers, the Floridan Aquifer System (FAS) produces 4.6 billion m3/yr of fresh and brackish water (Geddes et al., 2018). It serves as a primary source of drinking water to a local population of over 10 million people in Florida, and supports irrigation of over 2 million acres (Bellino et al., 2018). Additionally, the FAS's upward leakage nurtures thousands of lakes, springs, wetlands and the ecosystems they foster. Having low topographic relief with an altitude mostly below 30 masl, much of the region overlying this aquifer was cyclically inundated during the Pleistocene. The resultant changes in hydraulic head likely altered the rates and pattern of freshwater flow and delineated the landward seawater invasion that increased the salinity of the aquifer (Meyer, 1989). In support of this hypothesis, multi-tracer studies reported concomitant occurrences of colder noble gas recharge temperatures with D- and 18O-enrichments downgradient of the FAS compared to modern recharge in southeastern Georgia (Plummer, 1993; Clark et al., 1997) and in the broad region of south Florida (Morrissey et al., 2010). These observations were interpreted as a result of fossil meteoric water recharged during the last glacial period (LGP), followed by sluggish flow and limited recharge after the sea level rise.
Groundwater age structure of the aquifer would ideally provide information on the timing of recharge, mixing of different water bodies, and spatially- and temporally integrated groundwater flow rates to corroborate or refute the hypothesis. Such data would also allow an assessment of the origin of salinity, which is the primary obstacle of economic water use in the region. The most common dating tracers of groundwater covering Pleistocene timescales are 14C and 36Cl. However, application of 14C is hampered due to water-rock interaction with the carbonate reservoir rocks, making the interpretation complex (Plummer and Sprinkle, 2001). The high salinity of these groundwaters also complicates the interpretation of 36Cl data in terms of residence time. Thus, due to the lack of appropriate dating tracers, previous studies primarily relied on indirect time constraints as described above.
Krypton-81 ( kyr) is an emerging tracer in groundwater dating over the timescales of interest here (Lu et al., 2014). Its chemical inertness and simple source function make the age interpretation easier and more reliable compared to other tracers such as radiocarbon and 36Cl (Purtschert et al., 2013). A detection of the extremely low modern atmospheric isotopic abundance of 81Kr (81Kr/Kr, Zappala et al., 2020) and 85Kr ( yr, 85Kr/Kr ), which serves as an indicator of young water mixing, requires a dynamic range far beyond conventional noble gas mass spectrometers. Instead, it relies on Atom Trap Trace Analysis (ATTA; Chen et al., 1999), an isotope-selective, laser-based atom counting method. Since the first developments (Chen et al., 1999) the ATTA method has significantly improved in sensitivity (Jiang et al., 2012; Zhang et al., 2020). Along with advances in krypton sample collection and preparation techniques (Yokochi, 2016; Riedmann and Purtschert, 2016; Dong et al., 2019) it has developed into a practical groundwater dating approach. Application of the tracer in several studies unveiled the time scale of water flow in continental aquifers (Sturchio et al., 2004; Matsumoto et al., 2020). Furthermore, 81Kr served as an additional constraint to resolve mixing of waters of different age and origin (Gerber et al., 2017; Yokochi et al., 2019). First applications of radiokrypton isotopes in a coastal aquifer along the Mediterranean demonstrated the usefulness of noble gas radioisotopes in revealing the presence of hydraulic connections between the ocean and the aquifer that had long been thought isolated (Yechieli et al., 2019). In this study, we report noble gas radionuclide and radiocarbon isotopic abundances of groundwater in the southern portions of the Florida peninsula. While the data confirmed a major freshwater recharge during the LGP as inferred from other geochemical tracers, they also revealed an active freshwater recharge in the upgradient of the aquifer during the Holocene and the preservation of fossil seawater that encroached prior to the last glacial maximum (LGM).
Section snippets
Hydrologeological background
The Floridan aquifer system (FAS) underlies the entire state of Florida and beyond, consisting of a sequence of hydraulically connected carbonate rocks of Paleocene to Miocene age (Miller, 1986). The Upper Floridan Aquifer (UFA) contains the highly permeable Suwanee Limestone (Oligocene), Ocala Limestone (upper Eocene) and upper portion of the Avon Park Formation (middle Eocene). The Avon Park Formation includes a sub-regionally extensive high permeability zone (Avon Park Permeable Zone, APPZ;
Sampling sites and methods
In order to address the presence of LGP freshwater and Holocene seawater in the FAS, samples were collected at three sites (Fig. 1a), each of which had two or three wells at different depths (Fig. 1b). Site and well IDs used here conform with the database of the South Florida Water Management District, and are common with Morrissey et al. (2010). The first site (OKF105) is in the region where probably the youngest LGP freshwater resides according to 4He concentrations, as reported by Morrissey
Results
In all samples, 85Kr was below 1% of modern atmospheric activity, indicating that there was no significant downward leakage of shallow (younger, <50 yr) groundwater or atmospheric contamination during sampling and Kr purification (Table 1). Furthermore, this also strictly eliminates the possibility of major contamination from post-1970 drilling fluids. The isotopic abundances of 81Kr were between 89 pMKr (percent Modern Kr) and modern, suggesting relatively young (<40 kyr) apparent groundwater
The rate of C exchange
As reviewed by Plummer and Glynn (2013), the first generation of attempts to correct for geochemical dilution of the 14C signals focused on the processes occurring in the recharge zone under open-system conditions, where 14C-depleted soil carbonates and other minerals were dissolved. Subsequently, a geochemical inverse modeling code, NETPATH (Plummer et al., 1994), additionally tracked time-integrated aqueous reactions in the aquifer to deduce ‘adjusted’ 14C abundances. However, data for
Conclusion
The first application of noble gas radionuclide analysis in water samples from the Floridan Aquifer System identified freshwater recharged during the LGP, confirming previous studies based on other geochemical tracers. The new data, which were interpreted in a novel comprehensive multi-tracer framework, also suggest a possibility of fresh groundwater recharge during the Holocene in the upgradient of the aquifer where the confining unit is thinner. Unlike simple aquifer systems where groundwater
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Ben Gurion University–Argonne National Laboratory–University of Chicago Collaborative Water Research Initiative. J. C. Z. and P. M. are supported by U.S. Department of Energy, Office of Nuclear Physics, under Contract NoDE-AC02-06CH11357. We thank Emily Richardson, Steven Krupa and Brian Collins from South Florida Water Management District for letting us join their field campaign, sharing their profound knowledge on this aquifer, and for providing input to the
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