Groundwater of the Crimean peninsula: a first systematic study using stable isotopes

ABSTRACT Karst springs in the Main Range of the Crimean Mountains and the Crimean Piedmont show a restricted range of values (δ18O = –10.5 to –8.0 ‰, δ2H = –72 to –58 ‰), somewhat more negative than the weighted mean of meteoric precipitation. This suggests preferential recharge at higher elevations during winter months. Groundwater tapped by boreholes splits in three groups. A first group has isotopic properties similar to those of the springs. The second group shows significantly lower values (δ18O = –13.3 to –12.0 ‰, δ2H = –95 to –82 ‰), suggesting recharge during colder Pleistocene times. The third group has high isotope values (δ18O = –2.5 to +1.0 ‰, δ2H = –24 to –22 ‰); the data points are shifted to the right of the Local Meteoric Water Line, suggesting water–rock exchange processes in the aquifer. These boreholes are located in the Crimean Plains and discharge mineralized (ca. 25 g L−1) thermal (65°C) water from a depth of 1600–1800 m. Groundwater associated with mud volcanoes on the Kerch peninsula have distinct isotope characteristics (δ18O = –1.6 to +9.4 ‰, δ2H = –30 to –18 ‰). Restricted δ2H variability along with variable and high δ18O values suggest water–rock interactions at temperatures exceeding 95 °C.


Hydrogeology
Two main hydrogeological domains are distinguished within the Crimean peninsula (Fig. 2), the Crimean Mountains groundwater system and the southern part of the Prichernomorsky groundwater system (artesian basin) which encompasses the Crimean Plains and the Piedmont (Shestopalov et al., 2010). Structural depressions in the basement of the Scythian Plate within the Crimean part of the Prichernomorsky system form second-order artesian basins (North-Sivash, Belogorsky and Alminsky). The Kerch peninsula hosts a suite of small-scale artesian basins. Schematic hydrogeological cross-sections are presented in Fig. 3.

Crimean Mountains groundwater system
The main groundwater resources in the Crimean Mountains are associated with the karstified Upper Jurassic limestones, which reach a thickness of up to 1 km. Terrigenous and terrigenousvolcanogenic deposits form the basement of the karstified plateaus in the Main Range (yailas) and show low hydraulic conductivities. The plateaus serve as the main recharge area and possess a mature epikarst. Limestone massifs of the Main Range provide relatively small storage of karst water in the phreatic zone, which discharges via more than 2000 springs located on the periphery of the plateaus. More than half of these springs are located within the Supplemental materials to: Groundwater of the Crimean peninsula: A first systematic study using stable isotopes. Isotopes in Environmental & Health Studies. 7 200-600 m a.s.l. altitudinal interval. Discharge of the karst springs ranges from a few to hundreds of liters per second. Springs exceeding 10 L s -1 comprise only 4 % of the springs. Their cumulative discharge, however, is 83 % of the total spring discharge (Barabanov et al., 1970).
Out of those, 19 springs with a discharge exceeding 100 L s -1 account for ca. 75 % of the total discharge. These numbers indicate that the groundwater flow is focused into a few highly transmissive karst conduits.
Groundwater in the Upper Jurassic limestones, even when confined (e.g., in the Baydarsky depression), belongs to the zone of active water exchange and shows 0.3 to 0.5 g L -1 total dissolved solids (TDS). Some boreholes located outside the massifs at intersections of tectonic faults tap confined Na-Cl-type (e.g., Krasnaya cave area) or Na-HCO 3 -type (e.g., Chernye Vody area) waters exceeding 3 g L -1 . These waters are commonly enriched in He, Rn, H 2 S, CH 4 , and higher hydrocarbons, suggesting admixture of deep-seated fluids. High TDS, high contents of dissolved gases (CO 2 and H 2 S) and minor elements were also reported from some springs in the eastern part of the Crimea Mountains associated with faults (Barabanov et al., 1970, Gor'kova, Lymar, 1987, Lushchik et al., 1981.

Southern part of the Prichernomorsky system
The Prichernomorsky system extends as far south as the Crimean Piedmont, where Upper Cretaceous, Paleogene and Neogene strata are uplifted, tilted toward north-northwest and exposed within the Inner and the Outer Ranges and on the northern slope of the Crimean Mountains. This is the recharge area of the groundwater system located underneath the Crimean Plains.  (Barabanov et al., 1970, Lushchik et al., 1981. The Aptian-Albian and some Upper Cretaceous strata, the thick Supplemental materials to: Groundwater of the Crimean peninsula: A first systematic study using stable isotopes. Isotopes in Environmental & Health Studies.

Neogene aquifers
The Neogene sequence in the Crimean Plains hosts three aquifers: Middle to Upper Pliocene sands and gravels, carbonates of Sarmatian to Pontian age, and undifferentiated Middle Miocene carbonates. The upper aquifer is only 10 to 40 m thick, separated into distinct horizons by shale aquitards, and confined by the overlying Pleistocene and Pliocene shales. The aquifer is recharged primarily by leakage from the underlying confined aquifer. The Sarmatian-Pontian aquifer is subdivided by internal aquitards and is underlain by Lower Sarmatian clays forming a regional aquiclude. The Middle Miocene aquifer comprises a layer of limestone and underlying sands. The transmissivity of the Neogene deposits is highly variable (Lushchik et al., 1981). The Pontian beds have the highest transmissivity reaching 2.310 -1 m 2 s -1 . The hydraulic conductivity of this horizon may be as high as 6.410 -7 m s -1 . The transmissivity of the Sarmatian and Maeotian horizons varies between 3.510 -3 and 1.410 -2 m 2 s -1 , and that of the Middle Miocene horizon from 1.210 -4 to 5.810 -4 m 2 s -1 (Lushchik et al., 1981).
The Crimean Piedmont forms the outer recharge area of the Neogene aquifers and the groundwater flows north-and northeastward. Approximately at the latitude of the town of Dzhankoy the flow splits; one branch continues northward and another one is deflected to the west, flowing north of the Tarkhankut uplift (the latter represents an additional recharge area).
In the North-Sivash region, near the axial part of the Karkinitsky depression, this southerly flow meets the flow of freshwater from the north which recharged in the northern (continental) part of the Prichernomorsky system (Lushchik et al., 1981).
Hydraulic heads in the Middle Miocene aquifer typically exceed those in the Sarmatian-Pontian aquifer by 10 to 15 m, whereas heads in the latter exceed those in the Pleistocene aquifer by up to 10 m. This leads to upward leakage of water between the aquifers, documented along some faults as local hydrodynamic anomalies marked by natural gas emissions (Lushchik et al., 1981;1987).

Paleogene aquifers
The Paleogene deposits host aquifers in the Middle Eocene and the Lower Paleocene, dominated by calcareous rocks. The aquifers are recharged in the Crimean Piedmont via infiltration of meteoric precipitation and sub-river channel flow in streams. Additional inflows from adjacent stratigraphic units are related to cross-formational flow along tectonized and often karstified zones. In the Piedmont, the waters are unconfined down to a depth of 50 m.
Discharges of natural springs and wells typically do not exceed 2 L s -1 . The waters are of the Ca-HCO 3 -and Mg-Na-HCO 3 -type, containing up to 0.7 g L -1 TDS. As the aquifer rocks plunge northward toward the Crimean Plains, the Paleogene aquifers become confined, the groundwater acquires a substantial head, and TDS increase.
The Lower Paleocene aquifer is present on the southern sides of the Alminsky and the Belogorsky basins. It is confined by low-permeability marls and clays of Upper Cretaceous age below and by dense marls of the Kacha unit (Upper Paleocene) and Lower Eocene clays above.
Toward the centers of the basins the thickness of aquifers increases from 45 to 75 m and the head reaches 1000-1500 m. Discharge is highly heterogeneous, drastically increasing in tectonized and karstified zones. In the recharge area waters are of the Ca-HCO 3 -and Mg-Na-Ca-HCO 3 -type and TDS reaches up to 0.7 g L -1 . With increasing depth the composition changes to Na-HCO 3 -Cl-and Na-Cl-types, and the mineralization increases up to 18 g L -1 . These waters also contain minor components of Br and I and dissolved gases (mostly CH 4 , as well as N 2 , noble gases, and small amounts of O 2 and CO 2 ).
The Middle Eocene aquifer has a thickness of 20-40 m in the recharge area, increasing to 200-220 m in the submerged parts of the basins. It is mostly continuous in the southeastern side of the Alminsky basin and in the Simferopol uplift, and in the southern side of the North-Sivash depression. The water has a Ca-HCO 3 -type composition and TDS values of 0.4-0.6 g L -1 in the recharge areas. Its composition evolves via a Ca-Na-HCO 3 -and a Na-HCO 3 -type into a Na-Cltype, with up to 25 g L -1 TDS.
The two Paleogene aquifers are separated by marls and clays. This aquitard is discontinuous and is breached along fault zones. In areas where the aquitard is entirely absent the two aquifers merge. In the eastern sector, several kilometers to the north of the recharge area, there is a line of upwelling springs (Krivtsovo-Sennoe line), representing a marginal zone of discharge of this plunging Paleogene aquifer. Discharge diminishes to the north of this line, suggesting lower conductivities in the deeper submerged parts of the aquifer.

Cretaceous aquifers
The Upper Cretaceous aquifer has its outer recharge area in the depression that separates the Main Range from the Piedmont and stretches along the foot of the Inner Range (the Southern Longitudinal depression). This aquifer is comprised of marls and marly limestone and hosts water of the Ca-Cl-SO 4 -type with 1.7-3.2 g L -1 TDS. Toward north and northwest, the aquifer becomes confined and its composition changes to a Na-Cl-type with TDS values of 10-40 g L -1 .
The Lower Cretaceous aquifer comprises layers of sandstones, sands, Hauterivian and Barremian conglomerates and fractured limestones, as well as fractured sandstones and conglomerates within Albian shales. These rocks crop out on the northern slopes of the Main Range and, locally, in the Southern Longitudinal depression. The outcrops represent the outer recharge area for this aquifer. The thickness of the aquifer ranges from several meters to >100 m. The aquifer rocks plunge in northward direction down to a depth of 1500-2500 m (underneath the Novoselovsky uplift and the North-Sivash region). In these areas heads exceed 80-120 m above land surface (i.e., total head in the aquifer of 200-250 bar).
The water in the recharge area has a Ca-HCO 3 compositions and 0.3-0.6 g L -1 TDS. In the submerged parts, in the North-Sivash region, waters show a Na-Cl composition and 10-68 g L -1 TDS. The waters are also enriched in minor elements (Br and I). In the Novoselovsky uplift water contains CH 4 (up to 56 vol. %) and N 2 (up to 40 vol. %), as well as elevated amounts of He (Lushchik et al., 1987). CH 4 -N 2 -bearing waters are known from the Belogorsk area, and groundwater containing CH 4 and CH 4 -N 2 -CO 2 occurs in the northern and northwestern part of Sivash, respectively. In the western part of the Alminsky basin, in the Novoselovsky uplift, temperatures in the aquifers reach 50-58°C and in the northern Sivash region even about 100°C.

Stable isotope characteristics of groundwater in Crimea
Summary of the stable isotope analyses of the Crimean groundwater, discussed in the paper which is supported by this document ("Groundwater of the Crimean peninsula: A first systematic study using stable isotopes" Dublyansky et al., 2019) is shown in Fig. 4. The complete dataset is available at doi:10.17632/7bhp3v3wcs.1.