Hydrogeology
Major on-shore karst springs with individual average discharges of 1 to 2.5 m3/s (35 to 88 cu ft/s) are located at altitudes ranging from 1 m (3.3 ft) (Reproa Spring) to 540 m (1,770 ft) (Gegsky Vodopad). Two of them are located in the shore area; these are Reproa (average discharge 2.5 m3/s or 88 cu ft/s; altitude 1 m or 3 ft 3 in above sea level) and Kholodnaja Rechka (1.2 m3/s or 42 cu ft/s; 50 m or 160 ft a.s.l.). Two more major springs are located in the river canyons bordering Arabika to the east: Goluboe Ozero in the Bzyb canyon (2.5 m3/s or 88 cu ft/s; 90 m or 300 ft a.s.l.) and Gegsky Vodopad in the Gega canyon (1 m3/s or 35 cu ft/s; 540 m or 1,770 ft a.s.l.). There are also several smaller springs in the Gagra town.
Some boreholes located along the shore of the Black Sea yield karstic groundwater from depths of 40–280 m below sea level. Other much deeper boreholes tapped low-salinity karstic waters at depths of 500 and 1,750 m in the Khashupse Valley near Tsandripsh and 2,250 m near Gagra. This suggests the existence of a deep karst system and vigorous karst groundwater circulation at depth.
Submarine springs are known in the Arabika area, emerging from the floor of the Black Sea in front of the massif. Shallow springs at depths of 5–7 m can be reached by free dive near Tsandripsh. Kiknadze (1979) reported submarine springs near the eastern part of Gagra at depth of 25–30 m and Buachidze and Meliva (1967) revealed submarine discharge at depths up to −400 m by hydrochemical profiling. Recently an outstanding feature of the sea floor topography near Arabika has been revealed from a digital bathymetric map that combines depth soundings and high-resolution marine gravity data. This is a huge submarine depression in front of the Zhovekvara River mouth, which has dimensions of about 5 x 9 km and a maximum depth of about 380 m (1,250 ft). The Arabika Submarine Depression is a closed feature with internal vertical relief of about 120 m (390 ft) (measured from its lowest rim) separated from the abyssal slope by the bar at a depth of about 260 m (850 ft). It has steep northern and northeastern slopes (on the side of the massif) and gentle south and southwestern slopes. Its formation is apparently karstic. Presently this depression seems to be a focus of submarine discharge of the karst systems of Arabika.
The speleological explorations and a series of dye tracing experiments conducted during the 1980s under the coordination of Alexander Klimchouk have radically changed previous notions of the hydrogeology of Arabika, revealed its outstanding speleological perspectives and strongly stimulated further efforts for exploration of deep caves. Tracers injected in the Kujbyshevskaja Cave and the Iljukhina System were detected in the Kholodnaja Rechka and Reproa springs, proving groundwater flow to the south-southwest across major tectonic structures over a distance of 13–16 km as the crow flies (Figure 1). The tracer from Kujbyshevskaja Cave was also detected in a borehole located between these two springs, which yields groundwater from a depth of 200 m (660 ft) below sea level. This has been interpreted as an indication of the connection of the cave with the submarine discharge. The large "Central Karst Hydrologic System", which encompasses most of the southeastern flank of the Arabika anticline, had been identified in this way. The system became the deepest in the world with its overall vertical range of about 2,500 m (8,200 ft) (measuring to the borehole water-bearing horizon) or even 2,700 m (8,900 ft) (measuring to the deepest reported submarine discharge points).
Another tracer was injected in the Moskovskaja Cave (−970 m) and detected at the Gegsky Vodopad spring, indicating the presence of a karst hydrologic system comprising the northeastern flank of the Arabika anticline (the "Northern System"). No connections have been revealed with yet another major spring, Goluboje Ozero in the Bzyb River canyon, although it apparently drains a large area of the eastern sector of the massif (the hypothetical "Eastern Karst Hydrological System"). It is not clear where Sarma Cave (−1,550 m) drains to, Goluboje Ozero to the southeast or Reproa to the southwest, at the shore.
The results of the dye-tracing tests demonstrated that groundwater flow is not subordinate to the fold structure but is largely controlled by faults that cut across the strike of major folds, and that the large part of the central sector of Arabika is hydraulically connected to the springs along the seashore and with submarine discharge points.
Krubera Cave has an extremely steep profile and reveals a huge thickness of the vadose zone. The lower boundary of the vadose zone (the top of the phreatic zone) is at an elevation of about 110 m (360 ft) at low flow, which suggests a low overall hydraulic gradient of 0.007-0.008. Low-TDS groundwater is tapped by boreholes in the shore area at depths of 40–280, 500, 1,750, and 2,250 m below sea level, which suggests the existence of a deep flow system with vigorous flow. Submarine discharge along the Arabika coast is reported at depths up to ~400 m b.s.l.
It is difficult to interpret these facts in terms of the development of karst systems controlled by contemporary sea level, or within the range of its Pleistocene fluctuations (up to −150 m). In combination with the existence of the Arabika Submarine Depression, all these facts point to the possibility that karst systems in Arabika could have originated in response to the Messinian salinity crisis (5.96–5.33 Ma) when the Black Sea (Eastern Paratethys) could have almost dried up, as did the adjacent Mediterranean, where the dramatic sea level drop of ~1,500 m is well established.
Read more about this topic: Voronya Cave