Origin and Flow Dynamic of Groundwater

ORIGIN AND FLOW DYNAMIC OF GROUNDWATER IN MOVILE CAVE AREA

Origin and flow dynamic of Groundwater

In MOVILE CAVE AREA, ROMANIA, based on

Environmental isotopes

Lucia Feurdean, Victor Feurdean, Mihai Gligan

National Institute for R&D of Isotopic and Molecular Technologies, PO Box 700, 3400 Cluj-Napoca 5, Romania,

Abstract

As a conservative tracer in carbonate rocks deuterium was used to determine the un-elucidated problems of the water origin in cave. The values of 2H and 18O plotted on GMWL (Craig line) indicate the meteoric origin of groundwater in Movile Cave area. The 2H-dens correlation of samples delineated three isotopically water types. Time series of the deuterium concentration from groundwater samples exhibit a distinct seasonal effect with lower deuterium concentration in summer than in winter. The water of Movile Cave is the result of the mixing of two endmembers that have different hydrogen isotope ratios. The spatial variation of the deuterium content delineated the areas with vertical inflow and the flow path of groundwater from deep aquifer. The discharge of the deep aquifers occurs through faults and karstic channels into overlying aquifer. The Movile Cave and Karaoban Lake are connected and the cave water discharge by overflow mechanism that offers to cave the isolation of atmosphere.

The stable isotopes content of waters collected from study area provided important information about the unusual subterranean ecosystem of Movile Cave that is based on chemoautotrophic conditions. Located near the Mangalia City, the Movile Cave is closed system, which has groundwater rich in H2S (8-12 mg/l) and an atmosphere poor in oxygen (7-10%), rich in CO2 (2-3.5%) and containing significant amount of CH4 (1-2%). Climate is semiarid; the land topography is smooth. The detritic Palaeozoic formation is overlain by a 450-500m thick stack of mainly carbonate rocks belonging to the Jurassic, the Cretaceous and the Eocene; over them the fosiliferous limestones of Sarmatian age have been deposited in a thickness of 60-150m. Loess deposits covering limestones represent the Quaternary. The tectonic structure indicates the three faults (figure 1). Rocks classified according to their hydrological function include:

The unconfined aquifer in the Sarmatian karstified limestones.

The confined aquifer in the Eocene and Mesozoic limestones, which are fractured and karstified and represent a unitary complex interconnected with upper lying aquifer.

As a conservative tracer in carbonate rocks, deuterium was used to determine the un-elucidated problems of the water origin in cave. The average value of 2H for all samples is –85‰, for the most samples (76%) the 2H values are lower than -80‰ and is isotopically more light than the waters from the semiarid lowland of the Dobrogea district that are over –76‰ (figure 2 and 3).

Fig. 2. Histogram of 2H valuesFig. 3. 2H-Dens diagram

The 2H-Dens correlation. The distribution of the average values in the diagram delineate the existence of three isotopically water types:

-Type I that have low deuterium content (2H<-88‰) and the samples come from wells concerning the Barremian-Jurassic aquifer and from the springs emerging in Karaoban area.

-Type II represents waters with deuterium content about mean recorded value of about –85‰. These samples are from domestic wells, Movile Cave and few springs.

-Type III represents waters with higher deuterium content (2H>-80‰) that come from drink water wells and from southwestern part of area, in proximity of Mangalia Lake.

The 2H values lower than –80‰ are not specific to low land of Romania. The low 2H values can be attributed to the following factors:

-the infiltration of winter meteoric water or high altitude water,

-the paleoclime effect,

-the isotopic exchange between water and other hydrogen compounds.

The2H-18O covariance. The data of hydrogen and oxygen isotopic contents for the samples from Movile area (fig. 4) form a linear band that can be described by equation with the slope of 8. The isotope data for samples collected from the Sarmatian aquifer () fit the Craig line () reasonable well despite the mathematical differences between the local line and global meteoric line. The 2H-18O covariance indicates the meteoric origin of groundwater from Movile area end reflects small kinetic effects during evaporation-condensation of precipitation that are the input of groundwater. The recorded isotopic content ranges between isotopically very light water and isotopically normal water for the lowland of Romania.

The 2H-time correlation. The seasonal variation of deuterium content from precipitation input is reproduced after damping and delay in the seasonal variation of 2Hvalues from groundwater.

The seasonal effect is great preserved in groundwater flowing in fractured rocks (conduit flow or flow through subterranean passage partial filled with water) and lamped in groundwater moving in homogenous porous media and/or long travel way. Similarity in the behavior of deuterium time series of different samples shows the same origin and path flow of groundwater. The different distances between recharge and sampling points or the different flow rates can produce the 2H value shift in time and the peaks of discharge can be recorded at different moments. The recorded peaks on the background isotopic concentration of the 2H- time series are caused by the storm discharge that strongly affects the flow system by the input concentration at the discharge moment.

The time series of 2H values from groundwater of Movile Cave area indicate the following:

-Well outlined in time series (fig. 5), the seasonal effect of 2H values from precipitation excludes the possibility of the paleoclimatic origin of groundwater and indicates a strong link with isotopic composition of precipitation, but a lower deuterium concentration in summer than in winter;

-This six-month shift of the isotopic composition variations with respect to normal succession of seasonal maximum and minimum values of precipitation and low 2H average values are due to the water recharge from higher altitude (from the Balkan Plateau) and the long way of subterranean movement.

-The analogy of the variation in time (fig. 6,7), with or out temporal shifting of the peaks, emphasized the membership at mixing system and/or the genetic relationship between water sources;

-The large peaks of 2H values recorded in late spring months and in early summer months for the F1 and F11 samples (fig. 7) are due to the event water (rain falling on the site with altitude of 1000m and a rapid flow through alternative pathway is the most probably).

The mixing lines. On the 2H - Dens average values, from possible straight lines are corroborated with other criteria represent a mixing system. The alignment of samples suites must be corroborated with evolution in time of 2H. The diagram (fig. 8) suggests that the water from Movile Cave belongs to a mixing water system. One end member is isotopically light water and another end member
is isotopically enriched water.

Fig. 7. Time variation.

The spatial variation of 2H values. The closed isolines of equal concentration of deuterium indicate vertical movement of water. The most low 2H values were recorded for samples F1 and F11 from aquifer caught in Mesozoic limestones. The map of isolines of equal concentration (fig. 9) delineates the zones with the movement of water by vertical (closed lines) and horizontal (open lines). The water movement by vertical is for the following samples:

-from the aquifer caught in Mesozoic limestone (F1, F2);

-from the springs and well on the Karaoban site;

-for samples from Movile Cave.

The horizontal movement of water was determined for water caught in Sarmatian limestone. The movement direction is along the faults toward Mangalia swamp for deep groundwater and from WSW toward Black Sea from Sarmatian aquifer. The water of Movile Cave is a mixing of waters from Karaoban site, WSW wells and domestic fountains. Movile Cave is the buffer system between the confined aquifer (from depth) and unconfined aquifer and the connection between them is of siphon type. The isolines show the same mixing system as a mixing lines.

Fig. 9. Spatial variationof 2H average values.

Conclusions

-Water of Movile Cave area is of meteoric origin.

-Water from cave area come from high altitude (Balkan Plateau) site and flow in deep is slow and discharge occurs through faults that establish hydraulic connections with Sarmatian aquifer, but the springs across the impermeable formation, too. The Movile Cave is the buffer system between the confined aquifer (from depth) and unconfined aquifer and the connection between them is of siphon type.

References

1. Craig H.Isotope variations in meteoric waters. Science, 133 (1961): 1702-1703.

2. Sarbu S. M., Kane T. C. and Kinkle B. K.A chemoautotrophically based groundwater ecosystem. Science, 272 (1996): 1953-1955.

3. Feurdean L., Feurdean V. and Oraseanu I. The use of deuterium as natural tracer to point out groundwater flow from karst area. Case study. In New Approaches Characterizing Groundwater Flow (eds. Seiler & Wohnlich): 2001, 59-63.