The Danube Delta is a dynamic wetland system where surface and groundwater are closely interconnected, primarily influenced by the water levels of the Danube River. Given the shallow depth of the water table, the fluctuations in river discharge can impact groundwater levels in the delta. This study shows hydrostatic levels along the banks of several canals in the Danube Delta through field measurements conducted during high-water (spring) and low-water (autumn) periods. Shallow boreholes were drilled using a hand auger manual set to determine groundwater levels and analyze sediment composition. During fieldwork, it was observed that when groundwater was encountered in the boreholes, the water often began to rise until reaching a stable level. The extent of this rise was influenced by the sediment type affecting the distance the water travelled before stabilizing. This highlights the role of sediment composition in controlling groundwater movement, particularly in relation to capillarity and permeability. The collected data were used to generate spatial representations of hydrostatic variations. Furthermore, correlations were established between sediment granulometry, groundwater fluctuations and hydrological extremes, revealing some relationships between sediment characteristics, surface hydrology and subsurface water dynamics in this complex deltaic environment. The results contribute to highlighting the groundwater behaviour in response to seasonal water level changes, with potential implications for wetland management.

Danube Delta, groundwater, Hydrology, hydrostatic level

The hydrostatic level of groundwater is the height at which water naturally stands in a well or borehole, reflecting the pressure in the aquifer. It indicates the water table, which is commonly used in hydrogeological studies (Brassington 2017).

Infiltration is an important process in hydrogeology and hydrology by which water from the land surface penetrates the unsaturated zone, which consists of soil, sediment, or permeable rock. During this process, infiltrating water may encounter an impermeable layer, which promotes water accumulation and subsequently forms an aquifer (Scrădeanu and Gheorghe 2007).

The phreatic aquifer of the Danube Delta has formed a close connection with the waters of the Danube River. It represents the first aquifer layer, located at relatively shallow depths, and its water level marks the upper surface of the saturated zone within the aquifer. Phreatic waters are recharged through the infiltration of precipitation and surface water.

The main factor influencing infiltration is soil/sediment permeability (Kreitler 1989). Groundwater movement is faster in permeable soils, such as sands and gravels, and slower in clay-rich soils or compact rock formations. As permeability depends on porosity, grain size, and particle shape (Shepherd 1989; Kamann et al. 2007), the sediments hosting the aquifer directly influence the hydrostatic level variations, storage capacity, and overall behaviour of the aquifer system. Additionally, variations in permeability and porosity affect how capillary water is retained and transmitted (Benavente et al. 2015).

Other important factors influencing infiltration include precipitation, floods, surface water levels, and evaporation (Fetter 2018). Precipitation and floods affect the hydrostatic level, which is closely linked not only to the amount but also to the frequency of these events, directly influencing aquifer recharge. During periods of heavy rainfall or flooding, groundwater levels rise due to the infiltration of surface water into the soil.

Since groundwater is closely connected to surface waters, the hydrostatic level will fluctuate in response to seasonal variations. When the Danube River level rises, water can infiltrate the soil, increasing the groundwater hydrostatic level. Conversely, during droughts or periods of low river levels, the hydrostatic level may decrease.

Evaporation is the process by which water changes from a liquid to a gaseous state and rises into the atmosphere. This process is influenced by factors such as temperature, humidity, wind, and the exposed surface area. In the Danube Delta, evaporation is combined with plant transpiration (evapotranspiration), contributing to water loss from the soil and phreatic aquifers. High evaporation rates, particularly during the summer months, can reduce surface water levels and indirectly impact the hydrostatic level by decreasing aquifer recharge (Lubczynski 2011).

The phreatic aquifer of the Danube Delta is important in maintaining the ecological health and water resource sustainability of the region. It supplies water to numerous aquatic and terrestrial habitats within the delta, which host a diverse range of species, including rare and protected plants and animals. In wetland areas, the phreatic aquifer helps sustain these ecosystems by filtering and preserving water quality. Additionally, soils and sediments within the phreatic aquifer act as a natural filter, purifying water from various contaminants (Keesstra et al. 2012) before they reach deeper layers or surface water bodies. This natural filtration system helps prevent soil and water salinization in the Danube Delta, especially by limiting the infiltration of saltwater from the Black Sea.

The measurements for determining the water level (WL) were conducted on Crânjală and Mitchina canals, on March 27, 2023, during high water levels, and on October 26, 2023, during low water levels. Similarly, on April 4, 2023, when water levels were high, and on October 25, 2023, during low water levels, measurements were carried out on Letea and Sidor canals. In total, 80 shallow boreholes were drilled, with depths ranging between 15 and 240 cm. On Crânjală canal, 15 boreholes were performed, with 7 in the spring and 8 in the autumn, while on Mitchina canal, 5 boreholes were drilled, 2 in the spring and 3 in the autumn. On Letea canal, the number of boreholes reached 24, evenly distributed between the two seasons, whereas on Sidor canal, the highest number of boreholes was recorded, totalling 36, with 18 performed in spring and another 18 in autumn.

On the designated canals, measurements were taken at three specific locations, marked as Profiles 1, 2, and 3 (see Fig. 1). At each location (profile), it was initially planned to drill six boreholes (three per bank), perpendicular to the canal direction, at a maximum distance of 160 meters from the bank. However, due to the unstable nature of the terrain and the constant presence of water in the deltaic system, completing all six planned drillings for each profile was often not possible (Figs 2, 3).

The drillings were carried out using the Eijkelkamp manual auger set for heterogeneous soils (Fig. 4 Left). To perform the drillings, the appropriate Edelman auger must be selected based on the soil/sediment type encountered (sandy, clayey, or a combination of both). With the Eijkelkamp drilling device, the operator advanced approximately 20 cm at a time, corresponding to the length of the auger, which was then pulled out filled with sediment or soil. The auger was cleaned and reinserted into the borehole repeatedly until the water table was reached. The corresponding coordinates of the drillings can be found in Table 1.

The data collected in the field was used to develop hydrogeological profiling and analyze the seasonal variation of the water level in the studied canals (Figs 5–7).

Figure 1. 

The location of the studied canals (blue line) – profiles and drillings. Legend: L. – lake, c. – canal, F – drilling, white dot – profile 1, light blue dot – profile 2 and red dot – profile 3 (on each canal).

Figure 2. 

Ponds on the banks of the Mitchina (left image) and Sidor (right image) canals. Photos by Livanov O., 2023.

Figure 3. 

Shallow water on Mitchina canal – northern bank. Photo by Livanov O., 2023.

The hydrogeological profiling of the Crânjala Canal on the three surveyed profiles is presented in Fig. 5. Access to both banks proved difficult due to the presence of surface water as ponds, especially in spring when water levels were high. During high water conditions, two drillings were performed on Profile 1, four on Profile 2, and one on Profile 3. At lower water levels, an additional drilling was possible, as access became easier due to the receding waters. General information about the drillings and the recorded water levels is provided in Table 2.

During high water levels, the water table was encountered at depths ranging between 30 and 70 cm, eventually stabilizing closer to the surface, at depths between 15 and 50 cm. The distance travelled by the water level before stabilization varied from 15 to 35 cm. A stable water table was observed in only three drillings. At low water levels, the water table dropped significantly, being identified at depths ranging from 70 to 240 cm and stabilizing between 60 and 210 cm, with a fluctuation range of 10 to 55 cm. Stable levels, where water did not rise after drilling, were recorded in four boreholes.

The seasonal variations in the water table ranged between 5 and 170 cm, corresponding to a canal water level difference of 138 cm (210 cm during high water levels and 72 cm during low water levels).

The hydrogeological profiling of the Mitchina canal on the three surveyed profiles is presented in Fig. 5. Access to this canal proved to be much more difficult than on Crânjală. The only stable area was the southern bank, where two drillings were conducted during high water levels and three during low water levels. Moving further inland was demanding due to the presence of water ponds, muddy terrain, and dense vegetation typical of the area. The northern bank of the canal could not be approached as dry land, as it presented itself as a water expanse (Fig. 3), making docking and drilling impossible.

During high water levels, as mentioned above, two drillings were carried out, both on the southern bank, on Profiles 1 and 2. On Profile 3, drilling was not possible due to surface water, but it was successfully conducted in autumn, during low water levels, when the water had partially receded. At high water levels, the water table was encountered at a depth of 50 cm on Profile 1 and 30 cm on Profile 2. In both cases, the water level remained stable and did not rise. During low water conditions, the water table dropped to 140 cm on Profile 1, 130 cm on Profile 2, and 150 cm on Profile 3. While the levels in the first two boreholes remained stable, the water table in the borehole on Profile 3 rose by 10 cm, stabilizing at 140 cm. General information about the drillings and the recorded water levels is provided in Table 3.

Regarding seasonal differences, only the first two profiles can be compared, showing a fluctuation of 90 cm on Profile 1 and 105 cm on Profile 2, in relation to a canal water level difference of 138 cm (210 cm during high water levels and 72 cm during low water levels).

The hydrogeological profiling of the Letea canal on the three surveyed profiles is presented in Fig. 6. Access to the eastern bank of the canal was unimpeded, as its proximity to Letea village facilitated movement and allowed drillings to be carried out safely. On the western bank, however, only one drilling per profile was conducted due to the presence of water ponds and muddy terrain.

In spring, during high water levels, the water table was encountered at depths ranging between 30 and 70 cm, stabilizing between 20 and 30 cm. The distance travelled by the water level before stabilization was minimal, between 5 and 10 cm in four boreholes, while in the remaining ones, the water table was stable. During low water levels, the water table dropped to depths between 65 and 120 cm, stabilizing in four boreholes at depths between 60 and 100 cm, with a fluctuation of 5 to 10 cm. General information about the drillings and the recorded water levels is provided in Table 4.

Seasonal variations in the water table ranged between 25 and 90 cm, corresponding to a canal water level difference of 60 cm (119 cm during high water levels and 59 cm during low water levels).

The hydrogeological profiling of the Sidor canal on the three surveyed profiles is presented in Fig. 7. Access to both banks of the canal proved to be very easy, regardless of water levels, as difficult terrain and water ponds were located outside the targeted areas. Although some isolated flooded zones were present during high water levels, they did not hinder the survey, as they could be easily bypassed.

During high water levels, the water table was encountered at depths ranging from 15 to 50 cm, stabilizing between 10 and 30 cm. The fluctuation before stabilization varied between 5 and 15 cm. During low water levels, the water table dropped to depths between 80 and 120 cm, stabilizing between 85 and 110 cm, with a fluctuation of 5 to 10 cm, values very close to those recorded in spring. General information about the drillings and the recorded water levels is provided in Table 5.

Seasonal variations in the water table ranged between 45 and 80 cm, corresponding to a canal water level difference of 60 cm (119 cm during high water levels and 59 cm during low water levels).

Regarding the lithology encountered in the boreholes, fine-grained sediments (clay-silt with slight sand content) were identified on the Crânjală and Mitchina canals, while coarser sediments, primarily sands, were found on Letea and Sidor canals. In almost all boreholes, the uppermost centimetres revealed an organic layer (soil) rich in plant remains, with its development varying depending on local conditions within the Delta (Livanov et al. 2023). A rich organic surface layer can be found along the banks of Crânjală and Mitchina canals, where Delta-specific vegetation is abundant. However, on the sandy banks of Letea and Sidor canals, this layer was poorly developed and, in many cases, completely absent.

Figure 4. 

Left: Drilling a borehole near Letea Canal with Eijkelkamp manual auger set for heterogeneous soil. Right: Edelman augers: a, b. Sand type; c. Combination type; d, e. Clay type. Photos by Livanov O., 2023.

Figure 5. 

Hydrogeological profiling on Crânjală and Mitchina canals. Legend: WL – water level, F1–6 (green) – drillings, red line – high water level (spring 2023), blue line – low water level (autumn 2023), continuous lines (red and blue) – stabilized WL, dashed lines (red and blue) – encountered WL.

Figure 6. 

Hydrogeological profiling on Letea canal. Legend: see Fig. 5.

Figure 7. 

Hydrogeological profiling on Sidor canal. Legend: see Fig. 5.

Hydrostatic level measurements revealed significant differences depending on the specific characteristics of each canal. These variations allowed for comparative analysis, providing a detailed perspective on hydrostatic fluctuations and the factors influencing them for each canal.

As expected, substantial seasonal variations in hydrostatic levels were observed. The correlation is straightforward and directly proportional to the water levels of the Danube: when river levels are high, the hydrostatic level of the phreatic aquifer rises, whereas during low water periods, it decreases.

The Crânjală and Mitchina canals, located in the fluvial delta, have similar sedimentary deposits, as both are connected to Fortuna Lake and share a sediment composition primarily consisting of sandy clayey silts and their variations (Catianis et al. 2020). The presence of fine particles (clay and silt) significantly influences the behaviour of the phreatic aquifer and the distribution of its hydrostatic level. Since clay and silt have much lower permeability compared to sand and gravel (Lewis et al. 2006; Evirgen et al. 2015), aquifers composed mainly of fine particles restrict groundwater movement, affecting recharge and discharge rates. Due to its extremely small pore spaces, clay has a high water retention capacity, which can lead to the formation of impermeable layers, acting as barriers that separate different aquifer units and influence groundwater distribution.

In contrast, the Letea and Sidor canals are situated on sandy deposits. Sand has a much higher permeability than clay and silt, allowing for faster groundwater movement. Aquifers primarily composed of sand exhibit higher recharge capacity and water flow rates. Additionally, sand promotes a more uniform hydrostatic level distribution due to its high permeability, facilitating the equalization of hydrostatic pressure within the aquifer and ensuring a consistent water flow.

This study aimed to demonstrate the dependence of multiple factors influencing the hydrostatic levels of the phreatic aquifer in the Danube Delta. Seasonal variations of the Danube River, precipitation patterns, sediment composition, grain size distribution, permeability, and porosity significantly affect the dynamics of both groundwater and surface water within this complex aquatic-terrestrial system. Long-term seasonal measurements will contribute to the development of a comprehensive database, forming the foundation for quantifying fluctuations in the hydrostatic level of the phreatic aquifer and hydrological variations.

Wetland management is closely tied to the characteristics and behaviour of the underlying aquifer, as groundwater is important in hydrological balance. Aquifers contribute to wetland water levels through groundwater discharge, ensuring a stable water supply during dry periods and influencing wetland hydrodynamics. The permeability and porosity of aquifer sediments determine the rate of water exchange between groundwater and surface water, affecting nutrient cycling, vegetation patterns, and habitat stability.

This work was supported by the project “Research on the evaluation and analysis of the clogging rhythm of canals subjected to engineering interventions to improve hydrological conditions from the territory of the Danube Delta Biosphere Reserve” at Danube Delta National Institute for Research and Development of Tulcea, financed by the Ministry of Research, Innovation, and Digitalization of Romania in the framework of Program “Danube Delta 2030”, code PN 23 13, Project PN 23 13 04 01, Contract 35N/2023.