Correlation Between Entrance Velocities, Increase in Local Hydraulic Resistances and Redox Potential of Alluvial Groundwater Sources - page 03

Study Areas
Given that the well ageing problem is not encountered solely in connection with groundwater sources, but extends to other riparian well systems with diverse functions, wells used for different purposes and of different designs were selected for the present study. The study areas are related to three rivers: the Danube, the Sava and the Velika Morava, whose aquifers differ in terms of origin, evolution and current characteristics. Systems of wells were analyzed on five locations in alluvial sediments, Fig. 6:

  • The Sava: groundwater source that services the City of Belgrade,
  • The Velika Morava: groundwater source that services the City of Požarevac (called “Ključ”); and
  • The Danube: series of wells that are part of a drainage scheme which protects riparian lands, sections Kovin-Dubovac, Knićanin-Čenta and Veliko Gradište.

There is voluminous aquifer and groundwater information about these locations. However, the availability of necessary data and the analyzed periods were quite different. Consequently, the results of chemical analyses for the Veliko Gradište site were disregarded as field investigations and laboratory campaigns need to be intensified.

 

The Sava River

Belgrade Groundwater Source

The Belgrade Groundwater Source is in part surrounded by urban fabric. It is comprised of 99 radial wells and about 50 tube wells, located along the Sava's bank upstream from its confluence with the Danube. The length of the series of wells is about 50 km, Fig. 7.

 

The alluvial aquifer was developed through several sedimentation cycles and sequences: sandy gravel, sands of various grain sizes, and silty and clayey sediments. The thickness of the Quaternary strata is up to 25 m. With regard to the grain sizes of the sediments, two cross-sectional zones have been distinguished, Fig. 8:

  • Lower zone: coarse-grain sediments, in which radial well laterals are installed. These sediments occasionally feature clay, sandy clay and silt interbeds and lenses: and
  • Upper zone: fine-grain sediments, with poorer filtration properties.

The grain sizes of the lower zone, in which the well screens are installed, range from medium-grain sand to fine-grain gravel, Fig. 9.

The capacity of this source largely relies on the radial wells. Since its inception (the first well was built in 1953), the city's increasing water demand has been met by adding new wells. However, a continuous decline in discharge has been noted from the very beginning, Fig. 10. Rapid expansion of the source ended in the mid-1980's and, since then, it has registered a permanent capacity decline, along with a further decrease in well discharge. Periodic well regenerations (rehabilitations) increased well discharge only in the short term.

Some of the chemical parameters of the groundwater, derived from analyses of samples collected from 2005 and 2013, and aquifer grain sizes in the vicinity of the wells, are summarized in Table 1. The values are related to the entire source and reflect data on all the wells.
Figure 11 shows the results of interpretation of several groundwater parameters, average entrance velocities to the wells and d50 (50%) of the grain-size distribution, relative to selected redox potential intervals. The horizontal axis, apart from the Eh interval, shows the number of samples used in the given interval. The main characteristic of this source is a relatively low Eh, generally below 150 mV.

 

fig06
Figure 6: Study areas along the Sava, Danube and Velika Morava rivers (www.Google Earth).

 

fig07
Figure 7: Location of the Belgrade Groundwater Source along the Sava River.

 

fig08
Figure 8: Belgrade Groundwater Source: typical lithological section through the Sava riverbank.

 

fig09
Figure 9: Belgrade Groundwater Source grain-size distribution curve envelope of the setting in which radial well laterals are installed (Dimkić et al, 2007).

 

fig10
Figure 10: Average annual discharges of the wells and of the entire Belgrade Groundwater Source.

 

fig11
Figure 11: Average values of selected Belgrade Groundwater Source parameters, by redox potential segment. (Legend: Eh – redox potential, d50 – aquifer grain size (50% of the grain-size distribution curve), v – entrance velocity, O2 – oxygen concentration in water, NO3 – concentration in water, Fe2+ – bivalent iron concentration in water, SO4 – concentration in water, and Ec – water conductivity).

 

Tab01