Correlation Between Entrance Velocities, Increase in Local Hydraulic Resistances and Redox Potential of Alluvial Groundwater Sources - page 9

Figure 32 is one of the outcomes of the study, which illustrates the correlation between Eh and well entrance velocities with regard to the considered alluvial aquifers in Serbia. The conclusion was that the velocities that ensure filtration stability of the near-well region do not meet the criterion of the allowable increase in local hydraulic resistances at low values of Eh (especially Eh < 200 mV). When Eh is greater than 300 mV, the two criteria become virtually equal.

In view of the fact that the focus was on well ageing caused by iron clogging, the correlation between Eh and Fe2+ was analyzed. Figure 33 shows averaged results for the studied wells along the three rivers. The correlation between these two parameters could not be doubted. For values of

Eh of 150 to 200 mV, the average Fe2+ concentration was of the order of 0.5 mg/l. If Eh was lower, the Fe2+ concentration increased rapidly.

Based on the established correlation, a plot was constructed of entrance velocities as a function of Fe2+ concentration, Fig. 34.

It was found that for the quantification of maximum allowable entrance velocities, the variability of local hydraulic resistances also needed to be taken into account. At nearly all the wells, this quantity was much smaller than if estimated using the formulas that ensure filtration stability of the near-well region. This was especially true of anoxic groundwater conditions, or in the present case for Eh ≤ 200 mV and Fe2+ ≥ 0.5 mg/L, Figs. 32 and 34.

The above shed some light on the phenomenon that has long puzzled specialists who studied well ageing at the Belgrade Groundwater Source. In well design, the entrance velocity criteria applied were those that ensured filtration stability of the near-well region and well screen. Here a clearer emphasis is placed on the impact of biochemical factors (above all the groundwater redox potential and iron concentration) on the intensity and rate of well clogging.

Data collected from the other study areas were added to the data from the Belgrade Groundwater Source, to provide a more complete picture of the effect of iron concentration and oxic state of groundwater on well ageing. The data presented above are related to study areas along three rivers (the Sava, the Danube and the Velika Morava) and they confirm the general prevalence of the same well clogging processes in alluvial environments.

For values of Eh greater than 200 mV and iron (Fe2+) concentrations less than 0.5 mg/L, the criteria derived from the filtration stability condition become dominant, while the effect of biochemical clogging is reduced and ultimately ceases.

 

Conclusions

Well ageing due to well-screen clogging is a long-known problem. The economic and technical significance of the clogging phenomenon is enormous. Mechanical clogging, due to excessive entrance velocities, has been studied extensively and the relevant criteria defined by old masters (Abramov, 1952, Cistin, 1965, Gavrilko and Alekseev, 1985, Johnson, 1975, Kovacs and Ujfaludi, 1983, Pietraru, 1982, Sichardt, 1928 and others).

 

 

Biochemical clogging (biofouling) occurs as a result of processes that take place in anoxic groundwater in the near-well region. A number of authors worldwide have considered the anoxic state and the processes that lead to well ageing. However, the objective of the long-term and broad-based research reported here was to establish a correlation between some of the most important biochemical parameters of groundwater (Eh, Fe) and mechanical parameters (groundwater entrance velocities, rate of clogging). As far as we are aware, this constitutes pioneering work on a global scale.

Research was conducted on several sites along three rivers in Serbia (the Sava River – Belgrade Groundwater Source; the Danube – riverbank sections from Kovin to Dubovac and from Knićanin to Čenta; and the Velika Morava – Ključ Groundwater Source near the City of Požarevac). All the tests revealed that there was virtually no biochemical clogging under oxic groundwater conditions (where the redox potential was greater than 250 mV).

In anoxic conditions (the redox potential range above 50 mV was studied), the main parameters that drive well clogging, apart from well entrance velocities, were found to include the groundwater redox potential and iron concentration.. The general expression is:

for10

where: v –entrance velocity, Fe – iron concentration in well water, Eh – groundwater redox potential, B – function of the rate of bacterial growth in the well, Γ – function of several parameters related to well design (with or without gravel pack, gravel pack characteristics, type and characteristics of screen slots), as well as the grain-size distribution of the aquifer.

Given the large number of parameters, different condition of the wells and a diversity of hydrogeological settings, it was impossible to establish very stringent correlations between the rate of clogging, and groundwater iron concentrations and redox potential. Still, the correlation outlined above is proposed. Allowable entrance velocities were also specified. Plots of the vcr = f(Eh, Fe) type are proposed as suitable and recommended for use in the design of both tube wells and radial wells. This is also deemed important for assessments of well regeneration and reconstruction methods.

The tests, both completed and ongoing, address a large number of other important aspects (nature of sediment formation, microbial activity of different micro-organisms, difference in redox potential between different parts of the porous medium, effect of the structural characteristics of well screens, well regeneration methods, etc.). The results of some of the tests will be reported shortly, while other tests are still to be conducted.

 

Acknowledgment

The present paper is an outcome of Project TR37014 "Methodology for the Assessment, Design and Maintenance of Groundwater Sources in Alluvial Environments Depending on the Aerobic State", which is funded by the Ministry of Education, Science and Technology Development of the Republic of Serbia.

 

 

fig32
Fig. 32: Well entrance velocities as a function of groundwater redox potential at a controlled annual increase in local drawdown of ∆S = 0.35 m/year.

 

fig33
Figure 33: Correlation between groundwater  Eh and Fe2+ in the alluviums of the Sava, the Danube and the Velika Morava (points represent average values by well).

 

fig34
Figure 34: Well entrance velocities as a function of Fe2+ concentration for a controlled annual increase in local drawdown of ∆S = 0.35 m/year.