Correlation Between Entrance Velocities, Increase in Local Hydraulic Resistances and Redox Potential of Alluvial Groundwater Sources - page 8

Assessment of the Generalized Plot
of VCR = F(EH)

In engineering practice to date, great care has been taken to ensure the aquifer's filtration stability. In other words, entraining and removal of aquifer particles and their entry into the well need to be prevented. If not, normal operation of the well becomes threatened to the point of decommissioning. Filtration stability is ensured by limiting well screen capacity and entrance velocity. Allowable entrance velocities, which ensure the aquifer's filtration stability in the near-well region, have been studied by Istomina, 1957, Abramov, 1952, Cistin, 1965, Johnson, 1975, Gavrilko, Alekseev, 1985, Truelsen, 1988, Vuković, Soro, 1990, Vuković, Pušić, 1992, Dimkić, Pušić, 2008 and others.

However, the entrance velocities quantified in this manner do not ensure the absence of well ageing due to clogging, nor do they in the very least slow down this process. An LHR increase, manifested by an increased "parasitic" well drawdown, is an important consideration in well maintenance planning.

A large database is available at the Belgrade Groundwater Source on the history of well discharges, regenerations, numbers and lengths of radial well laterals, pumping tests, and groundwater redox potential and bivalent iron concentrations. The number of analyses varies from one to more than ten, depending on the well in question.

Forty-seven wells were selected for the present study: 42 "old wells" (whose laterals had not been replaced) and five new wells, whose laterals were replaced between 2005 and 2008 with stainless steel laterals, and where three or more analyses had been performed.

The focus was on finding out whether there was a correlation between Eh and LHR increase due to iron clogging. Local losses were determined and then their variation over time analyzed, with special emphasis on the variation (increase) in local drawdown (∆S) on an annual basis. The wells were categorized as shown in Table 6.

 

It was found that 15 wells exhibited a change of ∆S < 0.5 m/year and 35 wells a change of ∆S > 0.5 m/year. Average entrance velocities (v) were calculated for all the wells and plots constructed of entrance velocities as a function of redox potential, v = f(Eh), Fig. 31. The representation also included the allowable entrance velocity criterion, which is deemed to ensure filtration stability of the near-well region (several authors: Abramov, Sichardt, Kovacs, Gros...; Sichardt and Kovacs criteria were considered here, as they are the most stringent). Also depicted in the figure are the other study areas: Kovin-Dubovac, Knićanin-Čenta and Veliko Gradište along the Danube and the Ključ Groundwater Source adjacent to the Velika Morava, with a total of 22 wells.

The correlation between the groundwater redox potential, well entrance velocities and the rate of well ageing (or the rate of increase in local resistances) was used to determine the so-called critical, maximum allowable entrance velocities. In view of its nature, this concept is of both technical and economic nature. In the BGWS case, wells that exhibited an annual increase in local drawdown of ∆SAV = 0.35 m were deemed to be "good" wells. This was also the allowable level (i.e. a higher rate of well clogging should not be permitted) (Eq. 7).

Wells whose ∆S ≤ 0.5 m were used as benchmarks for the quantification of the critical entrance velocity, based on the adopted well ageing criterion. A table was generated, which also included select wells along the Velika Morava and the Danube. A total of 15 wells were assessed and used for this purpose. No re-calculation for ∆S = 0.35 m/year was made for the wells along the Velika Morava and the Danube. Instead, actual data were used as the wells along the Velika Morava exhibited virtually no losses, while those along the Danube measured extremely low discharges and could therefore have no substantial losses.

Given that the availability of data in the considered range was not uniform, the data were re-distributed as average values in selected intervals. The outcome of this interpretation is shown in Table 7.  The resulting points were plotted (Fig. 32) and connected by two lines: one for v (filtration stability), and the other for
v(∆S = 0.35 m/year.

 

Tab06

 

fig31
Figure 31: Well entrance velocities as a function of groundwater redox potential.

 

Tab07