Correlation Between Entrance Velocities, Increase in Local Hydraulic Resistances and Redox Potential of Alluvial Groundwater Sources - page 7

LHR Variation: Case Study of the Belgrade Groundwater Source

A total of 18 new laterals were installed on five radial wells (hereafter: new wells) at the Belgrade Groundwater Source from 2005 to 2008. Their individual lengths were between 40 and 50 m. The pre-existing, "old" laterals were shut-off and decommissioned.

Since the very beginning, the new wells exhibited a capacity decline. A long-term study has been conducted to assess the rate of clogging of the new laterals, expressed via the increase in local hydraulic resistances. LHR variation monitoring, or the quantification of KLHR, indicated biochemical iron clogging of the five new wells.

Figure 28 shows the variation in LHR over time for the new wells. The relative temporal axis (horizontal axis) represents the period of operation of the new wells. The laterals of most of the wells were regenerated at least once during the study period (denoted by broken lines and R in the figure).

It is apparent that in the 6-7 years of operation of the new wells, LHR increased about 20 times, on average.

Initially, all wells exhibited an LHR of the same order of magnitude, which actually represented hydraulic resistances without the clogging effect. The resistance created due to clogging over time differed from well to well. Its magnitude was proportional to the groundwater Eh and Fe2+. The rate of clogging, KLHR, can readily be obtained from the plots in Fig. 28, via Eq. (9).

for09(9)

where: t1 is the selected point in time when the process is observed, and t0 is the time immediately after installation of laterals (or after regeneration). The estimated KLHR values of the five new wells are shown in Table 5.

 

Since 2005, the water utility has undertaken several regenerations of laterals on four of the five new wells. Based on available data, two intervals were selected and two KLHR calculations performed for well RB-5m, before and after regeneration. The somewhat slower post-regeneration increase in KLHR was attributable to the pre-existing microbial quasi-equilibrium resulting from prior operation of the well.

The assessment of the five new wells revealed an undoubted correlation between the rate of clogging and the chemical properties of the groundwater, primarily the iron concentration and redox potential. Figures 29 and 30 contain plots which indicate that the rate of clogging (KLHR) was proportional to the iron concentration in the groundwater, and inversely proportion to the redox potential.

It clearly follows from this example that iron clogging is of overriding importance for the well ageing process. Iron clogging dominated other types of clogging of radial well laterals. Based on assessments of the other wells at the same site, it is possible to generalize that in alluvial environments the rate of well ageing is accelerated in anaerobic and semi-anaerobic conditions (Eh < 150 mV). The analyses involved a large number of samples, ensuring a relatively high degree of confidence in the conclusions. However, the large areal extent of the Belgrade Groundwater Source (50 km of riverbank) and the diversity of geological and other conditions in its various parts resulted in data scatter, as apparent in the figures.

The rate of well clogging was found to be higher under conditions that corresponded to natural anoxic, rather than mildly aerobic, conditions. Estimation of the rate of well clogging is important for selecting the location of a future groundwater source and designing wells (screen type and material). The previously-discussed indicators also provide insight into the cost of maintenance.

 

fig28
Figure 28: LHR change after installation of new laterals, R – regenerated (Dimkić et al., 2011b, 2012 revised, supplemented).

 

tab05

 

fig29
Figure 29: KLHR as a function of iron concentration (new wells at the Belgrade Groundwater Source), Dimkić et  al., 2011b, revised.

 

fig30
Figure 30: KLHR as a function of redox potential (new wells at the Belgrade Groundwater Source),  Dimkić et  al., 2011b, revised.