Correlation Between Entrance Velocities, Increase in Local Hydraulic Resistances and Redox Potential of Alluvial Groundwater Sources - page 02

Well Ageing by Iron Clogging and Bank Filtration Stages

Fine-grain fractions of alluvial aquifers along lower courses of large rivers often include minerals that contain iron. As a result, there are massive populations of iron bacteria on such locations. The sediment created is the main cause of well ageing and well capacity decline. Such ageing occurs where the redox potential is relatively low and there is enough dissolved iron in the groundwater. Apart from natural conditions, some of the important considerations include the material of the well, its initial capacity, operating conditions and maintenance.

For large-scale development of sediment, the redox potential in the near-well region needs to be elevated (assuming there is more oxygen than in the groundwater beyond the near-well region). Then a reaction of the following type takes place:

For01(1)

This reaction is catalyzed by bacteria, which use the energy of the reaction and/or its products for their needs. According to Jurgens et al., 2010, oxidation-reduction processes proceed as follows:

for02(2)

River (surface) water, on its way to the well, undergoes several chemical transformation stages. Depending on the oxic state, changes occur that result in well ageing due to iron clogging. Four stages of groundwater flow can be distinguished, as shown in Fig. 4 (Dimkić, 2012, Dimkić et al., 2012).

 

The first stage is filtering of river water through the riverbed. The occasionally mobile bedload is generally highly oxic and characterized by extensive sorption of most organic and other dissolved substances, as well as high biochemical activity.

In the second stage filtration takes place under oxic aquifer conditions and dissolved oxygen is used for the oxidation of organic substances and volatile minerals. Of interest here is the effect of dissolved oxygen on the oxidation of volatile aquifer minerals, which contain bivalent iron, and the conversion of iron into insoluble trivalent forms.

In the third stage, which takes place at a somewhat greater distance from the river than the previous two, the oxic state of the groundwater is lower and the nature of the transformation processes different. Fe2+ might occur at this stage, which is highly soluble in water. If the aquifer is highly oxic, the third stage of groundwater transformation may be missing.

The fourth stage of bank filtration/groundwater transformation takes place in the immediate vicinity of the well, where mechanical, chemical and biochemical changes occur due to intensive processes at the aquifer/well screen interface. If the aquifer is anoxic around the well (with a somewhat higher redox potential inside the well), Fe2+ is again converted into Fe3+ at the aquifer/well screen interface.

 

fig04

Figure 4: Bank filtration stages (Dimkić et al., 2012).

 

Local Hydraulic Resistance and Entrance Velocity

The parameter generally used to quantify the efficiency of a well is its specific capacity, q:

for03(3)

where: Q – well capacity (L/s) and S - well drawdown (m). Here drawdown is defined as the difference between the static (natural, undisturbed) groundwater level and the (dynamic) water level in the well (assuming, of course, that the well is operating).

An inverse, modified quantity is referred to here as the local hydraulic resistance (loss) and abbreviated as LHR (Dimkić, Pušić, 2008, Dimkić et al., 2012):

for04(4)

where ∆S (m) is the local drawdown—difference between the piezometric head of the groundwater at the contour of the near-well region (adjacent to the well) and the water level inside the well, and v (m/s) is the average entrance velocity to the well (in practice, the discharge of the well can be used instead of v, and in the case of radial wells also the discharge of the well lateral). LHR is a parameter that can be used to quantify the extent of well clogging.

In practice, a piezometer installed in the immediate vicinity of the well, sometimes referred as the "nearby piezometer", is used to measure groundwater levels outside of the well, Fig. 5. At the same discharge, the difference between groundwater levels from the piezometer to the clogged well does not change over time and can be disregarded in ideal conditions. In practice, this parameter can be used to compute LHR without a large error.

The same assumptions are made for a radial well; the water level inside the well is equal to the water level inside its lateral.

An increase in LHR over time does not necessarily mean that well discharge will decrease. At the same discharge, the water level inside the well decreases until there is no more room in the well due to the position of the pump or other constraints.

The rate of clogging, KLHR (kinetics of local hydraulic resistance), is defined as "LHR variation in time interval ∆t":

for05(5)

 

Based on research conducted at the Belgrade Groundwater Source, the rate of clogging depends on several parameters, some of which can be used as well clogging indicators:

for06(6)

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

It is apparent that KLHR can, inter alia, be considered a function of the rate of groundwater extraction, or the magnitude of the entrance velocity. The question is raised of predicting the change in local drawdown, which in the given time interval should not exceed the pre-defined, allowed value - ∆SAV. For practical reasons, the time interval ∆t is one year, such that ∆SAV is defined as the maximum allowed drawdown on account of LHR increase and serves as a criterion. Compliance with this criterion is deemed to ensure long-term service of the well:

for07(7)

where: ∆SAV – specified LHR variation during the year, expressed via increase in well drawdown (m), vperm – allowable entrance velocity (m/s), and KLHRyearLHR variation over one year. Allowable entrance velocities to the well (or well laterals, in the case of radial wells), taking the annual LHR variation as the applicable criterion, can be calculated as follows:

for08(8)

Long-term service of wells requires the above criterion, as well as the filtration stability criterion, to be fulfilled. However, compliance with these two conditions will not guarantee the full cessation of well clogging. It only reduces it to a level which ensures that the annual increase in hydraulic losses in the zone between the outer medium and the inside of the well will remain below the specified value of ∆SAV .

The case study of the Belgrade Groundwater Source revealed that at nearly all the wells vperm was much lower than estimated using "conventional" experimental formulas related to the filtration stability of the near-well region. This was especially true of anoxic/suboxic conditions, in this case 70 mV ≤ Eh ≤ 150-200 mV.

 

fig05Figure 5: Increase in local hydraulic resistance over time, tube well (t3 > t2 > t1).