Correlation Between Entrance Velocities, Increase in Local Hydraulic Resistances and Redox Potential of Alluvial Groundwater Sources - page 01

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Figure 1: Change in grain sizes of an alluvial aquifer along a river course (Dimkić et al., 2011a).

 

Well ageing is generally a result of two processes: well clogging and well corrosion. They can take place in parallel or one of them may be entirely lacking. Both processes are mechanical, chemical or biochemical. Well ageing reduces well capacity at the same groundwater level maintained in the well, or the groundwater level declines at the same discharge. In most cases, the ultimate outcome of well ageing is decommissioning.

Mechanical clogging is a process by which fine particles of the aquifer matrix become entrained by the groundwater and are transported to the well, where they are unable to pass through well screen slots and, remaining there, form sediments (clogs). This reduces the active surface of the well screen and the throughput of the well.

Mechanical clogging has been directly linked to the filtration performance of the near-well region. Filtration instability occurs relatively quickly in intergranular porosity settings, where groundwater flow velocities are such that fine particles of the aquifer matrix are picked up and deposited on the well screen and in the near-well region (Istomina, 1957, Vuković, Pušić, 1992). Based on in-situ and laboratory research, many authors (Abramov, Sichardt, Kovacs, Truelsen, Gavrilko, Johnson and others) have provided instructions and criteria regarding allowable entrance velocities of groundwater to the well, which ensure the absence of this undesirable occurrence. Dimkić, Pušić (2008) reported a number of results of similar research. As defined, allowable entrance velocities are correlated with the grain-size distribution and groundwater flow characteristics of the aquifer, Vuković, Soro, (1990), Fig. 2.

The results of a review of available data on the Belgrade Groundwater Source (BGWS) indicate that a correlation can be established between the grain-size distribution of the aquifer and the redox potential of the groundwater, Fig. 3.

 

Although with some reservation, the redox potential (Eh) was adopted as the main parameter for well ageing assessments and the definition of allowable entrance velocities, keeping in mind the above-mentioned process (Dimkić et al., 2008, Petković et al., 2008, Dimkić et al., 2011c). The primary advantage of this parameter is the possibility of conducting in-situ measurements, and its disadvantage is a relative imprecision of the collected data. The data in the figure have been "evened out", by adopting representative intervals for d50 (50% of the grain-size distribution curve of the aquifer matrix) and constructing a plot based on calculated mean values.

In alluvial environments, chemical and biochemical well clogging is manifested by the formation of sediments, mostly compounds of trivalent iron, manganese and carbonates (Barbič et al., 1974, Cullimore, 1999, Dubinina, 1978, Houben, 2003, Houben, Treskatis, 2007). The development of such deposits is accompanied by an increase in local hydraulic resistances at the well screen or in the near-well region (each new well initially exhibits a local hydraulic resistance, LHR, whose magnitude depends on several factors), Dimkić et al., 2011b. Research conducted at several alluvial sites in Serbia has shown that a biochemically-induced LHR increase largely depends on the oxic state of the aquifer and groundwater. A correlation was established between LHR increase, iron concentration and redox potential of the groundwater. Based on a previously specified rate of well clogging, the correlation between the groundwater redox potential and the allowable (critical) entrance velocity to the well was quantified. It was concluded that under certain conditions, it is necessary to tighten the entrance velocity criterion which ensures the aquifer's filtration stability. This tends to substantially decelerate the rate of LHR increase, or well ageing, and is reflected in the nature of well maintenance.

 

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Figure 2: Allowable entrance velocities of groundwater to the well where filtration stability is maintained, Vuković, Soro, (1990).

 

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Figure 3: Correlation between the grain-size distribution of an alluvial aquifer (d50) and the groundwater redox potential (Eh) at the Belgrade Groundwater Source.