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

Milan A. Dimkić1, Milenko Pušić2

 

 

1 Jaroslav Černi Institute for the Development of Water Resources, 80 Jaroslav Černi St., 11223 Belgrade, Serbia; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it , This e-mail address is being protected from spambots. You need JavaScript enabled to view it

2 University of Belgrade, Faculty for Mining and Geology, Department for Hydrogeology, 4 Djusina St., 11000 Belgrade, Serbia; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it , This e-mail address is being protected from spambots. You need JavaScript enabled to view it

 

 

Abstract

Water well capacity decreases under certain conditions. This is a result of already extensively-researched mechanical clogging, but also of chemical and biochemical clogging (biofouling). In alluvial environments, deposits of trivalent iron and carbonates are often formed on well screens and in the near-well region, which increase the local hydraulic resistance (LHR) of the well. Research conducted on several locations in Serbia has shown that the LHR increase, of a biochemical nature, largely depends on the oxic state of the aquifer and groundwater. A correlation was established between LHR increase, iron concentration and redox potential of groundwater. Based on a pre-defined well clogging rate, the correlation between the groundwater redox potential, iron concentration and allowable (critical) entrance velocity to the well was quantified. It was concluded that under certain conditions it is necessary to tighten the allowable entrance velocity criterion which ensures the aquifer's filtration stability. This considerably decelerates the LHR increase, or well ageing, and is reflected in the nature of well maintenance.

Keywords: groundwater, biochemical well ageing, local hydraulic resistance (LHR), critical entrance velocities.

 

 

Introduction

Big cities (economic and administrative centers) are in many cases situated in the valleys of large rivers. Water supply to such cities is provided solely, or partially, through groundwater extraction from alluvial aquifers. The characteristics of such aquifers depend on the part of the river course in question. In general, the thickness of alluvial deposits increases from the source of the river to its mouth and the number and structure of the strata become increasingly complex. Downstream, fine-grain fractions gradually become dominant, Fig. 1.

In such environments, some of the main characteristics of water supply sources that rely on groundwater (groundwater sources) are as follows:

  • They are of the bank filtration type, the artificial recharge type (with infiltration ponds), or a combination of the two;
  • Groundwater is (in most cases) extracted by means of wells: either tube wells or radial wells;
  • In the case of bank filtration, wells are distributed along one or both riverbanks, while in the case of artificial recharge they are grouped around infiltration ponds;
  • The locations and configurations of the wells depend on the thickness and groundwater flow characteristics of the water-bearing medium (the aquifer);
  • The capacity of the groundwater source and of the individual wells is limited by the groundwater flow characteristics of the aquifer, the hydraulic link with the river, the hypsometric relationships between the riverbed, aquifer, river stage and well screen, the distance of the well from the river, and the structural characteristics of the well;
  • The presence of a semi-permeable aquifer roof ensures sound protection from surface pollution;
  • The self-purification potential of the aquifer ensures high and consistent groundwater quality;
  • Groundwater sources are often at a relatively small distance from the city and this reduces the cost of transportation of water to consumers. However, this situation sometimes creates a conflict between the interests of the city (expansion, development) and the existence of the groundwater source; and
  • Under certain conditions, well capacity decreases over time, more or less rapidly. This process is called well ageing. A correlation between entrance velocities to the well screen and well capacity was established a long time ago.