River sediment and flow characteristics near a bank filtration water supply: Implications for riverbed clogging

Summary

Riverbed clogging is an important issue related to the sustainable exploitation of riverbank filtration well fields. In this research, several complementary field techniques are employed to assess the current state and possible evolution of riverbed clogging at a site in the Saint John River, New Brunswick. The study is conducted in regions of the riverbed that have previously been identified as allowing recharge to the semi-confined aquifer that has been used since 1955 to supply water to the City of Fredericton. Flow velocity measurements, video imaging, and suspended sediment and bed sediment analyses conducted during the low flow (summer) period indicate that part of the recharge area closest to the well field, about 20% of the total area, is affected by bed armoring with cobbles and boulders. Consistent with previous studies, with increasing distance from the riverbank the sediment size decreases and the armor layer disappears. Previous research indicates that turbulent impacting of fine particles into the voids between the cobbles and boulders of the armor layer may reduce infiltration by up to 95%; however, the suspended sediment load in the river is mainly composed of organic matter, and the measured concentrations of suspended sediment (up to 3 mg/L) are not considered high enough to create such large reductions in infiltration. Additionally, the mineral fraction of the suspended sediment would not be expected to settle under the calculated average shear velocity of 0.012 m/s. Other sources of particulate matter, such as the degradation of aquatic vegetation on the riverbed, may be more significant with respect to riverbed clogging; however, annual peak flows may also create bed shear stresses that serve to limit long-term clogging effects.

Introduction

Riverbank filtration (RBF) systems consist of well fields that draw water from an aquifer that is hydraulically connected to surface waters. Pumping water induces a continuous flow of surface water towards the wells through the riverbank and/or the riverbed. Typical aquifers used for RBF consist of alluvial sand and gravel deposits with a hydraulic conductivity greater than approximately 10 m/day.

The benefits of RBF are multiple and include a reduction of turbidity, total coliform, microbial contaminants, natural organic matter (Tufenkji et al., 2002, Wang et al., 2002), and organic contaminants (Tufenkji et al., 2002). RBF is effective in removing some pesticides and some pharmaceuticals (Kuehn and Mueller, 2000) and is at least as efficient in removing disinfection by-product precursors as bench-scale conventional treatment (Weiss et al., 2003). Various authors also report that RBF is highly effective in reducing the risk of Giardia and Cryptosporidium contamination of drinking water when flow path length and filtration time are sufficient (Berger, 2002, Hiscock and Grischek, 2002, Gollnitz et al., 2003).

Some of the disadvantages of RBF include the difficulty of preventing river water from infiltrating the aquifer in instances of severe river contamination, the geochemical reaction of the infiltrate with aquifer materials that may raise the aqueous concentrations of Fe²⁺, Mn²⁺, As, ${\mathrm{NH}}_4^{+}$, CH₄, Ca²⁺ and ${\mathrm{HCO}}_3^{-}$, and clogging of the riverbed (Stuyfzand et al., 2004). Hubbs (2003), for example, has demonstrated that riverbed clogging may decrease the specific capacity of RBF wells by as much as 50–75% during the first three to five years of exploitation.

During RBF, clogging of the riverbed is considered unavoidable (e.g. Wang et al., 2002, Schubert, 2004) because river water containing particulate matter infiltrates the riverbed during flow of water towards the well. Two types of clogging may occur; mechanical clogging due to the deposition of inorganic or organic fine-grained particles in the riverbed, and chemical/biological clogging resulting from geochemical or biological reactions.

Clogging is both a desirable and an undesirable process. Clogging may increase the efficiency of natural filtration since it is believed to have comparable characteristics to the “Schmutzdecke” in engineered slow-sand filtration systems: rich in organic matter, with high concentrations of microbes (Ray, 2002b, Tufenkji et al., 2002, Schubert, 2004). However, it may be detrimental because it leads to a decrease in the hydraulic conductivity of the riverbed sediments. The yield of RBF systems can therefore be severely affected by clogging, which is a problem for the sustainability of the drinking water supply (Tufenkji et al., 2002, Schubert, 2004, Caldwell, 2005).

Although riverbed clogging is known to be a factor in the sustainability of RBF water supplies, there have been relatively few direct investigations of the sediment characteristics and river flow conditions in the immediate area of riverbed filtration. The objective of this research was to characterize riverbed characteristics, and the implications of these for riverbed clogging, at an operating RBF water supply. This was accomplished by using a combination of field investigations and analyses that consider velocity profiles, suspended sediment, and bed sediment characteristics. The study provides useful insight into the extent of mechanical clogging at the investigated site and illustrates an approach that is applicable for other RBF sites.