Importance of the Colmation Layer in the Transport and Removal of Cyanobacteria, Viruses, and Dissolved Organic Carbon during Natural Lake-Bank Filtration

Abstract

This study focused on the importance of the colmation layer in the removal of cyanobacteria, viruses, and dissolved organic carbon (DOC) during natural bank filtration. Injection-and-recovery studies were performed at two shallow (0.5 m deep), sandy, near-shore sites at the southern end of Ashumet Pond, a waste-impacted, kettle pond on Cape Cod, MA, that is subject to periodic blooms of cyanobacteria and continuously recharges a sole-source drinking-water aquifer. The experiment involved assessing the transport behaviors of bromide (conservative tracer), sp. IU625 (cyanobacterium, 2.6 ± 0.2 µm), AS-1 (tailed cyanophage, 110 nm long), MS2 (coliphage, 26 nm diameter), and carboxylate-modified microspheres (1.7 µm diameter) introduced to the colmation layer using a bag-and-barrel (Lee-type) seepage meter. The injectate constituents were tracked as they were advected across the pond water-groundwater interface and through the underlying aquifer sediments under natural-gradient conditions past push-point samplers placed at ∼30-cm intervals along a 1.2-m-long, diagonally downward flow path. More than 99% of the microspheres, IU625, MS2, AS-1, and ∼44% of the pond DOC were removed in the colmation layer (upper 25 cm of poorly sorted bottom sediments) at two test locations characterized by dissimilar seepage rates (1.7 vs. 0.26 m d). Retention profiles in recovered core material indicated that >82% of the attached IU625 were in the top 3 cm of bottom sediments. The colmation layer was also responsible for rapid changes in the character of the DOC and was more effective (by three orders of magnitude) at removing microspheres than was the underlying 20-cm-thick segment of sediment.

Residence Time Controls on the Fate of Nitrogen in Flow‐Through Lakebed Sediments

“…This suggests that over the duration of our experiments a local DOC source, such as buried POC, continued to be an important electron donor for aerobic respiration in the SWI. This POC sourcing is supported by the presence of POC in our SWI sediment cores (Figure ) and by observations of strong retention of POC in shallow sediments of nearby Ashumet Pond (Harvey et al, ).…”

“…The studied lake sediments and flow field were isolated from the surrounding environment using an open‐top injection ring installed in the lakebed and driven to a depth of 22 cm (Figure ). The ring was designed to mimic the predominantly vertical groundwater exchange at the shorelines of these lakes (Winter, , ) and encompass the most biogeochemically active layer of the SWI in these lakes (Harvey et al, ). The injection ring was built from a 55‐cm‐diameter polyvinyl‐chloride barrel.…”

“…A series of float switches were used in the intermediate bucket and ring to maintain a constant water level within the injection ring. The experimental flux rates were selected so that these experiments could be directly compared to previous SWI‐related studies in the nearby, and heavily polluted, Ashumet Pond (Bussey & Walter, ; McCobb et al, ; Rosenberry et al, ; Santelli et al, ; Smith et al, ; Stoliker et al, ; Walter & LeBlanc, ), which has similar hydrologic and geologic characteristics, and where downwelling seepage rates have been reported to be as high as 1.7 m/day (Harvey et al, ). A combination of measured water flux rates and specific conductivity (SpC) breakthrough curves were used to determine vertical flow path residence times during all experiments.…”

“…Samplers were driven to depths of 9.5, 14.5, 19.5, and 24.5 cm below the lakebed. These depths were chosen to sample below the gravelly upper 5 cm of sediment (see Figure 2a in Briggs, Day‐Lewis, Dehkordy, et al, ) and to sample the most biogeochemically active sediment zone in these lakes, as indicated by Harvey et al (). The 3.2‐mm OD tubing (Tygon ACF00002 E‐3603 Non‐DEHP tubing, Fisher Scientific, MA, United States) was attached to the end of each MINIPOINT, and the two shallowest lines were fed through two electrical conductivity (EC) microflow‐through cells (Amber Science, OR, United States).…”

Breakthrough of cyanobacteria in bank filtration

“…Although cyanotoxins have been measured in bank filtered water with coccoid cells and filamentous cyanobacteria cell fragments (Lahti et al, 2001), other studies have not reported any cyanobacteria cells in bank filtered water with the exception of Rachman et al (2014) in one well system with potential direct hydraulic connections to the water source. Previous research on the effectiveness of bank filtration for cyanobacterial removal focused on the physiochemical parameters involved in filtration, including sorption (Romero et al, 2014) or the importance of the colmation layer in the removal of cells (Harvey et al, 2015). The removal of cyanobacteria through drinking water treatment processes has shown that some species of cyanobacteria are more likely than others to pass through conventional sand filters and could result in the release of intracellular toxins into treated drinking water (Zamyadi et al, 2012c(Zamyadi et al, , 2013.…”