TRANSPORT OF MICROORGANISMS THROUGH THE GROUNDWATER/SURFACE-WATER INTERFACE OF A CAPE COD, MASSACHUSETTS KETTLE POND INTO A DRINKING-WATER AQUIFER

Engineered bank-filtration has proven to be a cost-effective means of removing pathogens from surface water. However, little is known about the efficacy of the groundwater/surface-water interface to remove microorganisms during movement of surface water into aquifers under natural-gradient conditions. Our study focused on removal of microorganisms by bottom sediments within the “pond shadow” of Ashumet Pond (kettle pond, pH 7.2, 118-123 µS cm^-1 specific conductance, 2.2-2.5 mg/L DOC). There are no inputs or outputs of surface water to the pond. However, groundwater continuously flows into the northern end of the pond. Pond water recharges the aquifer on the southern end and serves as a source of water for nearby wells. Injection-and-recovery studies were performed in the southern end of the pond and involved additions of bromide (conservative tracer); Synechococcus sp. IU625 (cyanobacterium, 2.6 ± 0.2 µm); a polydisperse (1.7 to 4.6-µm diameter) mixture of carboxylate-modified polystyrene microspheres; and uncultured, coccoidal-shaped cyanobacteria (~1 µm diameter) harvested from the pond. The latter were of a size, shape, and fluorescence consistent with Proclorococcus marinus, which dominated the autotrophic community in Ashumet Pond and constituted 28% of the total bacterial community as evidenced from subsequent 454 pyrosequencing. Bromide and colloids were added to a bag-and-barrel (Lee type) seepage meter and tracked as they moved across the water-sediment interface and through the underlying aquifer sediments past push-point samplers placed at ~30 cm intervals along a 1.2-m-long, diagonally (~45 °) downward flow path. The top 25 cm of near-surface bottom sediments removed ~44% of the pond DOC and was more than 9 orders of magnitude more effective at removing colloids than was the underlying sediment, underscoring the importance of the interfacial layer. Our study suggests that Prochlorococcus and Synechococcus spp., two common picocyanobacteria (less than 3 µm in diameter), may be useful indicators of groundwater under the direct influence of surface water (GWUDI) for wells located near kettle ponds.