MONITORING MICROBIAL AND SOLUTE TRANSPORT ACROSS THE SURFACE WATER-GROUNDWATER INTERFACE INTO AN AQUIFER WITHIN THE LAKE SHADOW OF A KETTLE POND IN CAPE COD, MASSACHUSETTS

Efficient transport of water across the surface water-groundwater (SW-GW) interface and underlying sediments is important to the effectiveness of bank-filtration operations, as is the removal of surface water pathogens. Our study involved Ashumet Pond, a kettle pond (flow-through groundwater lake) in Cape Cod, Massachusetts that recharges a sole-source, sandy, drinking-water aquifer, thus providing natural bank filtration for groundwater use downgradient. A “Lee-type” flow meter (simple bag-and- barrel) device was modified for introducing conservative (bromide) and fluorescent microsphere tracers to the sediment-water interface above the “lake shadow” where lake water recharges the aquifer near the south shore. Real-time specific conductivity measurements allowed adjustment of the locations of the push point samplers in order to sample the porewater and track the introduced microspheres along a 1.2 m long, diagonally downward flow path. In a subsequent study involving use of Illumina Mi-SEQ 16S rDNA sequencing, changes in bacterial community composition and percent abundance of the identified surface water indicator Actinobacteria ACK-M1 along with Synechococcus spp. (phylum Cyanobacteria), a phototroph, were tracked as they were advected by natural-gradient recharge from the lake to wells F722 and F631, located, respectively, 11.5 m and 52 m downgradient from the mean shoreline. Results of the study showed that the top 0.25 m of sediments immediately beneath the SW-GW interface were three log unit more effective at removing microspheres than was the underlying 0.2 m of aquifer sediments. Although highly attenuated in the sediments below the SW-GW interface, ACK-M1 was detected in both well F722 and F631 and may be a useful indicator of Groundwater-Under-the-Direct-Influence-of-Surface-Water (GWUDI). However, a strong seasonal influence was observed, i.e., more substantive transport of this organism occurred during the winter.