Influence of Depth on Enhancing Biofilm Extraction from Aquifers through In-Situ Sonication

Microorganisms in aquifers are often assessed by extracting groundwater samples. This technique though typically under-represents the fraction of microorganisms attached to sediment (biofilm). Pre-treatment of groundwater extraction with in-situ low frequency sonication can improve microbial representation. This treatment induces biofilm detachment from sediment through exerting a pressure variation on the local environment. Previously, it was unknown when sonication intensity, as a function of depth, would drop below the threshold required for mobilizing biofilm. A series of sonication pulses (2.43 kW) of increasing duration were applied to a shallow groundwater monitoring well (8.1 m deep, 1.7 m below ground level) in Canterbury, New Zealand at varying depths below the water table (0.7, 3.0, 5.4 m). Samples were collected during sonication pulses and at increasing intervals following sonication. Turbidity values were determined to examine the release of sediment fines and microorganisms. Heterotrophic colony counts were compared to background water samples (no sonication) and cells extracted from biofilms on collected sediment samples. Maximum colony counts were observed during the first sonication pulse. Values dropped rapidly after sonication ceased and subsequent pulses provided slightly lower colony counts. Sonication improved the extraction efficiency up to 84-fold compared to background values and were similar to the levels extracted from the in-situ biofilm samples. Colony counts were on the same order of magnitude for all three depths. Compared to the shallower depths, there was a statistically significant difference in extraction efficiency for the greatest depth during the first pulse, but its increase was marginal. Overall, in-situ sonication across all depths greatly increased extraction efficiency, making this technique a cost-effective tool for improved groundwater microbial assessment.