The capacity of natural microbial communities to transform a substrate in groundwater has been shown to increase after repeated exposures to the substrate; herein referred to as the “memory effect”. The objectives of this study were to determine: (1) how long the memory effect can last and (2) how the memory effect can alter the structure and function of natural microbial communities. Ethanol substrate was injected into a single groundwater test well for six consecutive weeks to establish a memory effect. The groundwater control well, located up-gradient of the test well, was not injected with ethanol during the first six weeks. Ethanol transformation in the test well was not significant the first week whereas ethanol transformation was significant during weeks two through six. The test and control wells were then monitored for eight additional weeks under ambient conditions. During week 14, ethanol was injected into both the test and control wells. Ethanol transformation was significant in the test well whereas ethanol transformation was not significant in the control well. These results demonstrated that the memory effect lasted at least eight weeks in the test well.
Here we present the hydrological, geochemical, and microbiological data and analyses in hand from the study site and the experimental well pair. This includes: (1) the magnitude and variability of hydraulic conductivity, hydraulic gradient, and effective porosity, (2) the potential for diffusive mass transport, (3) the temporal variability of specific discharge, (4) ethanol transformation to acetate and removal of nitrate and sulfate, (5) utilization/limitation of metal nutrients and/or co-factors, (6) microbial community structure (16S rRNA sequencing), and (7) microbial community function (GeoChip). Finally, we discuss the implications of the memory effort in terms of groundwater remediation with emphasis on the immobilization of redox sensitive metals and radionuclides.