Widespread Thermodynamic Limitations to Microbial Processes in US Aquifers

30 pagesWith the climate crisis threatening the potability of surface waters, studying groundwater issues is more important than ever. One important aspect of groundwater environments which impact water chemistry are microbes that dwell in aquifers and catalyze oxidation-reduction electron transfer reactions. These reactions can both introduce new contaminants to water and detoxify contaminants in groundwater. In aquifers, microbial processes are limited by the laws of thermodynamics, which dictate how much energy is available for microbial respiration. This study aimed to quantify the energy available to microbial life in U.S. aquifers using publicly available data from around the country obtained from the National Water Information System (NWIS) database. To do this, I simplified the microbial food chain into a model and calculated Gibbs free energy for each step in the food chain. First, I used species concentration data from the NWIS database to calculate the redox potential for each reaction with the Nernst equation. Then, I used a modified version of the Gibbs free energy equation to calculate free energy from the redox potential. Additionally, I calculated the Thermodynamic Potential Factor (TPF) for each reaction, which further describes the energy availability. While no broad-scale thermodynamic limitations were identified, this study concluded that the energy available to microbial life is highly dependent on pH. This is significant, as it indicates that pH could potentially be used as an indicator for microbial activity. I also concluded that the rate-determining step in the microbial food chain is likely the production of acetate and H2 from dissolved organic matter. This could have implications for groundwater chemistry in the future, as dissolved organic carbon levels in groundwater are expected to change because of climate change