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The Dutch Ministry has funded various (pre)feasibility studies to investigate the underground hydrogen storage potential (UHS) in depleted gas fields. Most of these studies focused on estimating the flow performance and storage capacity of subsurface reservoirs. However, their performances can be affected by biogeochemical reactions. The aim of this study is to characterize, quantify, and predict the potential impact of microbial activity and geochemical reactions in the most important geological formations in the Netherlands based on the composition of their formation waters, remaining natural gas, rock composition, and environmental conditions. So far, 700 laboratory analyses on deep groundwater samples have been collected from 161 onshore/offshore wells. Findings will provide practical recommendations on: 1) best sites for UHS and other types of underground storage and geothermal; 2) operational ranges that can reduce biogeochemical reactions; 3) optimal sampling and analysis practices for monitoring. Preliminary results show that for UHS many reservoirs included in this study present adverse conditions for microbial activity. This is due to high salinity and temperature, as well as low sulphate concentrations. Further determination of the rock matrix, and the remaining natural gas and the indigenous microbes in the reservoir will allow to complete the risk assessment.