The groundwater ecosystem fulfils functions that enable drinking water production. Here, we used a computational model to quantify one such services, namely the degradation or organic carbon. We ran scenarios with and without fauna and scenarios with (additionally) temperature elevated by 1.5 °C and 3 °C. Biodegradable organic carbon increased in scenarios without fauna. This may create problems in drinking water production. Fauna fulfils a service by promoting microbial carbon degradation.