The use of Horizontal Subsurface Constructed Wetlands (HSCWs) for treating wastewaters in small communities has increased in the last years due to HSCW’s ecological singularities. Unfortunately, the same singularities that differentiate HSCWs complicate any attempt to develop models and produce generic decision-support systems for them. Classical mathematical and statistical approaches used in other Wastewater Treatment Plants do not properly fit the particularities of HSCW and provide little insight in the domain of HSCW. We introduce a novel approach based on logic-based declarative specifications, i.e. non-monotonic causal logic, to capture explicit and implicit knowledge about HSCWs. By expressing all the relevant aspects of a HSCW in a declarative way, we produce a logic-based model which captures features that other approaches fail to formalize. At the end, we produce a complete decision-support system based on that model and test it against a set of realistic scenarios validated by experts. We discuss in which aspects this approach performs better than the most commonly proposed solutions in the bibliography and why it does so.