Lake health is a global ecological concern. As sources of drinking water and drivers of economic activity, lakes worldwide are experiencing increasing water quality degradation, largely driven by excessive nutrient inputs such as phosphorus, which accelerate eutrophication. Across Canada, an increasing number of authorities are reporting surface water quality deterioration, particularly in remote regions, where septic systems represent a significant source of phosphorus pollution. Despite their importance, septic systems remain underrepresented in water quality modeling frameworks. In this study, we developed an integrated water quality-quantity model to assess the impact of septic systems on lake quality in data scare regions. This methodological framework considers the potential failures exhibited by septic systems and how they influence contaminant transport toward surface waters. Due to the confidentiality related to location of septic system, a Monte Carlo approach was employed to account for uncertainty associated with poorly constrained variables in the water quality module. A phosphorus budget was conducted to estimate total phosphorus inputs to the lake, while a dynamic water budget was used to estimate variations in lake volume. Subsurface attenuation processes, including phosphorus adsorption, were also incorporated to model contaminant transport. Results for a remote lake in Quebec indicate that proper septic system maintenance, compliant installation practices, and soil types exhibiting high adsorption capacities delay phosphorus export to the lake. Finally, future climate projections, under both optimistic and pessimistic scenarios, reveal a deterioration of lake water quality if mitigation strategies are not implemented in the coming years.