<p>Onsite sewage treatment and disposal systems (OSTDS) are a major source of nitrogen loading to groundwater and represent a significant source of diffuse nitrogen loading that threatens groundwater quality and aquatic ecosystem integrity, particularly in regions with shallow water tables and high system densities. However, assessments of nitrogen attenuation from these systems have largely focused on nitrate (NO₃⁻-N), often overlooking the role of ammonium (NH₄⁺-N) in plume-scale transport and attenuation. In this study, we evaluated plume-scale attenuation of NH₄⁺-N and NO₃⁻-N from OSTDS by explicitly accounting for nitrogen transformations in the vadose zone and reactive transport in groundwater. Reactive transport was simulated for 26,510 OSTDS across eight study areas in Florida using a numerical modeling framework. The simulations indicated that incomplete nitrification in the vadose zone can lead to elevated NH₄⁺-N concentrations entering aquifers. Under the first-order reaction kinetics used in the model, NH₄⁺-N attenuates more slowly than NO₃⁻-N, producing larger plume extents and increasing the likelihood of plume overlapping. Under the superposition approach used here, plume interactions can lead to underestimation of attenuation when concentrations from multiple plumes cannot be distinguished, with average underestimation of horizontal attenuation of approximately 17% for NH₄⁺-N and 6% for NO₃⁻-N across the study areas. These results highlight that coupled vadose zone–groundwater processes play a critical role in controlling nitrogen persistence in OSTDS-impacted aquifers, and that neglecting plume interaction and NH₄⁺-N persistence may lead to systematic underestimation of nitrogen attenuation rates at receiving water bodies. The findings also highlight the importance of adequate vertical distance to promote nitrification in the vadose zone, thereby reducing NH₄⁺-N persistence in groundwater.</p>