The study examines the impact of evaporation on the fate of ammonium-N reactions in blackwater-contaminated soils. During evaporation, ammonia (g) volatilization is the preferred route of NH4-N transformation and nitrate formation is initiated thereafter. Ammonia volatilization ceased at residual blackwater contents of 16–40% owing to loss of air-void connectivity. Experimental results indicated that owing to ammonia volatilization and reduced blackwater content only 23–35% of initial NH4-N concentration was transformed to NO3-N. This study also predicted the nitrate accumulation in Mulbagal town aquifer due to blackwater discharge from pit toilets. The prediction indicated that the permissible (45 mg/L) nitrate concentration in the aquifer may have been breached several decades ago, exposing the populace to prolonged drinking water contamination.