The concentration of ammonium has been greatly increased in freshwaters flowing into the coastal zone. In this paper, the combination of a paper-based analytical device (PAD) and a smartphone application (app) for on-site quantification and mapping of ammonium (NH4-N) in river water samples have been described. The developed app can capture, analyze and map the PAD colorimetric outputs. The developed app used an image processing algorithm for analyzing color intensity and relating the RGB elements to the ammonium concentration. The GPS-tagged data can be sent via social networks. Under optimized conditions, our method provided linear range between 0.2 and 10 mg L-1 with R2=0.9998. The limit of detection (LOD) of the method for quantification of ammonium was 0.10 mg L-1. The developed technique was effectively applied to determine and map the ammonium concentration in Chalus river samples (Iran). The obtained results were approved by the lab-based conventional methods using spectrophotometer.