The discharge of phosphorus associated with wastewater has decreased significantly in Europe over the past 25 years, however the problem of diffuse pollution persists. Studies have shown that regulatory monitoring can miss elevated spikes in phosphorus concentrations and high frequency monitoring is required. Such programmes are resource intensive, requiring effective tools which enable appropriate water quality data collection and quality assurance. A low cost, portable, and rapid phosphate detection system is needed to enable the quick detection of phosphate in areas affected by high phosphate levels6. A new optical sensing system is being developed by evolving a colorimetric detection system using microfluidic lab-on-a-disc technology which has previously been demonstrated. It utilizes a micro-spectrometer, the molybdenum blue method, and has been built with the intent of requiring limited training for in-situ analysis. The analytical range of the system is 5-400 µg/L, which encompasses the threshold value of 35 µg/L P for Irish rivers and groundwaters. The system is extremely portable due to its compact size and weighing less than 2 kg. With a run time of 15 minutes per ten samples, it enables the in-situ detection of phosphate for rapid on-site monitoring. To demonstrate the sensor, rivers in the northwest of Ireland were identified. Three of these rivers have historical orthophosphate readings in the range of 5 - 47 µg/L and two others were reported considerably higher at 84 µg/L. With this microfluidic phosphate detection system, rapid in-situ detection and reliable, real-time monitoring of phosphorus in freshwater systems can be achieved.