The research objective is to develop a cost-­effective event detection sensing system to inform targeted sampling by traditional means and act as a decision support tool. The Optical Colorimetric Sensor (OCS) uses an array of LEDs to detect change in water coloration and alert to events. A prototype comprises the following features: an LED array light source, photodiode detectors, robust deployable design, GSM communication and antifouling measures. The system has been evaluated using laboratory and field measurements. The system is robust and deployable in the aqua<c environment. The OCS shows potential to detect events in the environment due to a pollution.