The optical characteristics of a simple, planar, single layer, dielectric Mg-based guided mode resonance filter (GMRF) were investigated by means of rigorous-coupled wave analysis (RCWA). This filter has great potential for real-life applications, especially as bio- and environmental sensors. The structure of the proposed sensor is compact, and all of its layers can be grown in a single process. In this paper, we present results on the design of a water pollution sensor in the violet region of the visible spectrum. The spectral and angular sensitivities of the sensor for both the transverse electric (TE) and transverse magnetic (TM) polarization modes were estimated and compared for various regions in the violet spectrum. A spectral response characterized with a narrow bandwidth and low reflection side bands was realized by carrying out extensive parameter search and optimization. Optimal spectral and angular sensitivities were found for the sensor with a grating thickness of 100 nm in the TM polarized mode where we found them to be 100 nm and 40 degrees, per index refraction unit, respectively.