Microstrip and planar microwave sensors offer a compact solution for water quality monitoring, changing the electromagnetic characteristics of a liquid into measurable changes in resonance frequency, reflection or transmission coefficients, phase and quality factor. In this critical review, the representative peer-reviewed studies on salinity, chlorine, ammonia, iron, lead, trace-metal pollution and chloride detection are discussed with focus on sensor topology, operating frequency, liquid-sensor interaction, calibration strategy and evidence quality. The literature demonstrates that patch, split-ring resonator (SRR), complementary SRR (CSRR) and related planar structures can sense controlled changes in aqueous composition with small sample volumes and, in some cases, without direct electrical contact. The electromagnetic response is however generally non-specific as several dissolved species can alter the complex permittivity and conductivity simultaneously. Therefore, a large resonance shift or a low fitted error should not be interpreted in itself as a validated limit of detection or selective chemical identification. A trend away from laboratory salinity measurement toward in-situ monitoring, portable instrumentation and chemically assisted selective detection is seen in recent work. The main remaining issues to be resolved are matrix effects in real water, temperature dependence, packaging and substrate variability, calibration drift, independent validation, uncertainty reporting and multi-analyte discrimination. The review therefore recommends differential or multi-frequency architectures, controlled microfluidic interfaces, selective chemical reactions or recognition layers where appropriate, and portable VNA/LiteVNA readout and statistically rigorous calibration and validation protocols.
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