  A feasibility study of microplastic detection and quantification in soil and
water using resonance microwave reflectometry is carried out using artificially
created samples with high volumetric concentration of microplastic with
50{\mu}m-0.5mm particles size. A mathematical model expressing microplastic
concentration in soil and water as a linear function of the measured S11
resonance frequency shift and relative permittivity contrast is developed and
is found to be in a very good agreement with the experimental data. Next, this
model is applied to find the best achievable theoretical resolution of
microplastic concentration in the natural environment using microwave sensing
technology which is shown to be at around 100ppm (parts-per-million) level in
the linear signal detection regime. It is demonstrated that the best achievable
level of microplastic contaminant resolution depends on the sensor probe
Q-factor and sensitivity of the microwave receiver. The bound for the
achievable contaminant concentration resolution is found in the analytical form
for high-Q resonance microwave sensors of arbitrary geometry. Even though
several well-established protocols based on optical, infrared and X-ray
spectroscopy are currently being used for microplastic detection in the natural
environment, microwave spectroscopy could offer additional possibilities,
especially for low-cost, real-time in-situ microplastic detection in diverse
environmental conditions outside of the laboratory space.
