Snapshot Sampling May Not Be Enough to Obtain Robust Estimates for Riverine Microplastic Loads

Area of Science:

• Environmental Science
• Aquatic Ecology
• Environmental Chemistry

Background:

• Wastewater treatment plants (WWTPs) are recognized sources of microplastics (MPs) entering aquatic environments.
• Temporal dynamics of MP concentrations and loads downstream from WWTPs are poorly understood, leading to potential inaccuracies in emission estimates.

Purpose of the Study:

• To investigate the impact of different sampling frequencies (hourly, weekly, monthly) on microplastic load estimations in a stream receiving WWTP effluent.
• To identify factors influencing temporal variability in microplastic concentrations and loads.

Main Methods:

• Utilized fluorescence microscopy and Raman spectroscopy for microplastic identification and quantification.
• Collected samples at hourly, weekly, and monthly intervals from a stream downstream of a WWTP.
• Analyzed microplastic concentrations, polymer composition, and discharge rates.

Main Results:

• Discovered significant hourly variations in microplastic concentrations, while polymer composition remained consistent.
• Temporal variability in microplastic loads was attributed to fluctuations in MP concentration, discharge rates, or both.
• Sparse sampling combined with annual discharge data resulted in substantial uncertainties (over/underestimation by 3.8 billion MPs annually).

Conclusions:

• Higher-frequency sampling is essential for accurately assessing microplastic loads and understanding transport mechanisms.
• Accurate quantification of microplastic dynamics is critical for effective downstream impact assessments.
• Reconnaissance campaigns are vital for designing representative monitoring programs and improving large-scale trend predictions.