This dataset accompanies a paper submitted to Environmental Toxicology and Chemistry titled "Spatial patterns of microplastics in freshwater bivalves (Bivalvia: Unionidae and Sphaeriidae) relative to municipal wastewater effluent discharges.". This study examined whether microparticle levels in resident mussels and fingernail clams and water samples were elevated downstream of wastewater treatment plant facilities in the communities of Hespeler, Ayr, and Caledonia along the Grand River, Ontario, Canada. All sampling for this work was done in August through September of 2021 and September of 2022. This dataset includes sample information for biotic and abiotic samples collected from the Grand River, a list of microparticles isolated from samples (including particle colour, morphology, and measurements), and information on chemical composition for a subset of particles.