Plastic production and consumption keep increasing through time and so the problems related to this increase. More and more plastics find their way to water bodies through different means such as rain runoff, accidental loss, directly thrown in the environment or wastewater treatment plants. Due to those processes, plastics mass in oceans is estimated to surpass the fish biomass in the near future. Plastics do not only stay in the water column as they can also sediment and get buried at the bottom of a water body. Three sediment cores were taken in Lake Geneva to be analyzed on their microplastic concentrations evolution through time. Gamma spectrometry was used to give dates to the different layers of the core using ²¹⁰Pb, ¹³⁷Cs, and ²⁴¹Am. Due to the impact of the Vidy wastewater treatment plant on the ²¹⁰Pb present in the sediments of the Bay, ¹³⁷Cs and ²⁴¹Am were used to determine the dates of each layer. The microplastic analysis was performed using digestion, density separation, microscopy and chemically analyzed with ATR. Two fractions were determined for this study (> 1 mm and 500 μm - 1 mm).

The results from the gamma spectrometry clearly show the ¹³⁷Cs and ²⁴¹Am peaks which give an accurate dating of the core. The sediments were successfully dated from 1896 to present. The microplastic concentrations per layer ranged from 100 microplastics/kg of dry sediment to 2325 microplastics/kg of dry sediment. Fiber was the morphology with the highest concentrations throughout the core while polyethylene was the most frequent chemical composition of plastic found in the samples followed by polyester and polyvinyl alcohol.

The results from this study show quantities of microplastics higher than most studies. The lower depths of the core have fewer microplastics than the top layers which shows the consumption and the evolution of the greater Lausanne area. The influence of the Vidy wastewater treatment plant on the microplastic accumulations in the sediments was also shown in this study as synthetic and cotton fibers were recovered from the samples in high numbers.