Microplastics (MPs) have become increasingly popular in the scientific community due to increasing abundance. Microplastics form from the weathering and erosion of common single use plastics, and are often deposited through atmospheric and aquatic processes. Microplastics pose a threat to human health and surrounding ecosystems. It is important to understand the spatial trends in microplastic deposition to better understand who is at risk of exposure. Additionally, understanding the mechanism in which microplastics are deposited is important to answer knowledge gaps in the scientific community. Finally, microplastics are an increasingly important contaminant, and it is important to understand their behavior and methods of exposure for future populations. 

See methods of exposure from common single use plastics here

For our project, we wanted to analyze methods of exposure and concentrations of microplastics in a recreational coastal environment. For our study area, we selected Pacific Beach located in San Diego, California. Pacific Beach is a popular beach with high recreational use. We decided to analyze the quantity of microplastics in sediment at varying distances from the sidewalk to the ocean at Pacific Beach. Greater microplastic concentrations are expected closer to the shoreline due to the deposition of MPs by the water. This information will help San Diego policymakers make informed decisions to better serve their communities. For example, creating “safe times” where microplastic exposure is expected to be at a minimum. Additionally, this information is beneficial to the scientific community to better understand mechanisms of deposition of MPs. 

In order to test our hypothesis, we needed to create a systematic method to sample sediments at varying locations. Two datasets were collected at each site and were three feet apart from each other (Figure 1). Additionally, we collected two water samples. Samples were then processed following a standardized procedure to dissolve organic matter using 15% HOOH, and NaCl for density separation of microplastics. Once processed, samples were filtered then analyzed under a Fourier Transform Infrared (FTIR) microscope. This microscope allows for high resolution imaging of the microplastics. Microplastics were quantified by counting visible microplastics across the 47mm2 diameter filter, then estimated using an area to quantity ratio equation. Finally, data was graphically analyzed using Microsoft excel. 

In general, greater microplastic concentrations were found as the distance sampled from the water decreased. There was a net increase in microplastics for datasets A and B from the sampling sites furthest from the water to closest. There were no microplastics identified in the water samples. Additionally, there was a sharp increase in microplastic concentrations where high tide extends to. We can therefore assume that tidal behavior deposits microplastics, and the sediment traps them (Figure 2).

In conclusion, our hypothesis was ultimately supported. Greater microplastic concentrations were found as the distance sampled from the water decreased. This information allows for informed decision making for individuals looking to reduce their microplastic exposure. Visiting Pacific Beach during high tide is suggested to avoid MP exposure, as the highest concentrations would be trapped underwater. It is still largely unknown as to how humans are directly exposed to microplastics. Additionally, exploring controlled variables in a lab setting could provide further insight into these methods of exposure. Finally, quantifying the type and size of microplastics would allow for better understanding of potential health implications and predominant sources of microplastics. Understanding the methods of deposition of microplastics is necessary to preserve ecosystems and future generations. 

Figure 1. Site map of sampling locations. Distances between sites were 15ft, unless otherwise noted.

Figure 2. Microplastic count at various distances from the sidewalk for Datasets A and B, and their average. Black line indicates high tide line.