Due to the great longevity of plastic in water, any prediction of its propagation in the environment is still hardly possible. This is mostly true for microplastics. Their high mobility in geophysical flows and susceptibility to change of their properties during their stay in the environment contribute to this uncertainty \citep{Chubarenko_2018} 
That being said, several studies have been made to understand the physical and dynamical properties of microplastics in marine environments. Their general motion is driven by forces that depend on the morphological characteristics of the particle: size, shape and density \citep{Chubarenko_2016}. Low density microplastics are likely to spend more time on the sea surface where they can be transported in remote locations, distant from their sources \citep{Gregory_1977,Gregory_1978,Gregory_1983}. These very same type of plastics (e.g. polypropylene from bottle caps, polyethylene from plastic bags, polystrene from fishing floats), although able to float, having a lower density than sea water (1020–1029 kg/m3), can be also found in sub-tidal sediments \citep{Thompson_2004}. This unexpected behaviour can be explained by the biofouling of the particles. As previously explained in 2.3, once physical degradation (e.g. UV light)  has changed the external characteristics of some particles, it is easier for organisms to attach to their surface changing their density and ultimately their behaviour in the water column, making it sink \citep{Kooi_2017}
The observed behaviour of microplastics on sandy beaches on the other hand, is of notable importance. Being located between land and sea, beaches can be both the source and the place where microplastics end up after a journey through the seas, and can also be a good indicator of the level of pollution in the environment \citep{Chubarenko2018}. Most of the published researches on microplastics contamination of sandy beaches are based on surface sampling \citep{McDermid_2004} with the majority of studies sampling the upper 5 cm of the sediment, \citep{Cooper_2010} which leads us to believe that is where there would be a higher chance of finding microplastic particles. That being said, particles have been found as deep as 2 m, following a study by \citet{Turra_2014}, hinting to a uneven 3 dimensional distribution of microlitter in the deeper layers of the beach sands.
As seen in  \citep{Ruiz_Delgado_2014,Orr_2005,Hammann_2014} the greatest density of litter is found at wracklines, following strong winds  or storm events, which confirms the believe that the action of these weather activities is the main reason of the variability of litter concentration along the coastline. On the shoreline the microplastic concentration tends to be patchy and also highly variable due to the continuous current circulation in costal zones \citep{Chubarenko_2017}