Bull Island (BI) is a coastal sand spit that formed as an unintended consequence of the construction of two walls, built over 200 years ago in Dublin Port, Ireland to alleviate silting of the shipping route. A large lagoon, on the land side of the island was separated in 1964 by the construction of a causeway to produce two separate lagoons that are now impacted by different water sources. Here we investigate the influence of riverine inputs on the two adjacent but unconnected tidal wetland lagoons. The South lagoon (SL) is supplied by tidal water passing through the eutrophic R. Liffey and R. Tolka estuary zones, while the North Lagoon (NL) is supplied by seawater and to a lesser degree, freshwater from the R. Liffey plume. Within each of these zones a clear ecotone exists between the mudflats (MF) and vegetated saltmarshes (SM). We determined the quantity and distributions of bulk geochemical characteristics across BI’s sediments, including total organic carbon (TOC), total nitrogen (TN), metals, and also, 16 individual polyromantic hydrocarbon’s (PAH’s) as an indication of anthropogenic input. Primary focus was placed on studying the blue carbon sediments of the lagoon zones. Significant differences in analytical results showed major influences exerted on sediment geochemistry within each lagoon. This study highlights the ability of a functioning coastal wetland to flourish and sequester elevated levels of carbon, metals and pollutants under the constraints of increasing anthropogenic impact. As the inadvertent result of geo-engineering, BI and its environs is a very important site to investigate the potential of artificially constructed wetlands to act as blue carbon reservoirs.