Transport Local and Global

Advection, which refers to movement with flowing groundwater, surface water, or air, can transport a chemical substance through the environment. That dry technical definition only hints at the dramatic global scale and consequences of such transport. Let s begin to understand the mechanisms and scale of advection by looking at the water cycle. 

Water cycles continually through our environment. The movement of water within a region or more broadly within the world s oceans can carry contamination far from its point of origin, as we see in an example to follow. 

Great Ocean Conveyor Belt. (From Philippe Rekacewicz, UNEP/GRID-Arendal (http //www.grida.no/graphicslib/detail/world-ocean-thermohaline-circulation 57ea). With permission.) 

Human acfivifies can release chemical substances at several points in the water cycle. Leachate from an unlined landfill or sewage treat in a septic tank may infiltrate groundwater. Treated wastewater commonly discharges to surface water. And air pollutants enter the water cycle when they dissolve in precipitation and return to earth. 

Each step in the water cycle has the potential to convey chemical substances through the environment by advection. Precipitation can wash chemical substances out of the air and carry them to soil or surface water. On land, chemicals may then re-evaporate or infiltrate groundwater, or run off to surface water either sorbed to particulates or dissolved in water. Once in surface water, compounds may evaporate or be carried far downstream with flowing surface water, even to remote marine environments. 

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