As rivers flow into estuaries, they carry with them the accumulated load of pollutants from the streams, rivers, and lakes throughout the whole drainage basin. Thus, an estuary shares all the same water quality problems as the freshwater systems in its watershed - point source pollution from industrial and municipal discharges and private septic systems, and nonpoint source pollution from urban and agricultural runoff. According to the National Directory of Environmental Monitoring Programs , rivers are the environment most commonly monitored by volunteer groups. Lakes follow, and estuaries come in third. This article is aimed at volunteer groups who are familiar with freshwater monitoring and want to find out what's different about monitoring an estuary.
Estuaries are chemically and hydrologically different from lakes and rivers.
As zones where fresh water and seawater meet and interact, estuaries are subject to tidal influences and characterized by salinity gradients. Estuarine monitoring groups must take these differences into account when designing their studies. Since most of the contaminants that pollute estuaries come from fresh water, it is useful to understand the differences between fresh and saltwater chemistry. However, when they encounter the alkaline saltwater of an estuary, they often flocculate or attach to other particles. Thus, instead of being flushed out to sea, these chemicals precipitate in the estuary, where they accumulate in bottom sediments or become concentrated in the food chain.