Water Pollution

 There are a wide variety of water pollutants. Organic compounds include petrochemicals (oil, gasoline, etc.), toxic organic chemicals such as TCE (trichloro-ethylene), and oxygen-demanding organic waste (sewage, e.g.).  Inorganic compounds include radioactive waste, water soluble inorganic chemicals (such as soluble metals) and inorganic plant nutrients (such as phosphorous or nitrogen). Biologic pollutants include pathogens (disease causing organisms such as E. coli) and non-native organisms (zebra mussels in the Great Lakes, e.g.).  Hot-water discharges from manufacturers or power-generating plants can cause thermal pollution.  In some cases, suspended sediment (silt and clay) can act as a pollutant.

 Pollutants can have many different effects.  Some pollutants are simply toxic, interfering drastically with biochemical processes in the organism; some heavy metals such as lead can affect the central nervous system, for example.  Some pollutants are cancer causing (carcinogenic); certain water-soluble organic compounds like TCE, for example.  Some pollutants cause disease; cryptosporidium in the Minneapolis water supply sickened thousands of individuals.  Some pollutants alter the physical nature of the habitat or ecosystem and create less than ideal living conditions; high quantities of phosphorous will create algal blooms which will deplete the oxygen supply, for example.

 A recent horrific example of water pollution was caused by various cyanide compounds and heavy metals from the Baia Mare mine in northwest Romania, operated by Esmeralda Exploration Ltd. of Perth, Australia.  Cyanide compounds (particularly sodium cyanide, NaCN) are poured over crushed rock containing gold, the gold bonds to the cyanide, the gold- and cyanide-bearing leachate is recovered and further processed to extract the precious metal.  This "heap leach" technique produces large quantities of waste rock (tailings) and water contaminated with cyanide compounds and heavy metals associated with the gold in the original rock (such as cadmium, mercury and lead).  The contaminated water and tailings are often stored behind earthen dams creating tailings ponds.  The dam impounding the tailings at Baia Mare was overwashed and breached during heavy rains on 30 January 2000.  About 100,000 m3 or about 3 million cubic feet of impounded materials washed down a stream leading into the Lapus River in Romania, and thence into the Somes River or Szamos in Hungary, then into the Tisza River south through Hungary into Yugoslavia and into the Danube River.  All of these rivers were contaminated, up to 500 km or more from the point source.

 The pollution of these rivers came from a point source of contamination.  Point sources represent localized concentrations of pollutants.   Non-point sources are large areas where pollutants are widely distributed and often in low concentrations.  The classic non-point source is distributed runoff from agricultural fields, which are often contaminated with pesticides, herbicides, fertilizers and animal wastes.

 The physics and chemistry of water pollution is not simple.  A pollutant dumped into a stream will mix into the stream water and become more dilute if the turbulence of the stream is high, if the stream is well stirred.  Lowland streams tend to exhibit more non-turbulent flow, especially at moderate discharges, and pollutants can travel downstream for great distances without mixing and diluting.  Pollutants such as volatile organic hydrocarbons may simply evaporate.  Water-soluble pollutants may remain dissolved in the water column or may precipitate out, depending upon the local water chemistry.  Suspended particles may stay in suspension if turbulence is high enough or if the particle is small enough, but will eventually settle out in quiet waters such as lakes and resevoirs.  Large particles may be temporarily deposited on the bed or the banks of the stream.
 Pollutants may be ingested or absorbed by plants and animals, and may be subsequently excreted from the organism, processed and chemically altered to a new compound, or fixed and concentrated in tissues.  DDT, for example, is a notoriously stable and persistent organic compound, concentrating in fatty tissues and easily transmitted from organism to organism in the food web.

 In the exercise, a pollutant is present in the model that is thoroughly mixed, and does not degrade, disappear or even settle out of the water column (does not leave the system).  Which pollutants might show these characteristics?

Streams and Lakes

 Water flows downhill in narrow channels called rivers or streams.  The size of the river is usually measured by the amount of water flowing through the river in a certain amount of time.  The volume of water per time is discharge, usually given in cubic feet or cubic meters per second.  A small stream might have a discharge of a 100 cubic feet per second, equivalent to about 3 cubic meters per second.  The Mississippi River at Baton Rouge, Louisiana, has a typical discharge of 100,000 cubic meters per second.  The bigger the channel (wider and deeper) and the faster the water flows, the higher the discharge.

 Lakes can be thought of as simply wide points in the river.  The width and depth of the "channel" has increased dramatically, so for the discharge to be more or less the same, the velocity must be decreased substantially.  That makes sense, water in lakes IS flowing, albeit very slowly.

 Most water in rivers and lakes comes from groundwater, water flowing slowly underground in the small gaps between sand and gravel particles or in cracks in bedrock.  Loose sand and gravel can accomodate up to 30% by volume of groundwater.  The groundwater flows downhill and feeds rivers and lakes from their sides and bottoms; this important process goes essentially unnoticed by you and I.  The groundwater originates as rainfall that soaks into the ground, or in some places as snowmelt.

 Rivers and lakes can be also fed by melting glacier ice, if the river has a valley glacier at the head of the valley.  Rivers and lakes can also be fed directly by overland flow of rainwater, that cannot soak into the ground, because the ground is already completely saturated.  This overland flow or sheet wash can sometimes be observed during heavy rainstorms.  But compared to groundwater, these two sources are pretty trivial.

 In the exercise, we neglect the contribution from groundwater and overland flow, and consider a very simple system, where water that flows into the lakes comes directly from the rivers, whose sources are not specified.  We also neglect loss of water from the system due to evaporation under the sun, or due to recharge of gravels beneath the rivers and lakes.  Our model is very much a closed system of water "pipes" and "bathtubs."

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