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Estuarine Chemistry/Physical :

Estuarine Chemistry/Physical : Estuaries are where rivers meet the sea - the exact nature of the chemical processes occurring in an estuary generally depends on :

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Estuarine Chemistry/Physical :

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  1. Estuarine Chemistry/Physical: • Estuaries are where rivers meet the sea - the exact nature of the chemical processes occurring in an estuary generally depends on : • 1) the quantity and kind of materials transported by the fresh and salt water sources; 2) different chemical reactions that occur in fresh vs. salt water; 3) the residence time of river water in the estuary

  2. Examples using Dissolved Oxygen and Inorganic Nitrogen: • DO and NO3- can change considerably over 24 hours -- mostly related to physical forces: • rising tide high in oxygen but low in nitrogen • tidal draining from fringing marshes low in oxygen but high in ammonia

  3. In Barataria Basin (eutrophic), 24 hour change nutrients largely due to biological activity: • Phytoplankton increase DON during day; community respiration lower at night • During peak phytoplankton blooms NO3- almost disappears

  4. Estuarine Mixing Bowl: • As river water mixes with sea water during its retention in an estuarine basin, many of the most important reactions are transformations between dissolved and particulate forms • These processes include: • 1) adsorption or desorption on particle surfaces • 2) coagulation, flocculation, and precipitation • 3) biotic assimilation or excretion

  5. Ion Exchange: • Oxides, especially Si, Al and Fe abundant components of rock • Hence most of the solid phases in natural waters contain oxides and hydroxides • Intereactions of cations and anions with hydrous oxide surfaces are important in natural water systems and colloid chemistry • Linus Pauling showed that most clays behave as weak acids • Al tetrahedron with corners shared by silica tetrahedron

  6. Ion-exchange capacity determined by surface change: • Gouy Theory - sum of change due to excess cations and deficiency of anions • Preforming of clays and most other natural ions exchange one for K+ over Na+. Remove K+ from solution ---> solid phase

  7. Aggregation of Colloids - Hydrophobic and Hydrophilic: • Particles with diameters <10 um are within the colloid range • Remain suspended because gravity settling 10-2 cms-1 • Biocolloids (viruses, bacteria) - hydrophilic • Particle agglomeration depends on frequency of collisions • Aggregation of colloids known as coagulation or flocculation • Aggregation of colloids is of great importance in the transport and distribution of matter in estuaries

  8. Fresh water - high turbidity, high amounts of Fe and humics. Negatively charged particles remain separated - stable suspensions! • In estuaries particles carried down with the river water will sink from upper to lower water layers • Concentrations much higher in oligohaline turbidity maximum. Destabilization of electrolytes. • Clays (neg. charged) become destabilized with increasing salinity • Interparticle forces become attractive

  9. Example: River-borne “dissolved” iron consists of iron oxide - organic matter colloids ca. <. 4 um Stabilized by DOC (humics) • Coagulation occurs on mixing because the seawater cations, especially Mg2+ and Ca2 + (neutralize negative charges) destabilize the negatively charged iron-bearing colloids. This allows van der Waals forces to dominate as particles collide • i.e. intermolecular attraction in water 1/3 of the surface tension due to van der Waals - remainder due to hydrogen bonding • entrapping mechanism entering estuaries

  10. 2 approaches to investigate physical and chemical removal of dissolved and particulate substances from river and sea during estuarine mixing • Reactant and Product Approaches: • Reactant: compares observed distributions of a given dissolved constituent to predicted from the simple mixing model, which assumes that the constituent remains conserved (total amount unchanged)

  11. *Reactant approach has 2 assumptions: • 1) concentration and flux of constituent at riverine end member constant over time • 2) only 2 end member sources river and sea (no tributaries) • disadvantage - no mechanism inferred • *Product approach - river water is mixed with varying amounts of seawater to yield a series with salinities • disadvantage - only 1 mechanism considered - flocculation

  12. Major ions in: SEARIVER chloride 19 .008 sodium 10 .006 sulfate 2.7 .011 magnesium 1.2 .004 *all values in 0/00 and g/kg-1

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