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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:

  • 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

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

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

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

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

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

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

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

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

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)

*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

Major ions in:


chloride 19 .008

sodium 10 .006

sulfate 2.7 .011

magnesium 1.2 .004

*all values in 0/00 and g/kg-1