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 NO 3 - 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 NO 3 - 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 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 Mg 2+ and Ca 2 + (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: SEA RIVER chloride sodium sulfate magnesium *all values in 0/00 and g/kg -1