Chemical processes in estuaries 1

Question 1

What is the significance of estuaries?

Answer 1

• most river borne material passes through estuaries
• important transition zones (“the estuarine filter”) for continentally derived material
• anthropogenic inputs, either directly or via rivers

Question 2

Why are estuaries important transition zones?

Answer 2

• they can change the flux of material entering the oceans (important for global budgets of biologically important elements)
• chemical distributions may influence biological processes (& hence productivity) in estuaries, and vice versa

Question 3

What are the types of estuarine mixing?

Answer 3

• highly stratified (salt wedge); large river input, weak tidal input of salt water
• partially mixed; small river input, large tidal input
• well mixed; small river input, v large tidal input
• inverse estuary (neg circulation); salinity increases towards river mouth; hot, arid climates (evap exceeds freshwater input)

Question 4

What does chemical composition of the water body reflect?

Answer 4

The extent of mixing between fresh water and salt water.

Question 5

What does concentration of major ions co-vary with?

Answer 5

Salinity

Question 6

Are concentrations of major ions far higher in freshwater or seawater?

Answer 6

Seawater

Question 7

At what salinity does the greatest change in major ion proportion occur?

Answer 7

Around 5

Question 8

What is the pattern of major ion concentration in seawater versus river water?

Answer 8

in seawater, major ions have same concentration

in river water, concentration of major ions is highly variable

but this has very little effect on mixing diagrams

Question 9

Explain what the TDL/mixing diagrams show.

Answer 9

• if constituent chemical plots follow along the line linking river water end member (salinity = 0) to seawater end member, which is the TDL, this constituent is said to show CONSERVATIVE BEHAVIOUR in the ESTUARY.
• conservative behaviour means that mixing alone explains the distribution of the element.

Question 10

Which constituents do not show conservative behaviour in estuaries? What processes affect their distribution?

Answer 10

Most minor and trace constituents (e.g. nutrients/metals).

Processes like:

• biological processes (photosynthesis, remineralisation)
• adsorption/desorption on particle surfaces
• coagulation/flocculation/precipitation
• redox processes

Question 11

What is adsorption?

Answer 11

Binding of dissolved chemical species (solutes) to mineral surface by formation of surface complexes.

Ability of minerals to adsorp solutes depends on mineral and on the pH. Depends on surface charge, which is a function of pH.

Question 12

What are colloids? What are main colloidal phases in rivers?

Answer 12

Very small particles (nanoparticles) that remain suspended in aqueous solutions.

Clay minerals. Iron (hydr)oxides. Humic/fulvic acid.

Question 13

When will colloids flocculate?

Answer 13

When their surface charge is 0. Flocculation of clays takes place as soon as river water encounters seawater. Flocculation of humic acids and FeOOH takes place at salinities ~5.

Question 14

What does flocculation do?

Answer 14

Incorporates colloids and their sorbed metals into estuarine sediments.

Question 15

Why are estuarine sediments rich in organic carbon?

Answer 15

High rates of biological productivity/flocculation

Question 16

What does the reducing nature of estuarine sediments mean for the concentration of dissolved substances?

Answer 16

Conc’s may be higher or lower in sediment pore waters than overlying estuarine water.
Leads to diffusion in/out of sediments.

Question 17

What is non-conservative behaviour? What causes it?

Answer 17

When there is positive (addition) or negative (removal) deviation from the TDL.

Caused by biological processes, coagulation/flocculation, adsorption/desorption and redox processes, which add or remove solutes from the estuary.

Question 18

What are the assumptions of these mixing diagrams?

Answer 18

• estuary is in steady state; flux in = flux out.
• conc of end members is constant on timescales longer than the residence time of water in the estuary.
• only one river end member and one sea water end member
• no additional sources of material

Question 19

What are the limitations of these mixing diagrams?

Answer 19

• if the residence time is short (flushing time), non-conservative behaviour will be detected
• mixing diagrams indicate apparently non-conservative behaviour if:
  residence time is long & there’s temporal variability in the end members
  estuaries receive input from more than 1 river
  estuary receives additional sources of material e.g. sewage outflow.

they tell us nothing about the processes causing this behaviour