Chapter 14
The major Ions of Inland Waters
Why are there differences in the
composition of inland waters?
Five major factors
#Climate
#Geography
#Topography
#Biotic activity
#Time
#They all interact
Total dissolved solids (TDS) and
specific conductance
#TDS includes salts and organic residues
#Average TDS in world's rivers is 120 mg/l
#This is a meaningless number
#TDS ranges from 1mg/l in Waldo Lake Oregon to
220,000 mg/l in The Great Salt Lake
#In general - lower TDS results in lower lake
productivity
Specific conductance or conductivity
#Specific conductance (conductivity) is often used
as a comparable substitute for TDS
#It is easy to measure with a meter that measures
the flow of electrons through water.
#The greater the conductivity, the more dissolved
material is in the water
#It does not measure organic residues
Specific conductance or conductivity
#Conductivity increases with Temperature and
elevation
#Compensating factor is 0.25 per degree of
temperature increase
#Usually, a standard of 25 degrees C. Is used
Expression of chemical results
#1ppm is very nearly 1 mg/l.
sea water is 35,000 ppm
#1000 ppm = 1 part per 1000 (o/oo)
sea water is 35 ppt
#10,000 ppm = 1%
sea water is 3.5%
Expression of chemical results
#TDS is reported as mg/l or ppm
Salinity vs Chlorinity
#Chlorinity is a measure of chloride ions present
#Salinity is the sum of cations (+) and anions (-). It
includes chlolrinity
#Salinity can be measured as conductivity
#This works well in sea water but in freshwater
conductivity due to carbonates must be subtracted
from total conductance
Alkalinity
#Carbonate is the major anion of fresh water
#It is usually combined with calcium.
#Alkalinity is usually a measure of carbonate in
fresh water.
#Indirectly alkalinity measures cations that are
balanced against carbonate and other anions.
Alkalinity
#In a pure Ca CO3 system, multiply alkalinity by
0.4 to get calcium ions
#However, if the system contained CaSO4 there
would be no buffering capacity and thus no
alkalinity to be measured.
#A pure NaCO3 solution might have very high
alkalinity but no calcium.
Alkalinity
Calcite- calcium carbonate is present in lake
sediments due to accumulation of mollusc
shells, especially snails.
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Alkalinity
#Carbonate is also commonly bound with
magnesium - magnesite
#or Dolomite - Calcium magnesium carbonate
CaMg(CO3)2
Alkalinity
• In arid regions- precipitates of carbonate
combined with sodium are found
• #commonly called natron and trona
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Alkalinity
#Lakes rich in sodium compounds are called alkali
or soda lakes
#Soda waters are not considered "hard" because
they do not contain calcium or magnesium.
#Potash ponds can be found in Nebraska that
contain potassium carbonate
#Iron and ammonia bind with carbonates in anoxic
hypolimnions
#High alkalinities are measured here. Only some
of the cations are calcium
Biota of extreme alkaline carbonate
waters
#Very few species
#Two distinct groups of organisms
#one group inhabits chloride and sulfate waters
#ex. brine shrimp
Biota of extreme alkaline
carbonate waters
• second group inhabits alkali and carbonate
waters
• ex. Corixid bugs, certain blue-green algae
• some organisms survive or can be found in
both environments
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Sulfate
#sulfate ion SO4 - - 2nd to carbonate in abundance
#Free elemental sulfur is inactive at ordinary temperatures
#It is abundant in areas of recent volcanic activity
Sulfate
• Hydrogen sulfide and iron
sulfide are the most
important un-oxidized
forms of sulfur
• Sulfur dioxide and sulfates
are the most common
oxidized forms of sulfur
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Sulfate
Sulfur combined with hydrogen or alkali earth
metals are the most common forms of sulfur
in lakes and streams.
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Sources of Sulfate
#Atmosphere - SO2
#industries put more than
2.4X as much sulfur into the
atmosphere as volcanos do.
#Coal combustion, copper
smelting and paper making
produce large amounts of
sulfur containing compounds.
Sources of Sulfate
• Intense evaporation concentrates sulfate in
some lakes in hot, arid regions.
• Some sulfur can come from sea spray and
rainfall.
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Sources of sulfates
#Soil and rocks are the greatest contributors of
sulfate.
#High sulfate levels reflect the presence of old
marine sediments in the water shed.
#Two most important stores of sedimentary
sulfates are:
#calcium sulfate and iron sulfide
#Sulfate is formed from evaporation
#Sulfides are formed from microbial activity
Sulfate lakes
#Many sulfate lakes are acidic
#Many may be caused by acid rain
#Gypsum (CaSO4) in the water shed is responsible
for high sulfate concentrations in some lakes
Hydrogen sulfide
#H2S can be found at high levels in the hypolimnion of
eutrophic lakes
#or monomolimnion of meromictic lakes
#This gas is poisonous to aerobic organisms
#A very stagnant hypolimnion can look like dilute india
ink
#
Hydrogen sulfide
• Black sea gets its name from
dark waters about 150m
below the surface
• It is produced by sulfate
reduction to hydrogen sulfide
and bacterial decomposition
of organic matter.
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Black Sea
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Thiobios
#Thiobios - organisms that metabolize
sulfur compounds for growth, mostly
bacteria
#environment is dimly lit, anoxic, low
pH and high in sulfides.
#Ex. Anerobic marsh muds
Sulfate reducers
#microorganisms that in the absence of oxygen
reduce sulfate to hydrogen sulfide and sulfur and
precipitates CaCO3 at the same time.
#CaSO4 + 2C B> CaS + 2CO2
#CaS + CO2 + H2O B> CaCO3 + H2S
#The carbon comes from decaying organic matter
Sulfate reducers
#Another group of bacteria attack protein in
decaying organic matter directly, releasing H2S
#The H2S gas is released back into the atmosphere
#Fossil fuels now put more H2S into the
atmosphere than natural processes
#Wetlands muds breakdown sulfur dioxide and
sulfuric acid produced by industry.
Sulfur oxidizers
#use sulfide to produce sulfates
#green sulfur bacteria - live in anaerobic
environments with H2S levels above 50 mg/l
#They use light as an energy source much like
plants
#CO2 + 2H2S - -> light B> (CH2O) + H2O + 2S
#The sulfur from this reaction is excreted
Purple sulfur bacteria
#adapted to very dim light
#retain sulfur from the previous reaction , they can
oxidize it to sulfuric acid with a net energy gain
#Other bacteria do not need light. They can
convert H2S to H2SO4 with a net energy gain
End of Part 1
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Chloride
#chloride ranks 3rd in ion abundance
#molecular chloride is a heavy, yellow, lethal gas
#chloride most abundant in the sea and desert
pools
#usually in association with sodium
#salt cedar is one of the few plants that can use salt
water.
#It can extract and secrete sodium chloride
Salt Cedar
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Sources of Chloride
#evaporites - beds of salts formed from evaporation of
ancient bodies of water
#wind blown inland from sea spray and coastal
evaporated pools.
#Volcanic gases- HCl
#Pollution - domestic and animal sewage contains
chlorides.
#Septic tank sewage can contribute measurable amounts
of chlorides into ponds
#Salts from city streets NaCl and CaCl2 to clear away ice.
Evaporites
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Sources of Calcium
#calcium carbonates
limestone - made from shells
of once living organisms
#calcium aluminum silcate
oxides - feldspars - most
abundant mineral - 60% of
earths surface.
Feldspar - calcium aluminum silcate
oxides
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Solubility and deposition of calcium
compounds
#CaCO3 is the scale that is deposited on hot water
pipes - economically important
Calcareous deposits in lakes
#Marl - usually fine grained,
calcareous clay sediment.
Deposited in the littoral
region of hard-water lakes.
Calcareous deposits in lakes
#Accumulations of dead
Chara coated with lime
contributes to this
deposition
#marl is deposited where
pH is high and
photosynthesis is intense
Why?
Calcareous deposits in lakes
#Sinter - chemical sediments deposited by mineral
springs
#ex. Siliceous sinter - geyserite - Hot springs in
Yellowstone national park
#ex. Calcareous sinter - Travertine deposited from
mineral springs near Sulfur, OK
Yellowstone National Park
Mammoth Springs
• Geyserite – a form of sinter
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sulfate minerals of calcium
#Gypsum - hydrated calcium sulfate CaSO4
2H2O
#anhydrite - calcium sulfate CaSO4
sulfate minerals of calcium
#Both have marine origins
#gypsum is the 1st mineral deposited when sea
water evaporates
#anhydrite deposited under high temperature and
salinity
#solubility of gypsum is 120X greater than
carbonate solubility
#Therefore it is easily dissolved from rocks in
water shed.
Chloride minerals of calcium
#Calcium chloride is the most soluble of the
calcium compounds
#30,000X more soluble than carbonate
#dead sea has high calcium chloride levels
#most bodies of water that have high CaCl also
have much higher NaCl or MgCl
Biota of calcium waters
#all organisms need calcium particularly molluscs,
crayfish and insects
#All species decrease when calcium is scarce.
Magnesiun
#2nd most abundant cation after calcium
#magnesite - MgCO3 and dolomite CaMg(CO3)2
are common sources combined with carbonates in
rock
#Epsomite - magnesium sulfate MgSO4 - Epsom
salt is a common Mg source combined with a
sulfate. 150X more soluble than gypsum
#Ex. Hot Lake and Soap Lake in Washington state
Magnesium
#Magnesium chloride MgCl found in the dead sea,
other desert waters and bittern lakes
#Bittern results when sea water evaporates and
most salts have crystalized and precipitated
#The liquid that remains is called bittern and
contains bromides and magnesium salts
#Magnesium is the basis of the chlorophyll
molecule. Necessary for photosynthesis
#Too much magnesium harmful. MgSO4 acts as an
anesthetic on many organisms.
Bittern Lake
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Hardness
#Cations (+) that form insoluble compounds with soap
contribute to what is called "hardness"
#Soap consuming power of water is lowered by heavy
metals and alkaline earth metals
#Calcium and magnesium are most common and
account for most hardness.
#Hardness is reported as mg/l equivalent of CaCO3
#Hardness measures all the calcium and magnesium
whether it is bound with carbonates or sulfates
Sodium
#6th most abundant element in the earths crust
#very reactive and soluble - its compounds tend to
stay in solution
#3rd most abundant cation in lakes and streams
Sources of Sodium
#feldspars and evaporites
#Most common water soluble sodium mineral is
halite NaCl common table salt
#Glauber's salt - Na2SO4 10H2O
#natron and trona hydrates of NaCO3
#Agricultural irrigation in arid regions puts salts
back into solution ex. Salton Sea
Salton Sea California
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3 kinds of sodium lakes
#Salterns - rich in sodium chloride NaCl
#Saline lakes - rich in sodium sulfate Na2SO4
#Soda lakes - rich in sodium carbonate Na2CO3
#soda lakes have enormous phytoplankton
populations not so with other sodium rich lakes
Saltern
Salterns - Rich in sodium chloride NaCl
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Saline Lake
• Rich in sodium sulfate Na2SO4
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Soda lake in Turkey
• Soda lakes - rich
in sodium
carbonate
Na2CO3
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Potassium
#4th ranking cation
#High potassium levels are lethal to many aquatic
animals
#Source is potash potassium carbonate K2CO3
#Thought to be ashes of ancient fires
Desert limnology
Great Salt Lake, Utah
#Closed basins, evaporation greatly exceeds
rainfall
#ex. Great Salt Lake
#best described by evaporation - concentration and
precipitation of compounds
Order of precipitation of common
compounds
#CaCO3 and MgCO3 are the 1st to precipitate
#CaSO4 precipitates next. Leaving mostly Na and
Mg cations Calcium all precipitated
#NaSO4 precipitates next leaving mostly the
chloride compounds
#NaCO3 next in order precipitates if any CO3 left
#MgSO4 precipitates out all that is left is NaCl
Order of precipitation of common
compounds
#NaCl saltern is left. These are fairly common
(Great Salt Lake)
#MgCl and CaCl lakes are rare. Called bitterns
Dead Sea)
#If all water evaporates - bed of salt NaCl usually
results.
The Dead Sea – a Bittern
Magnesium chloride (MgCl), main ions
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The Dead Sea –
a Bittern
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Precipitates Summary
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The End
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