Download Origin of the Chemical Compositions of Some

Summation of Biogeochemical
Research of Sierra Nevada
catchments
Kate Samelson
Kendra Morliengo-Bredlau
Ben West
Corey Lawrence
Origin of the Chemical
Compositions of Some
Springs and Lakes
Robert M. Garrels
Fred T. Mackenzie
Introduction
• Analysis of spring water from Sierra Nevada
- known analysis of water constituents
- primary analysis of igneous rock minerals
and soil minerals derived from them
- the system is closed: little loss or gain of
water or CO2
- the chemical composition of the rocks
studied are representative of continental
crust, and a widespread application to rockwater systems can be used.
primary igneous rocks +
2soil water high in CO2 =
soil minerals + spring
water
Weathering Relations
• Parental granite rocks: quartz diorite and
microcline
• Dissolved content of spring water comes
from attack of CO2 rich soil water on the
silicates
• Kaolinite is the main weathering product
Weathering Reaction Test
1. back-react with kaolinite to determine
if original rock minerals can be formed
2. subtract cations and anions in snow
water from spring water to determine
the minerals derived form the rock
*HCO3 is a fudge factor
1. Reactions balance for the closed system
2. Weathering product is kaolinite
3. Original rock materials were reconstructed
*Reactions depend on depth and retention. Rock
material encountered will affect pH, rate and
weathering products
Weathering Reaction
Results
Assume: CO2 rich water + plagioclase
 kaolinite…  montmorillite
• If the above reaction • So, a water
occurs, then:
undersaturated
with montmorillite
[Ca2+][SiO2]8
and saturated with
K = -----------------kaolinite should
[H+]2
have a value < K
Evaporation Experiment
Procedure
Conditions
1. The water remains in equilibrium with a
CO2 pressure of 10 -3.5 atm
2. Temperature remains constant at 25°C
3. Pure water (except for a little CO2) is
continuously removed form the system
4. Assume that any solids formed remain in
equilibrium
Results
• Waters emerge from closed system when
they have reached compositions similar to
general Sierra water, but it will continue to
gain and lose different constituents.
• Difficult to deduce complete reactions using
evaporation technique because there are
reactions that occur in the presence of the
parent rock and processes are asymmetric
Geochemical and Hydrologic Controls
on the Composition of Surface Water in
a High-elevation Basin, Sierra Nevada,
California
Mark W. Williams
Aaron D. Brown
John M. Melack
Introduction
• Emerald lake watershed is a high altitude
basin in the southern Sierra Nevadas
• Solute Composition in lake changes due to
three distinct periods:
-snow pack runoff
-transition period between runoff and
summer flow
-low flow from late summer into winter
Background
• Traditional dogma: composition of surface water
is controlled by solutes in equilibrium with
bedrock weathering products.
if so, then, if groundwater discharge is major
source of stream flow during storm events in
granite basins, chemical weathering is the major
process that neutralizes incoming acidity…but
the chemical weathering process may be
overwhelmed and too slow at buffering in
reservoirs with low retention rates
• Determine sources of solutes in stream flow
•
•
•
-focus on origin of Ca+, reactive silicate and
HCO3Determine whether stream water is in
equilibrium with mineral weathering products
Investigate if acidity due to atmospheric
deposition is neutralized by weathering of other
processes
Investigate effects that subsurface routing has
on composition of surface water
Objectives
Site Information
• Soils are strongly acidic
• Bedrock: granite and granodiorite
• Soils are mainly derived from weathering
of the bedrock
• Snowfall accounted for ~95% of the
precipitation input during study year
-very vulnerable to acid pulse
Methods
1. All water samples were analyzed
for major inorganic ions and
reactive silicates (Si)
2. Buffering capacity determined with
Gran titration method
way to establish [ ] of HCO33. Garrels and Mackenzie approach
used to determine whether the
streamwater content was a
product of the basin/catchment
4. Cl- was used as a scaling factor to
determine base cations in
snowpack melt waters
5. Reservior residence time was
determined using a 6LiBr tracer
6. Ca2+:Na+ ratio was used to help
explain possible soil buffering
process
6. Ca2+:Na+ ratio was used to help
explain possible soil buffering
process
soil retains Ca2+ more than Na+, Na
leaches out faster than Ca
is cation exchange in soils is a
major buffering process, the ratio
of Na:Ca should be high in
snowmelt runoff and low in end of
melt season
7. Back reactions are difficult to
calculate due to differences in
retention time, depth (rock
encountered)
used contributions of Ca2+ to
calculate products of mineral
weathering
contribution % change with
seasonal changes in water flow
…hydrologic flow paths change
…relative importance of different
biogeochemical processes change
Residence Times:
• 4/10/1086 – 8/30/1986  143 days
•
Residence time: 7 – 23 days
-Averages in May ’86 and ’87 had almost daily
turnover in talus
-second method incorporating soil saturation
reported soil retention rates of minutes to hours
before being available to surface flow
Groundwater discharge in low flow was ordered
from months to years
Conclusions
• Mass balance did not work:
wrong weathering reactions used
other processes beyond weathering of
plagioclase contributed to dissolved solutes
deficit of HCO3- indicates additional sources of
alkalinity beyond mineral weathering
• Weathering model inaccurate:
model is good for low flow periods but not as
well with high flow
unknown and possibly synergistic effects during
high flow
stream water in ELW are in steady state with
weathering products during low flow periods
Conclusions
cont…
• Mineral weathering does not seem to be the
primary process of buffering
buffering of acidic cations occurs too rapidly to
be attributed to silicate weathering
cation exchange seems to be the major
buffering agent because their kinetic rates match
what is needed to buffer during such short
retention times
• ELW is subject to acid pulses from low pH snow
melt
buffering during “high flow” comes from soils
buffering during “low flow” comes from
weathering
buffering controls change
Conclusions
cont…
• Snow pack runoff in ELW infiltrates soils and
•
unconsolidated materials, undergoes reactions
with soil water and soil exchanges and is then
discharged to stream flow
Granitic basins are sensitive to atmosphic
deposition of acids due to low residence time
during melt periods