Stream water solute loads and mixinganalysis

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Stream water solute loads and mixing-analysis
27 February 2006
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Outline of Lecture

• What is our interest in stream water chemistry
  (solute loads?)
• Examples of variation in stream water chemistry
• Seasonal time series (and spatial variation)
• Storm-event time series
• Bivariate Solute Plots
• Stream water as a mixture of contributing sources
• What is a mixing model?
• Identifying geographical sources (end-members) of
  water
• How do our activities alter the mixture that is
  stream water (as identified by solute loads)?

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Our interests as hydrologists, managers and
researchers of watershed processes

• Stream water quality and quantify
• Stream water as a mixture of contributing sources
  in the landscape
• Use of solute loads to help us infer hydrologic
  flowpaths and contributing sources.
• Inference of runoff generation mechanisms
• Tell us about biogeochemical processes and
  nutrient fluxes (spatial and temporal variation).

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Mixing of source waters in a watershed
RAIN
SOIL WATER
Stream
GROUND WATER
Focusing on physical mixing (no chemical
reactions)
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Stream water chemistry Seasonal time series
Watershed 10 HJA 3-Week composite samples from
1994-2004
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Stream water chemistry Seasonal time series
Watershed 10 HJA 3-Week composite samples from
1994-2004
Concentrations peak at low flow
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CHLORIDE Seasonal time series
Multiple watersheds at HJA 3-Week composite
samples from 1994-2004
Comparison in space multiple catchments
Chloride (ppm)
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CHLORIDE Seasonal time series
Multiple watersheds at HJA 3-Week composite
samples from 1994-2004
Comparison in space multiple catchments
Chloride (Kg/ha)
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Stream water chemistry Storm-event time series
In-stream instantaneous grab samples from
multiple nested catchments, Mont Saint-Hilaire,
Quebec Storm 36 mm delivered in 2.5 hrs.
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Mixing of source waters in a watershed
RAIN
SOIL WATER
Stream
GROUND WATER
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What is a mixing-model?

• Water in the stream is a linear mixture of
  contributing sources (end-members).
• Simple mathematical equations describing physical
  mixing of water.
• No chemical reactions considered.
• Example 1 Temporal hydrograph separation
  mixing of time-based sources of water (event vs
  pre-event new vs old) 2-component mixing model.
• Example 2 Geographical-source separation
  mixing of geographically-based sources of water
  to create observed stream water potentially gt2
  end-members.

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A simple mixing example 2 end-members
RAIN
Example Stream water (at some time to) is a
mixture 30 rain and 70 groundwater. Rain
Solute A 5 mg/l Groundwater Solute A 10
mg/l In the stream Solute A 0.30
5 mg/l 0.70 10 mg/l 8.5 mg/l for
in-stream mixture
Stream
GROUND WATER
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Storm Hydrograph Separation using Isotopes
EVENT
d18O -5
d18O -12
PRE-EVENT
d18O -8
Stream
2 end-members Event and Pre-event water
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What is a mixing-model?

• A simple mathematical equation describing
  physical mixing of water.

• 1) Qstream Qold Qnew
• 2) Cstream Qstream Cold Qold Cnew Qnew
• Proportion of old water in stream channel

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Mixing Model Assumptions

• Solutes must mix conservatively
• Meaning what exactly?
• 1) No chemical reactions. Physical mixing only
  as water travels to the
• stream channel
• End-members bound observed stream water
  chemistry.
• End-members must be significantly different in
  concentrations.
• End-members are constant in time (for travel
  time).
• End-members are constant in space (e.g. across
  the watershed).

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Bivariate solute plots
HJA Watershed 10 3 week composite samples from
1968-2004
Slight curvature?
Fairly linear trend
Definitely curvature here
Linear trends in bivariate plots indicates
conservative mixing of solutes.
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Identifying geographical sources (end-members) of
water

• End-member examples
• throughfall, hillslope groundwater,
  valley- bottom groundwater, organic-soil horizon
  water.
• How many end-members are contributing?
• What do these end-members physically represent?
• Are they present from catchment to catchment?
• How do their contributions vary from catchment to
  catchment?

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Searching for End-members Independent Field
Sampling
Sampling of end-members in the field Lysimeters
(soil water) Riparian wells Natural
springs Hillslope wells
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Storm response at Mont Saint-Hilaire, Quebec
Stream water as a mixture of 3 end-members i)
Groundwater (GW) ii) Throughfall (ThFall) iii)
Perched water
38 mm storm
DOC (?mol/l)
d18O
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Mont Saint-Hilaire, Quebec
Notice the large variation in perennial spring
(Seep) concentrations
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Mont Saint-Hilaire, Quebec
U-space an alternative to bivariate solute
plots Redefining the axes using many solutes at
once.
A bivariate solute plot.
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How do our activities alter the mixture that is
stream water (as identified by solute loads)?

• Alteration of flowpaths
• Alteration of sources
• Logging
• Urbanization
• Agriculture