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Three topics of interest at the Shingobee River Headwaters Area
GW/SW interactionsPeeper studies (poster)
Watershed-scale processesSynoptic studies
Tritium Talesthe mystery deepens
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Lake and Groundwater Interactions in the Littoral Zone of a Closed-basin Lake
in North Central MinnesotaPaul Schuster, Mike Reddy, Jim LaBaugh, Don Rosenberry, Renee Parkhurst, and Tom Winter
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Sediment porewater sampler
“Peeper”
Two-componentmixing model
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Williams Lake
Littoral zone18
8
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Why do we care?
Because of its sandy character,the littoral zone is in direct hydrauliccontact with the local groundwatersystem (“hydrologic gateway”)
Because of its proximity to the shoreline, the littoral zone is especially sensitive to the impacts of human activity
Therefore, an understanding of physical and chemical processes inthe littoral zone are critical to waterquality issues of the lake
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The sharpness and direction of the non-linear isotopic and chemical gradients at the inflow side indicate advective solute transport
From head measurements, we know Site I (south side)is inflow and Site O (north side) is outflow
It appears the flow velocities are highenough as to promote advection of solutes as opposed to diffusion
The near-zero slope of theisotopic gradients at the outflow side also indicate advective solute transport What about Mg? Quazi-conservative? What about the “hump” in the 18O profile?
Winter?
Summer?
Summer?
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The Calcium Story40-50% of Ca entering the Lakeis retained (LaBaugh, 1995)But surficial seds contain minoramounts of Ca (Dean and Bradbury, 1997)
Where is the Calcium?
Can the peepers explain? Ca precipitates on aquatic plants; sloughs off And deposits on seds in the littoral zone. Peeper dataSuggest a dissolution process (PW [Ca] > GW [Ca] > LW [Ca])
Autumn: decaying plant matter-Organic acids-lower pH- Ca dissolution (also, DOM inhibitsCa precipitation, Hoch, et al, 1999)Spring: plants take up dissolved Ca, sloughs off and cycle begins all over
The Ca lives in the PW
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Shingobee RiverHeadwaters Area
Synoptic studies199519972000
Understanding the GW component Influence of the Springs Preferential flowpaths
Watershed scale-Evap. RatesD/18O plots
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-90
-85
-80
-75
-70
-65
-60
-55
-50
DE
L D
-13 -12.5 -12 -11.5 -11 -10.5 -10 -9.5 -9 -8.5 -8
DEL O18
LMWL
GMWL
WM EVAP LINE
SH EVAP LINE
DEEP WELLS
WM SHALLOW WELLS
SH SHALLOW WELLS
SH RIVER INLET
SH RIVER OUTLET
Shingobee River Headwaters Area
Minnesota
Note: Symbol size is equal to associated error. Cross hairs at LMWL-WM EVAP LINE intersection represent the calculated confidence interval.
PPT Vol.wt. mean
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Shingobee River Headwaters Area
-3.919
-10.918
-3.319
-9.328
-9.433
-9.449
-6.240
18OCl ueq/L
Assume:PPT and evaporation roughly the same for all lakes in the basin
Upland, closed lakes 18O signal dominated by evaporationHow do we estimate GW component?
Moving down gradient:Deep GW component increases (Cl)Does evaporation? (Howard lake)
GW 18O ~ -10GW Cl ~ 15-25(shallow)
~ 45 (deep)
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7.15(0.010)
7.22(0.011)
5.04(0.020)
6.13(0.144)
5.90(0.123)
DOC mg/L(SUVA)
5.74(0.115)
DOC decreases down gradientSUVA increases down gradient
SUVA: UV/DOCan indicator of the character or “quality” of the DOC
Generally,> SUVA > aromaticity
More reactive, especially with metals
Are there down gradient trends with Hg?
Shingobee River Headwaters Area
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Tritium Tales
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What is Tritium?• 3H (radioactive isotope of hydrogen)• Expressed in TU• 1 TU = 1 3H atom per 1018 atoms of stable H
Tritium approximates groundwater age• Prior to 1952 3H was < 10TU•Thermonuclear bomb fallout•T1/2 = 12.26 years
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Williams Lake
18
8
30
29
28
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Is the water in WL18 30 years old?
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