Three topics of interest at the Shingobee River Headwaters Area

GW/SW interactionsPeeper studies (poster)

Watershed-scale processesSynoptic studies

Tritium Talesthe mystery deepens

Lake and Groundwater Interactions in the Littoral Zone of a Closed-basin Lake

in North Central Minnesota

Paul Schuster, Mike Reddy, Jim LaBaugh, Don Rosenberry, Renee Parkhurst, and Tom Winter

Sediment porewater sampler

“Peeper”

Two-componentmixing model

Williams Lake

Littoral zone18

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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

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?

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

Shingobee RiverHeadwaters Area

Synoptic studies199519972000

Understanding the GW component Influence of the Springs Preferential flowpaths

Watershed scale-Evap. RatesD/18O plots

-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

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)

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

Tritium Tales

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

Williams Lake

18

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30

29

28

Is the water in WL18 30 years old?