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Three topics of interest at the Shingobee River Headwaters Area. GW/SW interactions Peeper studies (poster) Watershed-scale processes Synoptic studies Tritium Tales the mystery deepens. Lake and Groundwater Interactions in the Littoral Zone of a Closed-basin Lake

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Three topics of interest at the Shingobee River Headwaters Area

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Three topics of interest at the

Shingobee River Headwaters Area

  • GW/SW interactions

    • Peeper studies (poster)

  • Watershed-scale processes

    • Synoptic studies

  • Tritium Tales

    • the 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-component

mixing model


Littoral zone

18

Williams Lake

8


Why do we care?

Because of its sandy character,

the littoral zone is in direct hydraulic

contact with the local groundwater

system (“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 in

the littoral zone are critical to water

quality issues of the lake


From head measurements,

we know Site I (south side)

is inflow and Site O (north

side) is outflow

Summer?

Winter?

The sharpness and direction

of the non-linear isotopic

and chemical gradients at the

inflow side indicate advective

solute transport

Summer?

The near-zero slope of the

isotopic gradients at the

outflow side also indicate

advective solute transport

What about Mg? Quazi-conservative?

What about the “hump” in the 18O profile?

It appears the flow velocities are high

enough as to promote advection

of solutes as opposed to diffusion


The Calcium Story

40-50% of Ca entering the Lake

is retained (LaBaugh, 1995)

But surficial seds contain minor

amounts 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 data

Suggest a dissolution process

(PW [Ca] > GW [Ca] > LW [Ca])

Autumn: decaying plant matter-

Organic acids-lower pH-

Ca dissolution(also, DOM inhibits

Ca 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 River

Headwaters Area

Synoptic studies

1995

1997

2000

Watershed scale-Evap. Rates

D/18O plots

Understanding the GW

component

Influence of the Springs

Preferential flowpaths


Shingobee River Headwaters Area

18O

Cl ueq/L

-6.2

40

-9.4

49

GW 18O ~ -10

GW Cl ~ 15-25(shallow)

~ 45 (deep)

-9.4

33

Assume:

PPT and evaporation roughly

the same for all lakes in the basin

Upland, closed lakes 18O signal

dominated by evaporation

How do we estimate GW component?

Moving down gradient:

Deep GW component increases (Cl)

Does evaporation? (Howard lake)

-9.3

28

-3.3

19

-10.9

18

-3.9

19


DOC mg/L

(SUVA)

Shingobee River Headwaters Area

5.74

(0.115)

DOC decreases down gradient

SUVA increases down gradient

SUVA: UV/DOC

an indicator of the character

or “quality” of the DOC

5.90

(0.123)

6.13

(0.144)

Generally,

> SUVA > aromaticity

5.04

(0.020)

7.22

(0.011)

More reactive,

especially with metals

7.15

(0.010)

Are there down gradient trends with Hg?


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


30

28

18

29

Williams Lake

8


Is the water in WL18 30 years old?


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