1. 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

2. 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

3. Sediment porewater sampler “Peeper” Two-component mixing model

4. Williams Lake Littoral zone 18 8

5. 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

6. 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 high enough as to promote advection of solutes as opposed to diffusion 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? Winter? Summer?

7. 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

8. Shingobee River Headwaters Area Synoptic studies 1995 1997 2000 Understanding the GW component Influence of the Springs Preferential flowpaths Watershed scale-Evap. Rates D/ 18 O plots

10. Shingobee River Headwaters Area -3.9 19 -10.9 18 -3.3 19 -9.3 28 -9.4 33 -9.4 49 -6.2 40 18 O Cl ueq/L Assume: PPT and evaporation roughly the same for all lakes in the basin Upland, closed lakes 18 O signal dominated by evaporation How do we estimate GW component? Moving down gradient: Deep GW component increases (Cl) Does evaporation? (Howard lake) GW 18 O ~ -10 GW Cl ~ 15-25(shallow) ~ 45 (deep)

11. 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 gradient SUVA increases down gradient SUVA: UV/DOC an 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

12. Tritium Tales

13. What is Tritium? 3 H (radioactive isotope of hydrogen) Expressed in TU 1 TU = 1 3 H atom per 10 18 atoms of stable H Tritium approximates groundwater age Prior to 1952 3 H was < 10TU Thermonuclear bomb fallout T 1/2 = 12.26 years

14. Williams Lake 18 8 30 29 28

17. Is the water in WL18 30 years old?