This study evaluated seasonal changes in hydrologic connectivity in a tile-drained agricultural catchment using environmental tracers. Samples were collected from piezometers and lysimeters throughout the site and from a tile drain. The goals were to determine the contributions of precipitation, shallow soil water, and deep soil water to tile drainage over the water year. It was hypothesized that early in the year, the deep and shallow groundwater systems would be isolated due to wetting of the soil profile, but would later mix as the profile saturated. Findings showed the primary source of water to the tile drain was the deep system, and values converged in the deep, shallow, and tile drain waters later in the high-flow season, suggesting mixing.

1. SEASONAL EVOLUTION OF HYDROLOGIC CONNECTIVITY
IN A TILE-DRAINED AGRICULTURAL CATCHMENT:
AN ENVIRONMENTAL TRACER STUDY
By:
Bryan Donaldson

2. Purpose of Study

3. Site Description
• Location: Cook Agronomy Farm (CAF) is
approximately 8 km NE of Pullman, WA
• Extent: Tile-drained part of the CAF is
approximately 12 hectares (ha)
• Samples were collected at 12 piezometers and
11 lysimeters throughout the site.
• Discharge samples were collected at a tile
drain on site.

4. Site Map

5. Cross-section

6. Study Goals
• Evaluate how groundwater composition
evolves as the water year progresses and soil
saturation levels fluctuate,
• Determine the contributions of seasonal
precipitation and snow-melt, shallow soil
water, and deep soil water to tile drainage
• Understand vertical mixing processes of
shallow groundwater over the course of a
water year.

7. Hypotheses
• At the beginning of the water year the deep
and shallow groundwater system should be
isolated from one another
• This isolation is due to the loss of
interconnection of flow paths while wetting
up of the soil profile is taking place.

8. Hypotheses
• The values of δ18O in the shallow groundwater
early in the season should be reflective of
“new water” from depleted events early in the
winter season.
• The values for δ18O in the deeper system are
more reflective an averaging effect of winter
precipitation and snow, as it is being
recharged by these waters once the soil
profile is wetted up.

9. Physical
Hydrology

10. Stable Isotopes

11. Stable Isotopes

12. Electrical
Conductivity

13. Low-Flow
Season

14. Early High-flow
Season

15. Late High-flow
Season

16. Physical
Hydrology

17. Findings
• Primary source of water at tile drain discharge
appears to be from the deep water system.
• Water hydrogeochemistry values converge in
the deep, shallow and TD-12 outlet as the
high-flow season progresses.
• Tile drain δ18O, EC, and Temperature all
appear to react rapidly to large melt-water
and precipitation events. Macro-pore flow?

18. Rapid response to melt water events

19. Special Thanks
Thanks to the field crew that
Helped make this happen including
Christopher Kelley, Erin Brooks,
Todd Anderson, William Payne,
Ryan Boylan, and Rebecca Martin

20. Special Thanks
My Wife Christy Donaldson