1. Colloid Mobilization and Biogeochemical Cycling of
Organic Carbon, Nitrogen and Phosphorous in
Wetlands
Bruce Vasilas, soil scientist (hydric soils)
University of Delaware
Yan Jin, soil physicist (colloid transport)
Anastasia Chirnside, engineer (water chemistry)
Ronald Manelski, M.S. candidate
Jing Yan, Ph.D. candidate

2. Acknowledgements
• This project is supported by a Agriculture and Food
Research Initiative Competitive Grant (National
Integrated Water Quality Grant Program #2013-
67019-21361) from the USDA National Institute of
Food and Agriculture.
• USDA NRCS Cooperative Soil Survey-soil
characterizations
• EPA & MDE-funding for previous projects
• DE DNREC & DE DA-field sites on state lands

3. Background & Hypotheses
• Wetlands can serve as sources, sinks, or conduits for
surface water contaminants.
• Literature indicates that mobilization and export of DOM,
Fe, N, and P from wetlands do not result from
independent processes.
• We hypothesize that colloid mobilization plays a key role
in these processes.
• We hypothesis is that soil Eh shifts in redox-dynamic
wetlands can cause wide shifts in [colloids] and dissolved
materials due to Fe mineral dissolution and pH shifts
associated with Fe oxidation state changes.

4. Background
1. Water table rises
2. Soil Eh drops=reducing conditions
3. Fe3+ → Fe2+
4. Clays disperse
5. Transient spike in DOC
6. Increased colloids

5. Previous Research
• Field studies at these sites
– Long term water table and soil Eh data
– Site variability in Ni & OP removal from
groundwater
– Seasonal fluctuations in [Ni] & [OP] in
groundwater
• Previous lab studies: Onset of reducing conditions
coincides with a transient spike in DOC

6. Objectives
• To quantify temporal & spatial variability in
groundwater [colloid] in freshwater wetlands.
• To evaluate the impact of soil Eh & Fe on [colloid].
• To assess the role of colloids on mobilization &
export of DOM, N, & P.

7. Wetland Hydrology Criteria
The water table is ≤30 cm below the soil surface
for ≥14 consecutive days during the growing
season, at a minimum frequency of 5 years in 10
(USACE, 2005).

8. Hydroperiod: seasonal pattern of water
table depth in a soil or wetland.
Hydrodynamics: direction and energy of
water flow
winterwinter

9. Devil’s Hole
Piedmont; Toeslope seep
Permanently inundated
Vertically steady state, laterally static

10. Depthcm
Devil’s Hole Hydroperiod
Permanently Inundated
25
0
-25

11. Possum Hill
Piedmont; Backslope seep
Permanently saturated
Vertically static, laterally dynamic

12. Depthcm Possum Hill Hydroperiod
Permanently Saturated
25
0
-25
-50

13. Blackbird
Coastal Plain
Mineral soil flat + depression
Seasonally saturated
Vertically dynamic, laterally low energy

14. Depthcm
Blackbird Flat Hydroperiod
Seasonally Saturated
25
0
-25
-50
-75

15. Sampling Well Placement Across
a Hydrologic Gradient
Inlet
Center
Outlet

16. Groundwater Sampling Scheme
winterwinter

17. Groundwater Parameters
• Water table depth
• Soil Eh (Pt & reference electrodes, alpha-alpha
dipyridyl strips, IRIS tubes)
• Electrical conductivity
• [Colloid] (dispersed phase particles)
• [Dissolved organic carbon]
• [Fe2+]
• [soluble reactive phosphorous (OP)]
• [NO3
-, NH4
+]

18. Challenges
• Groundwater sampling wells typically sealed near
the soil surface with bentonite (shrink-swell clays):
contamination of colloidal clay minerals
– Wells sealed with polyurethane foam & plastic sheeting
• Well purging & groundwater sampling with a pump
could increase colloid levels through agitation.
– Purge and sample at low flow rate (~100ml/min) to avoid
suspending immobilized colloids
• Exposure of groundwater sample to air: Fe2+ → Fe3+
– Argon

19. Mean Groundwater [Colloid]
0
50
100
150
200
250
P.H. D.H. BB
mg/L
Inlet
Wet.
Outlet
Depr.

20. Groundwater [Colloid] at Possum Hill in 2015
0
20
40
60
80
100
Dec. Mar. June
mg/L
Inlet
Wet.
Outlet

21. Groundwater [Colloid] at Blackbird in 2014
0
25
50
75
100
125
150
175
200
Dec. Mar. June
mg/L
Inlet
Flat
Depr.
Outlet

22. Summary
• Water chemistry is the most consistent at Possum
Hill (short residence time & static water table) &
changes slightly over time and as water moves
through the wetland.
• Water chemistry changed the most as water moved
through Blackbird (long residence time & dynamic
water table).
• Data collected to date supports our contention that
hydrologic characteristics have major impacts on
water chemistry in wetlands.

23. Questions