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

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

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.

Background

1. Water table rises2. Soil Eh drops=reducing conditions

3. Fe3+ → Fe2+

4. Clays disperse5. Transient spike in DOC

6. Increased colloids

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

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.

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).

Hydroperiod: seasonal pattern of water table depth in a soil or wetland.

Hydrodynamics: direction and energy of water flow

winterwinter

Devil’s HolePiedmont; Toeslope seepPermanently inundatedVertically steady state, laterally static

3636536460

3655536650

3675336848

3694337038

3713337228

3732337429

3752437619

3771437809

3807038165

3832638421

3851738612

3870738802

3889738992

Devil’s Hole HydroperiodPermanently Inundated

Dept

h cm

25

0

-25

Possum HillPiedmont; Backslope seepPermanently saturatedVertically static, laterally dynamic

3636936471

3657336675

3679136893

3700737109

3721137313

3741537592

3769437796

3789838000

3810238270

3837338475

3857838680

3878238884

38986

Possum Hill HydroperiodPermanently SaturatedDe

pth

cm

25

0

-25

-50

BlackbirdCoastal PlainMineral soil flat + depressionSeasonally saturatedVertically dynamic, laterally low energy

3824338275

3830738339

3837138403

3843538467

3849938531

3856338595

3862738659

3869138723

3875538787

3881938851

3888338915

3894738979

39011

Blackbird Flat HydroperiodSeasonally Saturated

Dept

h cm

25

0

-25

-50

-75

Sampling Well Placement Across a Hydrologic Gradient

InletCenter

Outlet

Groundwater Sampling Scheme

winterwinter

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+]

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

Mean Groundwater [Colloid]

P.H. D.H. BB0

50

100

150

200

250

InletWet. OutletDepr.

mg/

L

Groundwater [Colloid] at Possum Hill in 2015

Dec. Mar. June0

20

40

60

80

100

InletWet. Outletm

g/L

Groundwater [Colloid] at Blackbird in 2014

Dec. Mar. June0

25

50

75

100

125

150

175

200

InletFlatDepr.Outlet

mg/

L

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.

Questions