marine and terrestrial influence on submarine groundwater ... · 2014-09-04, hier ggf....

2014-09-04, Hier ggf. Veranstaltungstage oder Autor oder beides © 2016 UNIVERSITY OF ROSTOCK | Baltic TRANSCOAST

Marine and terrestrial influence on submarine groundwater

discharge in coastal waters connected to a peatland

Miriam Ibenthal Rostock University

Supervised by M. Janssen, B. Lennartz (Rostock),

G. Massmann (Oldenburg), T. Ptak (Göttingen)


DFG - Research Training Group - Baltic TRANSCOAST

Hydrodynamics

Geochemistry

Biology

Rostock

Autumn school in Askö field station, Sweden

Understanding coastal interactions:


Submarine groundwater discharge (SGD)

SGD driven by:

• Hydraulic gradient

• Density gradient

• Thermal gradient

• Storm

• Waves

• Tides

Seawater mixing with freshwater can lead to desorption of ions, oxidation of organic carbon and

release of nutrients.


Importance of Peatlands – What is Peat?

• Porosity: ~90%, decreases with depth

• Drainage: compaction of peat, release of CO2

• Partly degraded plant material

• Growth under anaerobic conditions

• Carbon sink and methane production

Ecohydrology Research Group

Waterloo University, F. Rezanezhad

Back-scattered electron image of a

Sphagnum peat

1 cm

HNE Eberswalde

Phragmite peat

Peat with wood from Hütelmoor

(partly degraded)


Importance of Peatlands

Drösler und Freibauer, 2012

• Globally: storage of 10% of freshwater and 30% of land-based organic carbon

• 13% of land area in MV is peatland

• >90% of peatlands in MV are being drained → 25% of total emissions of the state

Lappalainen, 1996

MV (Mecklenburg-

Vorpommern)

Bog

Fen


Hydrology of the study site

Miegel et. al (2016)

• Extensively drained for centuries

• Dune dike as protection measure

from flooding from the sea

• Rewetted in December 2009

through closure with a groundsill

3 km


Tiepolt, July 2016

Study site Heiligensee und Hütelmoor


CH4 CO2 measurements

Frozen methane bubbles

Ks measurements

L. Gosch N. Geißler

What is happening on land?


Benthic chamber

H. Schade

Pore water sampling along the beach

J. Westphal

Core sampling to measure N20 and CH4

What is happening in the sea?


Objectives

Hypothesis: SGD varies spatially (geology) and temporally (event driven),

and interface is in a transient state (long-term)

→ Understand the driving forces for groundwater flow in the peatland

• Influence of drainage channels, heterogeneity of peat

→ Exchange processes at interface land-sea:

• SGD or saltwater intrusion (spatial and temporal distribution)

• Geology: Influence of (less permeable) peat layer and varying thickness of aquifer

→ Base for later studies on quantification of nutrient load


Groundwater flow

• Water level readings

• Drilling for geological characterisation

• Determination of K with slug test and grain size analysis

• Density-dependent transient GW flow model

Groundwater quality:

• Electrical conductivity readings

• GW age dating: 3H/He method

• GW chemistry: major ions

Overview Methods

Well installation

Slug test „Boreas“ drilling platform


Drilling and installation of observation wells

• 17 wells (3 in ditches), 1 to 6 m deep, in winter 16/17

• Loggers measure EC and water level (pressure) every 15 minutes

1 m


Profile Hütelmoor: Filter depths and K-values

Kr from Slug-Tests in m/sec

1.9E-05 1.8E-05

2.3E-05

1.5E-05 peat

aquifer

till


Water levels across Forest – Hütelmoor – Beach

Baltic

Forest

2LS dune

Peatland:7LS 7P 6LS


Groundwater ages (J. Sültenfuß)

95% older than 1950

2/3 older than 1950 Infiltration 2010

1/4 older than 1950

30 years


Electrical conductivity distribution

~0.6 ~ 8

~9.5 ~8

~7

5.0

6.4

~1.3

3-8

~12

Front of Heiligensee

EC in mS/cm

EC Baltic: ~20


Profile along beach


Salinity in permanent pore water lances (J. Westphal, IOW)

43 61


3-D conceptual model

Peat 1E-07

Based on 168 drillings from

literature and own work:

• 33 deeper drillings

• 71 peat cores

• 64 peat probings

Steady state simulation with

Visual Modflow Flex

K [m/s]


Testing the influence of drainage channels: Boundary conditions

Fo

rest

BC

Observation heads


Testing the influence of drainage channels: Head contour lines [m]

No ditches With ditches

2.5

m

2.5

m

Connectivity of ditches with aquifer?


• Drainage channels influence the groundwater

flow regime and the resulting hydraulic gradient.

• The hydraulic gradient further depends on dry

or wet periods, stormy winters with periodically

higher sea levels and the interaction of these

states.

• Electrical conductivity measurements show

heterogeneous distribution:

• Interface: complex mixing processes

due to geology and dune dike width.

• Peatland: peat structure and ditches.

Summary


Outlook

• Calibration of the model (steady state and later transient) within the peatland

• Connectivity of ditches with the aquifer

• GW flow in the peat: problem of heterogeneity (K peat)

• Extend the model to the seaside and include density transport (SEAWAT)

• Calibration of density-transport crucial: Transient interface due to drainage

• Scenario simulation: long-term drainage, rewetting, sea-level rise...

• Model helps to determine which factors lead to

SGD / saltwater intrusion

• Quantify amount of fresh and re-circulating SGD

Thank you


Geoelectric profiles

E. Erkul University Kiel

M. Gräber GeoServe


• Anthropogenic interferences as drainage channels influence the groundwater flow

regime and the resulting hydraulic gradient. The hydraulic gradient further depends on

dry or wet periods, stormy winters with periodically higher sea levels and the

interaction of these states.

• Electrical conductivity measurements show heterogeneous distribution: complex

mixing processes at the interface due to geology and dune dike width.

• The groundwater flow model will be further extended to the seaside to consider the

outcropping peat layer. Density-dependent flow will be implemented to simulate mixing

processes at the interface.

• The groundwater flow model will help to understand:

• Which factors (varying extend of geological layers at the interface, hydraulic gradient)

drive submarine groundwater discharge

• Hypothesis of a transient interface: How was the fresh- saltwater interface influenced

by long-term drainage (e.g. reversed hydraulic gradient landwards, hence saltwater

intrusion), how fast is it changing under rewetted conditions (higher hydraulic gradient

seaward)?


Comparison of simulated and observed heads

• Linear interpolation of BC Forest

• Nearby ditches not considered

• Density not yet considered

• Homogeneous K within the aquifer

Missing:

Water level measurements in main ditches

Discharge of the area


Baltic Sea

Water depth and salinity distribution

Ulf Gräwe, IOW


Importance of Submarine Groundwater Discharge

• Lost freshwater resource

• Nutrient and contaminant transport


Groundwater ages Hütelmoor

>1950

55 years average

2/3 >1950 Infiltration 2010

1/4 >1950

30 years


Field work

Outcropping peat

Well installation

„Boreas“ drilling platform

Slug test

marine and terrestrial influence on submarine groundwater ... · 2014-09-04, hier ggf....

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