20th SWIM
24. June 2008
Global land-ocean linkage:
direct inputs of water and associated nutrients
to coastal zones via submarine groundwater
discharge (SGD)
Hans Dürr(1), R. van Beek(1), C.P. Slomp(2), H. Middelkoop(1), M. Bierkens(1)
h.durr@geo.uu.nl
(1) Department of Physical Geography, Faculty of Geosciences, Utrecht University,
Utrecht, The Netherlands
(2) Department of Earth Sciences – Geochemistry, Faculty of Geosciences, Utrecht
University, Utrecht, The Netherlands
Effects of nutrient enrichment
(eutrophication) of coastal waters
more algal
blooms
red tide
(harmful algal bloom)
foam
Rationale
• Higher nutrient inputs (N, P) from rivers to coastal waters +
estuaries, inverse trend (decrease) due to damming for Si
• Global nutrient fluxes at the land-ocean interface: Terrestrial
controls, impact on coastal waters and response to global change.
Spatially explicit, multiple nutrient forms (dissolved / particulate,
inorganic / organic)
• Box modelling (Slomp & van Cappellen 2004) has shown potential
impact on coastal zone of nutrient SGD at global scale
• Typically: Groundwater N and P flux = total SGD flux x nutrient
concentration, but decisive for net balance: fresh GW flux x
nutrient concentration ( local scale effects of subterranean
estuary)
• Submarine Groundwater Discharge (SGD) of nutrients: spatially
explicit not yet estimated at global scale, comparison to river
inputs (water discharge globally ~5-10% of river discharge)?
Proximal
coastal ocean
N:P ~ 14:1
river input
eolian
input N
denitrification
groundwater
input
Phytoplankton (mostly N-limited)
Redfield N:P = 16:1
N:P mostly
>>> 16
recycling
export to
continental
shelf
burial
• Fresh SGD: mostly N, less P (elevated velocity seeps in shallow, oxic systems:
P removal, conservative behaviour of NO3), could drive systems to P-lim.
• Only in cases when both GW and sediment are anoxic and upon saltwater
intrusion: N/P ratio of SGD < Redfield
• Deep GW supply mostly minor & not contaminated
QChannel
PREC
Epot
PREC
Epot
Canopy
Eact
QDR
T
Snow cover
Store 1
P
QSf
Each cell:
- Conceptual model partly
River icebased on the HBV
model
RS
Climate
variables
are
Tw
from theTERA40
data set
A
- Transfer / Routing of
runoff toSnow/Rain
the drainage
network RS
LH(Epot)
TA
Store 2
For baseflow here:
Q
- Not fully calibrated yet
Snow/Rain
- Energy balance + lakes will be
considered in later stages
- Monthly values  annual means
QBf
P
Store 3
0,5° grid
QChannel
Courtesy R. van Beek
Model
structure
Concept of groundwater flow
• Kraaijenhof-Van de Leur (1958)
– Boussinesq – Dupuit assumptions to calculate the
reservoir coefficient kr
 2 kD
kr 
4 fB2
where: k is the hydraulic conductivity (L·t-1 e.g., m/day),
D is the aquifer thickness (L, 50m assumed), f is the
drainable porosity (obtained from lithology + some
Holdridge climate data) (dimensionless, 0-1)
and B is the aquifer width (L).
Using various global data sets
Courtesy R. van Beek
Result:
Residence time of Groundwater
Long residence times:
- Highly weathered soils
- Tropics
Model performance (no full calibration yet):
- Good in most temperate and tropical regions
- Weaker in Arctic + semi-arid regions: no evaporation from
surface waters yet, snowmelt driven by mean monthly temp.
Next step – full calibration on low flow conditions
Courtesy R. van Beek
Local groundwater discharge
High in:
Some tropical regions, esp. SE-Asia / Indonesia;
W Canada, Florida; Ex.: Patos Lagoon
Low:
Arctic, but % contribution to total flow is high
Courtesy R. van Beek
Next steps, including nutrients
• Coastal water divide where baseflow = SGD??
• Human GW extraction?
• Salt-water intrusion sites (low-lying areas)?!
• Range of GW nutrient concentrations for DIN ~NO3
• Test nutrient attenuation scenarios, e.g., based on half-life
approaches
• Coupling to information from landuse and population
Coastal ribbon definition for SGD:
distance to water divide at coast
Problem :
last
gauging
station
what is a stream ???
 every catchment
basin that has a
cumulated upstream
area > xx km2
subgrid variability
0,5° grid cell
 testing possible
Groundwater
abstraction
Water extraction:
- country based GW abstraction from IGRAC / TNO
- linked to total water use and population numbers
from Vörösmarty et al.
Courtesy Rens van Beek
(NO3)
Range of
GW nutrient
concentrations
in different
landuse
settings
Agricultural
Urban
Misc.
Undevel. land
USGS
Summary data
High Groundwater Nitrate sites
Data from IGRAC / TNO
Potential hotspots with
potentially elevated N input
Caution:
- Very local phenomena not detected
- Total Groundwater flow in coastal
cells, not SGD
- GW abstraction not considered
- Saltwater intrusion not considered
- Effect of residence time in GW
on nutrient concentrations
Some study sites for SGD including nutrients
Rivers
Similarities in regions, local differences
Groundwater
DIN-NEWS empirical model:
Dumont et al. 2005
Conclusions: first steps towards
• Spatially explicit estimates of SGD at global scale, using
baseflow estimates from new global hydrological model
• Potential hot spots of freshwater nutrient SGD
– SE Asia (esp. Indonesia) + Central America: hot spots for both
river and GW fluxes (high baseflow, high runoff and high levels
of human activity) – exact locations and time scales may be
different
• Field studies needed in high ‘risk’ areas, e.g. (sub-)tropical
areas of Africa, S America and SE Asia – some studies now
underway
• Examples have been shown to include amendments such
as GW abstraction, GW quality data, landuse + population
data
As source of ‘new’ nutrients,
especially N (less P), freshwater
SGD is potentially important for
coastal nutrient cycling
(N-limitation of most coastal
waters) at global scale (strong
response to human impact)
Thank you !
Questions ?
Hand-outs available, ask anytime or email:
h.durr@geo.uu.nl
Salt-water intrusion:
CaHCO3 freshwater is replaced by NaCl and SO42- rich sea water
Na+
Ca2+
Ca2+
Ca2+
Na+
Na+
Ca2+
Na+
-Na2+ replaces Ca2+, NH4+
-Cl- or SO42- replaces PO4
- Increased degradation of organic matter (Nyvang, 2003)
- Increased sulfate reduction & reduction of Fe-oxides by S2NH4 and PO4 release to the groundwater