Nitrogen transport and transformation at the
groundwater – surface water interface
How important is the Hyporheic Zone?
Stefan Krause
A.L. Heathwaite (2), A. Binley
(1)
(2)
(3)
(3),
(1)
D. Kaeser (3)
Earth Science and Geography Department, Keele University, UK
Centre for Sustainable Water Management, Lancaster, UK
Lancaster Environment Centre, Lancaster, UK
Rese arch Institute for the Environment, Physical Sciences & Applied Mathematics
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
Outline
• Introduction – Motivation
• Efficiency of nitrate attenuation at the groundwater – surface
water interface
• Transport and transformation of N in the Hyporheic Zone
- physical streambed controls on transport
- chemical controls on transformation
• Potential implications of HZ nitrogen cycling - upscaling
strategies
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
1
Motivation – The Nitrate Time Bomb?
Nitrate Concentrations in 40 Cumbrian GW- Boreholes
1972 - 2007
Nitrate Directive
WHO 10.3 mg l -1
Why are GW Nitrate concentrations still incr easing in man y aquifer s? !
Results of diffuse inputs and long r esidence times - The Nitr ate Tim e Bomb?
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
Motivation – Riparian Attenuation?
(Potential) impact of riparian nitrate attenuation
ammonificatio n,
nitrification
N-fixation,
mineralisation
Denitrification?!
Background:
Expectations for potential Nitrate attenuation in riparian
groundwaters
20 yrs of r esear ch:
What is the riparian nitrate retention capacity? How much
amelioration can be achieved?
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
2
Riparian Controls on Nutrient Delivery
Nitrate retention within riparian groundwaters
Model: IWAN: Coupled Groundwater - Surface Water Model
Rathenow
Berlin
Riparian Attenuation:
Havel - River
Brandenburg
Potsdam
(Krause and Bronstert, 2007, Krause et al. 2007)
Stefan Krause
- up to 40 % NO3 redu ction
in rip arian GW
- baseflo w contributions up
to 20% (from 1% of the
cat chment !)
Hydroecology meets Hydrogeology
BGS 2008
Motivation – The Hyporheic Panacea?
Hyporheic Transport and Transformation of Nutrients
dissolution
anaerobic
fine grain
upwelling,
sizes
infiltration of solutes
mineralisation,
binding
river
transformation,
degradation
aerobic
coarse s ediment
groundwater
The Hyporheic Zone (H Z) - reactiv e area of GW-SW mixing (SW> 10% ) with strong redox gradients
and poten tial for nutrient transformatio n in depende nce of:
- pattern of transmissiv ities and fluxes (contact and residence times)
- pattern of redox co nditio ns, nutrient co ncentrations and av ailability o f
reductiv e agents (FeS 2 , Corg)
Previous Studies of HZ processes (usually accounting for surface water
infiltrating and exfiltrating into/from the streambed) derived evidence for HZ
potential to moderately change N transported at GW-SW interface
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
3
Motivation – The Hyporheic Panacea?
Hyporheic Transport and Transformation of Nutrients
ammonificatio n,
nitrification
N-fixation,
mineralisation
Denitrification?!
GW-SW interface potentially c ontrols transport a n transformatio n of ni trogen
How much of the riparian nitrate finally reaches the riv er?
Specific hyporheic process d ynamics are not taken in to account i n modelling a pproaches
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
HZ Nitrogen Cycling – the GW Perspective
Investigation of the dynamic controls of physical streambed
conditions on hyporheic exchange fluxes and redox chemistry
The Leith field
site :
• N - Cumbria
• T ributary of the
River Eden
• Gaining section
in riparian
floodplain (222 Rn)
• Baseflow
conditions
(May to
October )
• GW & SW < 10
mg l -1 NO3 -N
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
4
HZ Nitrogen Cycling – the GW Perspective
Experimental Setup - River Leith
19
G
F
24
20
23
F
G
F
G
18
G
F
17
F
G
F
G
16
F
G
21
8
G
F
F
G
st rea mb ed eleva tio n ( m)
31
Valu e
G
F
32
F
G
25
30 G
F
F
G
14
F
G
110.32
9
F
G
11
F
G
F
G
²
Site B
26
7
G
F
15
10
F
G
F
G
29
27 G
F
0 0 .5 1
2
3
28 G
F
5
G 4
F
F 1
G
F
G
4
Meters
6
G
F
13
G
F
core, sampling point,
piezometer
22
F
G
F
G
12
F
G
109.49
Site A
2
G 3
F
F
G
Two sites - ph ysical and chemical properties:
- 42 cores:
- porosity, compaction, grain size distribution, hydr. cond.
- cores: NO3 , NH4 , T N/T ON concentrations (foc, CEC (J. Smith))
- ca. 140 piezometer + gw boreholes in riparian floodplain:
- hydraulic gradients, slug tests, temperature and radon survey
- NO3 , NH4 , T N/T ON, DO, ORP, ph (fortnightly)
- DET probes (sub cm scale)
- tracer injection: salt, rhodamine – flow velocities, residence times
Stefan Krause
cm
55 c m
5
5
DDeept
h( cm)
p ht (c m)
550
0 cc m
m
50 cm
M
embb ananrr ee FilF
il erertt
M em
D ept
h (m
c )
De p ht (c m)
rFr o
F
non tWi
tWinn d
odo w
wP
lPl teteaa
15c m
0
-5
-1 0
-1 5
-2 0
-2 5
-3 0
-2 5
-3 0
-3 5
-4 0
-3 5
-4 0
0 .0
1
cm
660
0 cm
cm
60 cm
D if
Di
f u
siusi eevv GGelel LaL
a ereryy
M embran e iF tl er
Dif fu s vi e Gel a
L yer
0
-5
-1 0
-1 5
-2 0
1. 0
2. 0
3. 0
F e (mg /l
Fe
( mg/ )l
0
0. 1
0. 2
0. 3
MMnn(( mg
m/ g/l)
Hydroecology meets Hydrogeology
BGS
Intr od
2008
ucti on
HZ Nitrogen Cycling – the GW Perspective
Identification of sources with different chem. background
W
E
?
NH4 high
SW
GW II
HZ
NO3 low
Nitrate
10
NO3 low
GW I
Ammoniu m
Dissolved Oxygen
2.0
20
18
16
1.5
4
1.0
12
10
8
6
0.5
2
14
-1
6
DO (mg l )
-1
N H4- N ( mg l )
-1
NO3 -N (mg l )
8
4
2
0
0.0
SW
Stefan Krause
HZ
GW
0
SW
HZ
GW
Hydroecology meets Hydrogeology
SW
HZ
GW
BGS 2008
5
Con c. NO3 - N ( mg l
-1
)
HZ impact on porewater N along upwelling pathway
grit, gravel
6.09E-04
ms-1
cs, gravel
org. mat.,
ms
Con c. NO3 - N ( mg l
-1
)
3.21E-03 ms-1
cs, gravel
grit,
red sst, ms
Leith B
(avg. sediment 50 cm)
NO 3-N concentrations
in piezometers
Co nc. NO3 - N (mg l
-1
)
8.24E-04 ms-1
10
shal low piezometers
8
6
4
2
0
ca. 20 cm
10
intermedi ate pi ezometers
8
6
4
2
0
ca. 50 cm
10
deep p iezometers
4
18/08/ 06
13/09/ 06
02/10/ 06
10/10/ 06
23/10/ 06
2
pt-sst, ca. 100 cm
0
p ools
8
6
lb20
Stefan Krause
riffles
l b27
l b28
lb29
lb21
veg. b an k E
lb22
l b25
lb26
lb24
lb31
veg. bank W
l b23
l b30
Piezometer location
Hydroecology meets Hydrogeology
BGS 2008
Hyporheic Zone – Spatial Extend of Nitrogen Cycling
HZ significantly impacts on nitrate delivery
Highly redox reactive
area
(efficient
transformations/turnover
rates)
exceeds the area usually
understood as HZ
HZ:
rather static +
arbitrary definition of
SW-GW mixing with ≥
10% SW
Fig.
Stefan Krause
Alteration of Nitrate concen trations in the
upwelling GW of 30 streambed piezometers
Hydroecology meets Hydrogeology
BGS 2008
6
Hyporheic Zone – Revising static concepts
Episodic Mixin g
Pe rma nent
Mix ing
Surfa ce
Water
W ater
Revised Concept of Hyporheic Zone Impacts on Nutrient Cycling
RIVER
diss olution
fine grain
sizes
anaerobic
episodic
mixing
aerobic
transfor mation
degr adation
coarse s edime nt
Low DO, Corg rich
DO r ich, low Co rg
episodic mixing
Denitrification
episodic
mixing
Nitrific ation
Ground
G rou nd
Water
W ater
upwelling,
infiltration of solutes
upwelling,
infiltration of solutes
GROUNDWATER
BEDROCK
Considering episodic mixing as sufficient source for organic matter and dissolv ed oxygen
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
Hyporheic Nitrogen Cycling - Impact of Mixing
12
E
20
19
18
17
16
15
14
13
52 4 36 6
52 4 36 4
12
11
10
9
8
7
6
5
4
3
2
1
0
52 4 36 2
52 4 36 0
Transect I
52 4 35 8
52 4 35 6
52 4 35 4
Transect II
W
3 58 76 0 3 58 7 62
35 87 64
35 87 6 6 3 58 76 8
hydr aulic gra dient (-)
hyd. grad.
52 4 36 8
52 4 35 2
Transec t I
10
8
6
4
2
0
West
Ce ntr al
20
Eas t
Transec t II
18
hydra ulic gradient (-)
52 4 37 0
16
14
12
10
8
6
4
2
Hydraulic gradient in the superficial bed
sediments at 18/08/2 006
Stefan Krause
0
West
Hydroecology meets Hydrogeology
Central
East
BGS 2008
7
Hyporheic Nitrogen Cycling - Impact of Mixing
E
52 4 37 0
52 437 0
E
hyd. grad.
52 4 36 8
52 436 8
NO3 - N (mg/ l)
20
19
18
17
16
15
14
13
52 4 36 6
52 4 36 4
52 436 6
52 436 4
12
11
10
9
8
7
6
5
4
3
2
1
0
52 4 36 2
52 4 36 0
Transect I
52 4 35 8
52 4 35 6
52 4 35 4
7.5
7
6.5
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
52 436 2
52 436 0
52 435 8
52 435 6
52 435 4
Transect II
52 4 35 2
10
9.5
9
8.5
8
52 435 2
W
W
3 58 76 0 3 58 7 62
35 87 64
35 87 6 6 3 58 76 8
3 58 760 3 587 62 35 876 4 358 76 6 3 58 768
NO 3 – N concentrations 0.5 – 1.0 m
Hydraulic gradient in the superficial bed
sediments at 18/08/2 006
Stefan Krause
18/08/20 06
Hydroecology meets Hydrogeology
BGS 2008
Hyporheic Nitrogen Cycling - Redox Conditions
Nitrate concentration
change along the
upwelling gro und water
passage:
High foc:
upwards decrease
3 .5
3 .0
up ward s de crease
upw ards in cre ase
foc (%)
2 .5
2 .0
Low foc :
R2 = -0.77
1 .5
1 .0
upwards increase or no
concentration chan ges
p iezome te rs
l ine ar fit
0 .5
0 .0
-4
Stefan Krause
-3
-2
-1
0-1
chan ge C NO 3-N (mg l )
1
2
Hydroecology meets Hydrogeology
BGS 2008
8
Hyporheic Nitrogen Cycling - Retention Time
NO 3 c ha ng e on hz p as sag e
( mg/ l NO 3- N)
a vg. re sid e nce time s ( hr s)
38
5
4 .5
5 24 37 0
5 243 68
4
3 .5
3
5 24 36 8
5 243 66
2 .5
2
5 24 36 6
5 243 64
1 .5
1
0 .5
5 24 36 4
5 243 62
0
-0 .5
5 24 36 2
18
5 243 60
-1
-1 .5
-2
5 24 36 0
16
14
5 243 58
-2 .5
-3
5 24 35 8
5 243 56
-3 .5
-4
-4 .5
5 24 35 6
36
34
32
30
28
26
24
22
20
12
10
8
6
4
2
-5
5 243 54
0
5 24 35 4
5 243 52
358 764
35 876 6 3 587 68
HZ residence time vs.  NO3
6
in upwelling groundwater
2
0
-2
-4
-6
-8
5 24 35 2
35 876 0 35 87 62
8
4
 N O3 -N (mg l-1)
5 243 70
3 587 60
358 76 2 3 587 64
Tresd = Ltopsed k -1
358 766
0
35 87 68
50
100
150
time (hrs)
200
250
Ltopsed - coring, geoph. exploration
k
- derived by slug tests
Longer residence times increase the efficiency of redox reactions – for both nitrification / de nitrification
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
Streambed Geomorphology - Parameterisation Proxy
Geomorphic controls on transport and transformation
Leith B
central riffle – interpolated NO3 co nc., hydr. head gradients
C SW = 5.0 mg l-1 NO3-N
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
9
Hyporheic Connectivity
The Principle of Hyporheic Connectivity
Chemical connectivity,
Redox reactivity
Physical connectivity,
Riverbed transmissivity
HYPORHEIC
CONNECT IVIT Y
=
+
High
moderate
low
controlling
controlling:
controlling:
- Exchange flow rates
- Mixing intensities
- Pathw ays and
residence, reaction
times
-Redox environment
-Transformation types
(Nitrification/Denitrification)
- Reaction rates
- Efficiency of transport,
exchange and
transformation rates
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
Seasonally Variable HZ Implications
Temporally variable implications of nitrate contributions
from hyporheic groundwater
8
7
C NO3-N (mg l-1)
D
6
5
4
M
S
SW Leith upstream
SW Leith midstream
SW Leith downstream
GW deep
GW intermediate
GW shall ow
D
M
D
D
M
S
M
D
S
M
D
S
M
S
D
3
2
1
7/
25/ 0
S D
M
S
D
M
D
S
M
S
M
S
S
D
M
S
D
M
D
M
sample period 07/2006 - 08/2007
0/0260075/20075/ 2007 6/ 20077/20078/20078/ 20079/ 2007 0/20070/2007
24
2006 08/2006 08/ 2006 09/2006 09/ 2006 10/2006 10/ 2006 10/0
31/ 18/ 09/0 30/0 20/ 0 11/ 0 01/0 22/0 12/ 0 03/ 1 24/ 1
17/
03/
19/
05/
22/
08/
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
10
Spontaneous Changes of HZ Conditions
-1
NO 3-N (mg l )
7
6
5
4
3
2
1
0
180806
130906
021006
101006
231006
020507
dat e
170507
170707
010807
150807
10
9
171007
301007
s tre amb ed 4 0 - 60 cm
-1
NO 3- N (m g l )
8
7
6
5
4
3
2
1
0
180806
130906
021006
101006
231006
020507
d ate
170507
170707
010807
150807
171007
301007
10
str eam bed 80 - 1 00 cm
9
8
3
Concentration
alteration
disappears
st ream bed 15 - 30 cm
8
-1
Impact of
sediment
restructuring
after storm
events
10
9
NO -N (mg l )
NO3 conc.
changes
along
upwelling HZ
flowpath
(due to
denitrification,
nitrification)
7
6
5
4
3
2
1
0
180806
130906
021006
101006
231006
020507
170507
170707
010807
150807
171007
301007
da te
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
Future Challenges - Model Upscaling
plot scale
local scale
hz
hz
cross section
stream reach
regional scale
hz
catchment
Knowledge
Impact ?!
How does the impact of hyporheic connectivity controls on exchange fluxes
and nutrient amelioration change with scale?
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
Hyporhei
c Zone
11
Conclusions
i.
The h yporheic flo w path can have a significant impact on the GW nitrate
concentrations (attenuation + enrichment) – effect can be lost du e to HZ
disturbance
ii.
Pattern of groundwater n itrate transform ations and contributions to surface
water are controlled b y H yporheic Connectivity:
a)
Ph ysical Riverbed Connectivit y:
•
Mixing (sources of different chem. signature)
•
Residen ce/r eaction time (ac tive/non- active areas)
•
Flow path ways (ex pos ure to redox reactive z ones)
b) Chemical Reactivit y:
•
Redox environment
Annamox….)
•
Reaction efficien cy
(Reacti on ty pe (Nitrification / Denitrification
/
iii. River(basin) manag ement requires assessm ent of HZ impact on at least subcatchment scale - Model based upscaling of exp erim ental small scale kno wledge
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
Special Thanks to:
P. Keenan (CSWM); J. W. Smith (EA); M. Schaerer (FOEN); J. Cox (CSIRO), S.
Skradde (UP); H. Rubilar (UP); R. Rees (LU); J. Keery (CEH); M. Morgner (UP); A.
Bauer (UP);
THANK YOU !
Stefan Krause
Hydroecology meets Hydrogeology
BGS 2008
12