Groundwater recharge interface and
nitrate discharge:
Central Canterbury, New Zealand
Vince Bidwell
Lincoln Environmental Research
Lincoln Ventures Ltd
Central Canterbury
40 km
Central Canterbury
C. R. Hanson (Environment Canterbury, 2002)
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1.0 - 2.9
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Me dian nitrate nitrogen
concentration (mg/L)
•
•
,
Lake
Ellesmere
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o
10
20 Kilometers
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0
Nitrate-N mg/L
5
10
15
20
40
MAV = 11.3 mg/L
Depth below
water table (m)
60
80
100
From ~ 600 wells in
Central Canterbury
(adapted from Burden, 1984)
Sample of groundwater nitrate-N concentration for
Central Canterbury (M. Stewart, 2003)
Nitrate-N (mg/L)
0
5
10
15
0
Depth below water table (m)
20
40
60
80
100
120
140
MAV = 11.3 mg/L
160
180
200
Groundwater age derived from tritium data
for Central Canterbury (M. Stewart, 2003)
Groundwater age (y)
0
Depth below water table (m)
0
40
80
120
160
200
10
20
30
40
50
60
70
80
Groundwater recharge interface
• Observed variation of nitrate concentration with depth
is due to a:
• mixing interface between groundwaters from
contaminated land surface recharge and from
uncontaminated river recharge (Hypothesis 1)
or
• descending “cloud” of contamination from many
years of agricultural land use (Hypothesis 2)
or both
Relevance
• Existence of a stable interface between groundwaters
from contaminated and uncontaminated recharge
implies continued access to high quality groundwater
for human consumption
• Pumped abstraction of this high quality groundwater
is likely to cause increased nitrate contamination of
groundwater discharge to surface waters
Method
1.
2.
3.
4.
Identify principal sources of groundwater recharge
by means of a steady-state groundwater flow
model and separation of piezometric effects
Model vertical distribution of groundwater flow by
stream function analysis
Assign nitrate concentration values (from literature)
to recharge sources
Model contaminant transport in groundwater with a
mixing-cell model
Principal sources of groundwater recharge
Waimakariri River
mean flow = 120 m3/s
River recharge
unobservable
Land surface recharge
200 mm/y (water bal.)
13 m3/s from 2020 km2
Rakaia River
mean flow = 210 m3/s
Separating the recharge effects on groundwater levels
300
Observed groundwater level
Predicted LSR effect
River recharge effect
Land surface recharge (LSR)
50
250
200
150
45
100
50
40
1972
0
1976
1980
1984
1988
1992
1996
2000
Land surface recharge (mm/mth)
GWL (m) above mean sea level
55
The separation of recharge effects was applied to
12 observation well records from Central Canterbury
L35/0163
L35/0163
M35/1080
M35/1080
M35/1878
M35/1878
L36/0063
L36/0063
L36/0092
L36/0092
M36/0255
M36/0255
L36/0023
L36/0023
L36/0124
L36/0124
M36/0592
M36/0592
M36/0424
L36/0258
M36/0424
L36/0258
M37/0010
M37/0010
The results show the dominant piezometric effect
of river recharge, relative to sea level.
100
Land surface recharge
River recharge
80
60
40
20
59
2
M
36
/0
01
0
M
37
/0
87
8
M
35
/1
42
4
36
M
/0
36
Observation wells
/0
25
5
8
M
/0
L3
6
/1
35
25
08
0
2
M
/0
09
3
L3
6
/0
06
4
L3
6
L3
6
/0
12
3
02
/0
L3
6
/0
16
3
0
L3
5
Piezometric effect (m) amsl
120
A 4-parameter, steady-state, groundwater flow model
was calibrated for prediction of river recharge effect…
100
Observed
Predicted
80
60
40
20
Observation wells
59
2
M
36
/0
01
0
M
37
/0
87
8
M
35
/1
42
4
/0
36
M
M
36
/0
25
5
58
/0
2
L3
6
M
35
/1
08
0
92
/0
0
L3
6
24
/0
1
L3
6
63
/0
0
L3
6
23
/0
0
L3
6
/0
1
63
0
L3
5
River piezometric effect (m) amsl
120
Observation wells
M
M
36
37
35
36
36
59
2
/0
01
0
/0
87
8
/1
42
4
/0
25
5
/0
8
25
80
M
M
M
2
08
0
/1
/0
35
L3
6
M
4
09
/0
L3
6
12
/0
L3
6
3
3
06
/0
L3
6
3
02
/0
L3
6
16
/0
L3
5
Total piezometric effect (m) amsl
… and total recharge effect.
120
100
Observed
Predicted
60
40
20
0
Groundwater model provides flow paths and recharge
inputs for analysis of vertical flow distribution
Groundwater flow path
Stream function analysis of vertical flow distribution along
the example flow path shows the pattern of recharge
contributions
Groundwater discharge
to streams and lake
Rakaia River
From land surface recharge
~ 300 m
From river recharge
50 km
Recharge and nitrate discharge for each land use
along the flow path are used as inputs to a 2-D vertical
groundwater transport model.
Combines stream functions and dispersive mixing.
Recharge (mm/y)
Nitrate-N (g/m3)
1m3/s/km
0
Dairy
250
12
Forest
100
1
Sheep
150
3
Crops
200
15
Land use
Rakaia River
Nitrate distribution for an example of typical land use
Rakaia River recharge
Discharge to surface waters
Nitrate-N = 2.9 mg/L
Sheep
Dairy
Forest
Sheep
Crops
300 m
50 km
0.00-1.00
5.00-6.00
10.00-11.00
1.00-2.00
6.00-7.00
11.00-12.00
2.00-3.00
7.00-8.00
12.00-13.00
3.00-4.00
8.00-9.00
13.00-14.00
Nitrate-N concentration (mg/L)
4.00-5.00
9.00-10.00
14.00-15.00
Nitrate v depth, from sampling of model results
at 5 km intervals along the flow path
Nitrate-N (mg/L)
0
5
10
15
0
Depth below water table (m)
20
40
60
80
100
120
140
MAV = 11.3 mg/L
160
180
200
Comparison of model example with Stewart (2003)
Nitrate-N (mg/L)
0
5
10
15
0
Depth below water table (m)
20
40
60
80
100
120
140
160
180
200
Model
Stewart (2003)
MAV = 11.3 mg/L
Conclusions
• It is likely that the variation of nitrate concentration
with depth in the Central Canterbury aquifers is due
to the presence of a dispersive mixing interface
between groundwaters from river recharge and from
land surface recharge
• This interface, although stable, may be in a transient
position
• The contribution of uncontaminated river recharge is
important for maintaining access to high quality
drinking water and for reducing the contamination of
groundwater discharge to surface waters