Predicting nitrate levels for Wessex Water`s long term water resource

Transcription

Predicting nitrate levels for Wessex Water's
long term water resource planning
Nick Rukin, Associate Director, Entec UK Ltd
Luke de Vial, Head of Water Resources, Wessex Water
UK Groundwater Forum Conference - 13 May 2008
Background
z Water quality can limit available water resources as
much as recharge, environmental requirements etc
z Nitrate is biggest water quality issue affecting
Wessex Water
z To ensure security of supply, Wessex Water need to:
– provide treatment or blending capacity
– reduce nitrate leaching through catchment management
z As part of AMP5 submissions, DWI are expecting
'twin track' approach to the nitrate problem of:
– preferably blending for short to mid term
– catchment management for mid to long term
z Nitrate treatment is DWI's least preferred option
(24) Sources at risk of exceeding
nitrate limits by 2020
Key
Key
Sources at risk from exceeding nitrate limits
Water supply and sewerage services area
Sewerage services only
Water supply services only
Nitrate Treatment
z Reliable solution
z Costly to build and
operate
z Waste disposal difficult
z Long term energy /
carbon footprint issues
z Wessex Water
– have built 4 so far
– average cost £4M
– possible 1 per year for
foreseeable future
Catchment Management
z Wessex Water have been working with farmers over
the last four years in a number of trial catchments
z Collection of samples and farmer liaison
z Dedicated Wessex Water catchment advisors meet
farmers each week / month
z Catchments are designated (2002) Nitrate Vulnerable
Zones, but Environment Agency have budget for
one visit every six years
z Early results look promising but ………..
……..how long until rising nitrate trends can be
reversed in the abstractions?
Typical Trend
12
Drinking Water Standard
10
?
Short Term
Spike
Nitrate Concentration (mg/l N)
Short Term Spikes related to
bypass recharge events
?
?
?
?
8
?
6
4
Seasonal Variation related to
change in water level - probably
due to changing the age range of
water containing historically
leached nitrate at the water table
Seasonal
Variation
2
0
1960
Long Term Trend controlled by
mixture of historically leached
nitrate which has travelled
through the unsaturated zone
Long Term
Trend
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
The Big Question
Can the observed trends be simulated to gain confidence in
controlling factors and forward predictions?
Model Development
z Constrain Historically Leached Nitrate
z Constrain Travel Times from Soil to Abstraction
z Factor in Seasonality and 'Spikiness'
Best Estimate of Historically Leached
Nitrate
1900
15
1920
1940
1960
1980
Concentration (mg/l N)
Managed
Grassland
- historical fertiliser use
- historical land use changes
- historical stocking densities
- observed nitrate in observation boreholes
- optimised model fit for NO3 at the PWS
10
Arable
5
5
0
0
100
80
60
40
Years before 2007
20
Concentration (mg/l N)
15
Derived by Considering:
10
2000
0
Note: Concentration in
440 mm/yr recharge
Unsaturated Zone Travel Times - Theory
Historically Leached Nitrate
Thick Unsaturated Zone Travel Time = 30 years
Thin Unsaturated Zone Travel Time = 10 years
Recharge Water
with Nitrate from
30 years ago
Recharge Water
with Nitrate from
10 years ago
Unsat Travel Time = Moisture Content x Unsat Thickness
Recharge
Moisture Content Data from Cores
Moisture Content (%)
0
5
Depth to Top of Sampled Interval (m bgl)
0
1
2
3
4
5
6
7
8
9
10
Lower moisture content in
top 1-2m - weathered Chalk
or upwards movement to
satisfy SMD
10
15
20
25
30
35
40
Tritium Profiles also used to constrain
moisture content
Tritium (T.U.)
0
100
200
300
400
500
600
Depth to Top of Sampled Interval (m bgl)
0
5
Tritium peak due
to 1963-64
infiltration
10
2.53m move in a period with 773 mm
infiltration recharge for this grid cell, so
~0.773/2.53 = 30.6% Matrix Moisture
15
Data provided by John Chilton of BGS, as
published in Geake & Foster, 1989)
OG07 (Lower Valley, SU 0000 1200, Sept 1970)
OG08A (Upper Valley, SU 0052 1198, July 1977)
20
OG09A (Upper Valley, SU 0052 1195, July 1979)
Infiltration Recharge
Less
Recharge
More
Recharge
Output from the 4R
South Wessex
Recharge Model
Unsat Zone Thickness & Travel Time
Less
Recharge
Long
Thick
More
Recharge
Short
Thin
(9) Age of Water arriving at Chalk Water Table in Friar Waddon Catchment
8%
120%
Excel processed GIS
outputs of travel times for
different land uses in a
selected catchment area
6%
5%
100%
80%
4%
60%
3%
40%
2%
20%
1%
0%
0%
0.4
3.4
6.4
9.4
12.4
15.4
18.4
21.4
24.4
27.4
30.4
33.4
36.4
39.4
42.4
45.4
48.4
51.4
54.4
57.4
60.4
63.4
66.4
69.4
72.4 >75.4
Age of Water (Categories are 9 months long)
Arable
Grassland
Semi-Natural Vegetation
Urban
Woodland & Forestry
Cumulative
Cumulative Proportion of Water <= Age
% Water of Given Age in Catchment Recharge
7%
Seasonal Variations in NO3 Related to
Water Level Variation
12
160
10
140
NO3
at
PWS
8
120
6
100
Water
Level
at
OBH
4
2
80
60
Site Measured Nitrate
0
1990
Woodyates OBH Water Level
1995
2000
2005
40
2010
Water Level at Woodyates O BH (m AO D)
Nitrate Variations at Eagle Lodge and Groundwater Levels
Spikes – Bypass Recharge is a plausible
explanation
Infiltration recharge drives
slow movement of
historically leached NO3
Bypass recharge sends
through spikes of this
months (?) leaching
Slow plug flow
through the
Chalk matrix
Rapid flow
through Chalk
fissures
Nitrate Predictions
Nitrate concentration at time t =
z Function of historically leached nitrate from
appropriate land use, infiltration recharge and delay
in unsaturated zone
+
z Function of groundwater level
+
z Function of bypass recharge
Nitrate Predictions – Long Term Trend
Nitrate level at time t =
z Function of historically leached nitrate from
appropriate land use, infiltration recharge and delay in
unsaturated zone
+
+
Model Calibration Step 1 –
Long Term Trend
Eagle Lodge Calibration - Step 1 - Long Term Trend
12
N itra te C o n c e n tra tio n (m g /l N )
10
8
6
4
2
0
1960
Min Long Term Trend (LTT) based on Legacy Nitrate
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
Nitrate Predictions – Adding Seasonality
z Function of historic application of nitrate / thickness
of the unsaturated zone
+
+
Seasonal Variations Link to Water
Levels
Eagle Lodge Calibration Step 2 - Seasonal Variations Link to Water Levels
12
10
8
6
4
2
0
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
Zoom in and Add Delay
Eagle Lodge Calibration Step 3 - Zoom In and Add Delay
12
10
8
6
4
2
0
1990
1995
2000
2005
2010
Nitrate Predictions – adding the spikes
z Function of historic application of nitrate / thickness
of the unsaturated zone
+
+
z Function of bypass recharge (4R Model Output)
Add Bypass Recharge Effect
Eagle Lodge Calibration - Step 4 Add Bypass Flow Effect
12
10
8
6
4
2
0
1990
1995
2000
2005
2010
Good Calibration with a Number of
Sites
Friar Waddon at Step 4 Calibration
12
10
8
6
4
2
0
1990
1995
2000
2005
2010
Poor for Upper Greensand Source
Dunkerton at Step 4
12
10
8
6
4
Greensand Source so different unsat zone
porosity and likely delayed flow and
possible stratification in saturated zone
Note: No bypass flow predicted by
recharge model
2
0
1990
1995
2000
2005
2010
But Generally Good for Chalk
Sources
Chirton at Step 4
12
10
8
6
4
2
0
1990
1995
2000
2005
2010
Modelled Scenarios
z With good model fit future leaching scenarios can be
assessed:
– Scenario 1 – nitrate leaching at 2006/7 rates
– Scenario 2 – nitrate leaching reduced to zero*
– Scenario 3 – nitrate leaching reduced to 50% of 2006/7
rates
(Note: *zero leaching is unrealistic for farmland but
demonstrates the best that could be achieved)
(Forward predictions replicate water levels and bypass
recharge data from period 1992-2006 twice)
Winterbourne Abbas Scenario 1
Forward Prediction of Nitrate at PWS
Future leaching at 100% of 2006/7 rates
12
10
8
Forward prediction based on
repeating water levels and rainfall
from 1992 to 2006
6
4
2
Min Long Term Trend (LTT) based on Legacy Nitrate (Min) and Unsat Zone Delay
Predicted Nitrate using Function of Woodyates Monthly Water Level and LTT - plus lag
Predicted Nitrate using Function of Rainfall, Ashton Farm Water Level and LTT - plus lag
0
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
12
10
8
from 1992 to 2006
6
4
2
0
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
12
10
8
from 1992 to 2006
6
4
2
0
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
Eagle Lodge Scenario 1
12
10
8
from 1992 to 2006
6
4
2
0
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
Eagle Lodge Scenario 3
12
10
8
from 1992 to 2006
6
4
2
0
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
Eagle Lodge Scenario 2 Forward
Prediction of Nitrate at PWS
12
10
8
from 1992 to 2006
6
4
2
0
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
Wylye Scenario 1
12
10
8
from 1992 to 2006
6
4
2
0
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
Wylye Scenario 3
12
10
8
from 1992 to 2006
6
4
2
0
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
Wylye Scenario 2
12
10
8
from 1992 to 2006
6
4
2
0
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
Lessons / Conclusions
z Historically leached nitrate difficult to constrain, but use of
one arable and one managed grassland trend has been
successfully applied to 10 Chalk catchments
z Leached concentrations are lower where recharge is
higher so would anticipate higher nitrate concentrations
moving more slowly further east on the Chalk
z Source Protection Zones do not typically match the likely
catchments – so may manage the wrong fields!
z Good model fit gives basis for making forward predictions
z Provides a tool to help decide how soon catchment
management could make a difference – supports AMP5
funding application
z Additional value of recharge model realised
Predicting nitrate levels for Wessex Water's
long term water resource planning
Nick Rukin, Associate Director, Entec UK Ltd
Luke de Vial, Head of Water Resources, Wessex Water
UK Groundwater Forum Conference - 13 May 2008
Conceptual Model For Nitrate Trend
Prediction
NO3 at borehole = Long term trend + seasonal trend + bypass spike
Inorganic Fertiliser Use (1952 – 2006)
Total N - Tillage & Grass
180
Total N - Tillage
Total N - Grass
160
140
120
100
80
60
40
20
Data fromFertiliser
British Survey
of Fertiliser
use increased
up to early 1980s –
Practice (DEFRA
cause Website)
of increased nitrate leaching ?
Cause of Long Term Trend?
0
1952
1954
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
kg/ha total nitrogen application rate, England & Wales
200
Observed NO3 = Delayed Leached
Fertiliser?
150
11
125
10
9
Note: * Predicted
Concentration in Recharge
8
7
6
5
100
Measured NO3 at PWS
Note: The Fertiliser Application Rate axis has
been scaled so that the data plotted as kg
N/ha can also be read as mg/l N on the
opposite scale assuming 40% arable leaching
and 500mm/yr total recharge.
75
4
50
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015
National Average Fertiliser Application Rate (kg N/ha
12
Predicted
NO from 40%
Monthly average NO3 3
Fertiliser leached in 500
National
Average Recharge
Fertiliser Application Rate
mm/yr

Predicting nitrate levels for Wessex Water`s long term water resource

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