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69th SWCS International Annual Conference July 27-30, 2014 Lombard, IL
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Monitoring and Modeling Nitrate Fate in Subbasins with the Choptank River Watershed,
Maryland, USA
Greg McCartyUSDA ARS
1
2
3
4
7
8 9
1011
13146
5
15
Metolachlor Fate
N
O
OS
O
OH
O
MESA {2‐[(2‐ethyl‐6‐methylphenyl) (2‐methoxy‐1‐methylethyl)amino]‐2‐oxoethanesulfonic acid} is a metabolite of metolachlor, which is a widely‐used pre‐emergent herbicide
Glutathione conjugation is the common detoxification method for metolachlor in plants and its microbial degradation pathway in soil unsaturated zones
MESA is like nitrate‐N very water soluble, 2.12*105 g/L low sorption coefficient (calculated log Koc =
1.13) classified as highly mobile
MESA is also very stable; its half‐life for all MESA processing has been estimated at 100 – 200 days
Metolachlor
MESA
Co‐movement of nitrate and MESA in Croplands
Drainage Condition of Subwatersheds
Sub‐watershed sampling• Samples (events = 12) were collected at the mouth of 15 sub‐watersheds within the Upper Choptank River and Tuckahoe Creek sub‐basins
• Spatial and temporal variation in biogeochemical processes and source water mixing will cause complex relationships between solute concentrations in stream and river waters
Subwatershed and Estuary SamplingBas
eflo
w (m
3 /s)
0
20
40
60
80
100
120
Tota
l flo
w (m
3 /s)
0
20
40
60
80
100
120
Baseflow Total flowSubwatershed samplingRiver sampling
03/01
/2005
06
/01/20
05
09/01
/2005
12
/01/20
05
03/01
/2006
06
/01/20
06
09/01
/2006
12
/01/20
06
03/01
/2007
06
/01/20
07
09/01
/2007
12
/01/20
07
03/01
/2008
06
/01/20
08
Bas
eflo
w (m
3 /s)
0
20
40
60
80
100
120
Tota
l flo
w (m
3 /s)
0
20
40
60
80
100
120
Tuckahoe USGS Gauge (01491500)
Greensboro USGS Gauge (01491000)
Variance of Nitrate and MESA in Sub‐watersheds
Nitra
te‐N (m
g/L)
CO NO DO BL KC PB OL
GB SF LM NF
BD BW OT0
3
6
9
12
Subwatershed
MESA ( g
/L)
CO NO DO BL KC PB OL
GB SF LM NF
BD BW OT0
3
6
WDUSFPDU
Collinearity of subwatershedproperties
% Hydric Soils in Subwatershed
20 40 60 80
% C
ropl
and
in S
ubw
ater
shed
40
50
60
70
80
90
Nitrate concentration vs. Percentage Cropland
% Cropland in Subwatershed
40 50 60 70 80 90
Mea
n N
itrat
e-N
(mg/
L)
0
2
4
6
8
10
• Collinearity confounds interpretation of nitrate‐N fate as influenced by land use and land condition
• No correlation was found between MESA concentration and percentage cropland in the sub‐watersheds.
Conceptual ModelLand use/ condition Land management Local hydrology Impact on water fate
Cropland on well drained soils (high permeability soils)
Low intensity ditch network and incised streams provide drainage required for crop production
Predominant movement of precipitation into shallow groundwater due to high soil permeability
Oxic groundwater flow paths to local streams through surficial aquifers; deeper flow paths to regional groundwater via high permeability sediments
Cropland with low permeability soils (prior converted wetlands)
High intensity ditch network provides drainage required for crop production
Predominant movement of precipitation by vadose zone interflow to drainage ditches; low percolation potential
Preferential ditch flow through landscape provides rapid transport to the local stream network, impacting water chemistry
A more sensitive metric
% Cropland on Hydric Soils in Subwatershed
20 40 60 80
Mea
n N
itrat
e-N
/(Mea
n M
ESA*
1000
)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Conclusion
A more sensitive metric is percent cropland on hydric soil MESA leaches into groundwater with nitrate‐N and acts as a conserved transport analogRatio of nitrate‐N:MESA is correlated with percent cropland on hydric soils (R2 = 0.54***)Thus, croplands on well‐drained soils are the predominant source of nitrate‐N in streamsMitigation strategies should target these well‐drained area
Estuary Sampling Transect
Station Median Salinity(range)
Water Depth (m)
1 9.5 (9.0 ‐ 12.3) 10
2 8.7 (6.6 ‐ 11.1) 7
3 5.0 (3.4 ‐7.8) 4
4 0.80 (0.17 ‐ 2.6) 12
5 0.12 (0.06 ‐ 0.53) 5
6 0.15 (0.06 ‐ 0.44) 2
7 0.10 (0.07 ‐ 0.17) 6
Eight synoptic sampling campaigns were performance along the transect
Temporal and Spatial Variance
Whiskers represent minimum and maximum values, the box encloses the interquartile range, and the line within the box represents the median
River Site
Nitrat
e‐N (m
g/L)
1 2 3 4 5 6 70
1
2
3
4
aa
b
b b b
b
Nitrat
e‐N (m
g/L)
30 Mar 2005
2 Dec 2005
6 Apr 2006
11 Jul 2006
24 Aug 2006
25 Sep 2006
11 Apr 2007
16 Apr 2008
0
1
2
3
4
River Site
MESA
( g/L
)
1 2 3 4 5 6 70
2
4
6
MESA
( g/L
)
30 Mar 2005
2 Dec 2005
6 Apr 2006
11 Jul 2006
24 Aug 2006
25 Sep 2006
11 Apr 2007
16 Apr 2008
0
2
4
6
Nitrate‐N concentrations relative to MESA concentrations along the transect were linear for all eight sampling events (0.95 ≤ R2≤ 0.99 for all events except 25‐Sep‐2006, where R2 = 0.91; p ranged from < 0.001 to 0.044)
MESA (g/L)
0 1 2 3 4 5
Nitr
ate-
N (m
g/L)
0
1
2
3
4
Conclusions
The strong correlation of nitrate‐N with MESA indicates that nitrate‐N was conserved in much of the Choptank River estuary and that dilution is responsible for the changes in nitrate‐N and MESA concentrations
An alternative mechanism, yet highly improbable, is that the processing rates in the river for nitrate‐N and MESA are exactly the same
Although somewhat unusual, nitrate conservation in estuaries has been documented elsewhere, e.g., Conwy Estuary and Waterford Harbor in Ireland (Raine and Williams, 2000) and the Delaware Bay (Fisher et al., 1988)
Sub‐basin Monitoring and Modeling
GreensboroTuckahoe
ET 5.2
Current Real Time data at the Greensboro Gage Station
Date/Time
11/1
/201
2
12/1
/201
2
1/1/
2013
2/1/
2013
3/1/
2013
4/1/
2013
Nitra
te (m
g-N/
L)
0
1
2
3
4
5
Turb
idity
(FTU
)
0
20
40
60
80
100
Stre
am d
isch
arge
(ft3
/s)
10
100
1000
NO3-N TurbidityStream Discharge
Hurricane Sandy
Nitrate loads
0
500
1000
1500
2000
2500
3000
3500
4000
0
1
2
3
4
5
6
7
7/6/2005
9/12/2005
11/9/2005
1/27/2006
4/18/2006
5/23/2006
7/12/2006
9/5/2006
11/7/2006
12/6/2006
1/10/2007
3/5/2007
4/17/2007
6/4/2007
7/30/2007
9/10/2007
12/3/2007
2/6/2008
3/11/2008
5/12/2008
7/8/2008
9/8/2008
11/16/2008
1/8/2009
3/12/2009
4/28/2009
6/4/2009
8/12/2009
9/28/2009
11/9/2009
12/10/2009
2/22/2010
4/8/2010
5/25/2010
7/12/2010
8/31/2010
10/2/2010
11/8/2010
1/10/2011
3/21/2011
5/23/2011
7/18/2011
8/30/2011
10/24/2011
12/5/2011
Stream
flow
(cms)
Nitrate (m
g/l)
TK_FLOW GSB_FLOW TK_Nitrate GSB_Nitrate
• TK_FLOW: Stream flow from the Tuckahoe watershed• GSB_FLOW: stream flow from the Greensboro watershed• TK_Nitrate: Nitrate load from the Tuckahoe watershed• GSB_Nitrate: Nitrate load from the Greensboro watershed
Preliminary Findings
• Both sub‐basins have similar amounts of cropland but nitrate export is twice as high in the Tuckahoe sub‐basin.
• Greensboro sub‐basin has greater percentage cropland on hydric soils.
Proportion of agricultural lands• Tuckahoe has higher % Agg in watershed than
Greensboro.
% Crop Forest Pasture Range Urban Water WetlandsTuckahoe 53.96 32.83 8.19 0.23 4.21 0.05 0.53
Greensboro 36.08 48.33 8.96 0.31 5.58 0.12 0.62
Soil properties
A B C DTuckahoe 0.99 48.09 5.58 45.33
Greensboro 4.51 24.54 12.92 58.02
Tuckahoe A (%) B (%) C (%) D (%)Cropland 0.39 57.10 6.58 35.92
Non‐cropland 1.70 37.54 4.41 56.35
Greensboro A (%) B (%) C (%) D (%)Cropland 3.07 33.85 19.26 43.82
Non cropland 5.33 19.29 9.35 66.04
Winter Cover Crop Modeling
• Currently using SWAT to model winter cover crop impacts on nitrate export.
• Difficulty calibrating the model to reflect differences in nitrate export.
• The denitrification parameter can only be set at the sub‐basin level.
• Need modification of SWAT to adjust to parameter based on drainage class for soils.
Recommended