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Basin-scale assessment of Basin-scale assessment of transport of water and agricultural transport of water and agricultural
chemicals through the chemicals through the unsaturated zoneunsaturated zone
Rick Webb, Randy Bayless, Tracy Rick Webb, Randy Bayless, Tracy Hancock, Chuck Fisher, Tom Nolan, Rick Hancock, Chuck Fisher, Tom Nolan, Rick Healy, Jack Barbash, Josh Linard, and Healy, Jack Barbash, Josh Linard, and
Mike WieczorekMike Wieczorek
ObjectiveObjective
Develop a mobility index that classifies the Develop a mobility index that classifies the potential leaching of agricultural chemicals potential leaching of agricultural chemicals to ground water.to ground water. The mobility is dependent on properties of the The mobility is dependent on properties of the
agricultural compounds and the soilsagricultural compounds and the soils The leaching of mobile compounds to The leaching of mobile compounds to
groundwater can be reduced by limiting groundwater can be reduced by limiting applications before rainy months or in areas applications before rainy months or in areas where the water table is near the surface.where the water table is near the surface.
ApproachApproachLeaching of nutrients and pesticides to groundwater are simulated with Leaching of nutrients and pesticides to groundwater are simulated with the following considerations.the following considerations.
Agricultural practices (crop rotation and pesticides)Agricultural practices (crop rotation and pesticides) Physical and chemical properties of compounds.Physical and chemical properties of compounds. Water BalanceWater Balance Soil propertiesSoil properties
Total leaching for source areas to wells or surface water are estimated Total leaching for source areas to wells or surface water are estimated as follows:as follows:
Identify clusters or groups of similar soils and crops (and or rotations).Identify clusters or groups of similar soils and crops (and or rotations). Construct a mechanistic model for each major cluster and run stochastically Construct a mechanistic model for each major cluster and run stochastically
for the range of soil characteristics properties unsaturated-zone for the range of soil characteristics properties unsaturated-zone thicknesses.thicknesses.
Aerially weight the results to estimate mass fluxes for the Morgan Creek Aerially weight the results to estimate mass fluxes for the Morgan Creek watershed.watershed.
Final goal is to construct a mobility index specific to different Final goal is to construct a mobility index specific to different hydroclimatic regions of the country.hydroclimatic regions of the country.
Corn-Soy-Wheat crop rotations at the Maryland field site showing timing and loads of nutrients and herbicides
0
50
100
150
200
250
1/1/02 1/1/03 1/1/04 1/1/05
0
0.5
1
1.5
2
2.5
Fertilizers Crops Herbicides
Explanation
WheatCornSoybeans
NitrogenPhosphorusPotassium
AtrazineGlyphosateMetolachlorSimazine
Cro
p h
eig
ht,
in c
enti
met
ers,
or
Nu
trie
nt
load
, in
kilo
gra
ms
per
hec
tare
Her
bic
ide
load
, in
kilo
gra
ms
per
hec
tare
Pesticide propertiesPesticide propertiesMetolachlor – Immobile
Soil adsorption coefficient (Koc)
10710510310 109
Soil metabolism half-life, in days1.0 10 100 1000 1,000,00010,0001000.01 1.0
Solubility in water, in mg/L
Potential groundwater risk
Metolachlor Oxanilic Acid (MOXA) – Fairly immobile
Metolachlor Ethane Sulfonic Acid (MESA) – Mobile
Higher
Lower
0
2
4
6
8
10
12
14
16
18
0
2
4
6
8
10
12
14
16
18
Act_ET
GW_loss
OF_tot
Qdir_flow
BF+Exfil
Precip
Streamflow
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean
Me
an
mo
nth
ly d
ep
th,
in c
en
time
ters
Seasonal Fluxes for Morgan Creek, WY1995 - WY2004
Water balanceWater balance
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
6/1/
02
9/1/
02
12/1
/02
3/1/
03
6/1/
03
9/1/
03
12/1
/03
3/1/
04
6/1/
04
9/1/
04
CALFRichard's - Free DrainingRichard's with WT
Rec
har
ge,
in
mil
lim
eter
s
Buckel Field (10-m model column) - Morgan Creek, MD
RechargeRecharge
MethodMethod (mm/day)(mm/day)
WEBMODWEBMOD 0.630.63
CALFCALF 0.830.83
Richards EqnRichards Eqn 0.870.87
DarcianDarcian 0.08 – 2.950.08 – 2.95
0
500
1000
1500
2000
2500
Jan-94 May-95 Sep-96 Feb-98 Jun-99 Nov-00 Mar-02 Aug-03 Dec-04
1-m
2.5-m
5-m
7.5-m
10-m
Recharge and leaching greater where the Recharge and leaching greater where the unsaturated zone is less then 2.5 metersunsaturated zone is less then 2.5 meters
Rec
harg
e, in
mm
Rec
harg
e, in
mm
(12000 mm of precipitation)(12000 mm of precipitation)
Approach (General)Approach (General)Leaching of nutrients and pesticides to groundwater are simulated with Leaching of nutrients and pesticides to groundwater are simulated with the following considerations.the following considerations.
Agricultural practices (crop rotation and pesticides)Agricultural practices (crop rotation and pesticides) Physical and chemical properties of compounds.Physical and chemical properties of compounds. Water BalanceWater Balance Soil propertiesSoil properties
Total leaching for source areas to wells or surface water are estimated Total leaching for source areas to wells or surface water are estimated as follows:as follows:
Identify clusters or groups of similar soils and crops (and or rotations).Identify clusters or groups of similar soils and crops (and or rotations). Construct a mechanistic model for each major cluster and run stochastically Construct a mechanistic model for each major cluster and run stochastically
for the range of soil characteristics properties unsaturated-zone for the range of soil characteristics properties unsaturated-zone thicknesses.thicknesses.
Aerially weight the results to estimate mass fluxes for the Morgan Creek Aerially weight the results to estimate mass fluxes for the Morgan Creek watershed.watershed.
Final goal is to construct a mobility index specific to different Final goal is to construct a mobility index specific to different hydroclimatic regions of the country.hydroclimatic regions of the country.
Grid sampling for crops, soils, Grid sampling for crops, soils, and unsaturated zone thicknessand unsaturated zone thickness
Central Nebraska PODL
Metolachlor ESA (MESA) and Metolachlor OXA (MOXA) have longer half-lives than Metolachlor.
MOXA is less mobile than MESA
0
20
40
60
80
100
120
140
Metolachlor MESA MOXA
0
0.2
0.4
0.6
0.8
1
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Metolachlor and metabolites in top 50cmMetolachlor and metabolites in top 50cm
Mas
s in
hor
izon
, in
mg/
m²
Mas
s in
hor
izon
, in
mg/
m²
Fra
ctio
n of
par
ent
+
Fra
ctio
n of
par
ent
+
met
abol
ites
met
abol
ites
50cm – 100cm50cm – 100cm
Mas
s in
hor
izon
, in
mg/
m²
Mas
s in
hor
izon
, in
mg/
m²
Fra
ctio
n of
par
ent
+
Fra
ctio
n of
par
ent
+
met
abol
ites
met
abol
ites
0
20
40
60
80
100
120
140
Metolachlor MESA MOXA
0
0.2
0.4
0.6
0.8
1
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
100cm – 250cm100cm – 250cm
Mas
s in
hor
izon
, in
mg/
m²
Mas
s in
hor
izon
, in
mg/
m²
Fra
ctio
n of
par
ent
+
Fra
ctio
n of
par
ent
+
met
abol
ites
met
abol
ites
0
20
40
60
80
100
120
140
Metolachlor MESA MOXA
0
0.2
0.4
0.6
0.8
1
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
0
20
40
60
80
100
120
140
Metolachlor MESA MOXA
0
0.2
0.4
0.6
0.8
1
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
250cm – 1000cm250cm – 1000cm
Mas
s in
hor
izon
, in
mg/
m²
Mas
s in
hor
izon
, in
mg/
m²
Fra
ctio
n of
par
ent
+
Fra
ctio
n of
par
ent
+
met
abol
ites
met
abol
ites
Grid sampling for crops, soils, Grid sampling for crops, soils, and unsaturated zone thicknessand unsaturated zone thickness
0
25
50
75
100
-5 0 5 10 15 20
Unsaturated Zone thickness, in meters
Per
cent
are
a th
inne
r th
an v
alue
Streams and wetlands
Depth of UZ, in meters
0
20
40
60
80
100
120
140
160
180
0 2 4 6 8 10 12
MESA
MOXA
Metolachlor
Lea
che
d to
gro
un
d w
ate
r, in
mg
/m²
Lea
che
d to
gro
un
d w
ate
r, in
mg
/m²
Leached after four applications of Metolachlor at 140 mg/m² Leached after four applications of Metolachlor at 140 mg/m² (1.4 kg/ha) over 10 years; Total applied = 5.6 kg/ha(1.4 kg/ha) over 10 years; Total applied = 5.6 kg/ha
Grid sampling for crops, soils, Grid sampling for crops, soils, and unsaturated zone thicknessand unsaturated zone thickness
Metolachlor
Applied Leached Residual
Units are kg/ha. Five applications of Metolachlor at 1.36 kg/ha = 6.8 kg/ha
Transformed
ApproachApproachLeaching of nutrients and pesticides to groundwater are simulated with Leaching of nutrients and pesticides to groundwater are simulated with the following considerations.the following considerations.
Agricultural practices (crop rotation and pesticides)Agricultural practices (crop rotation and pesticides) Physical and chemical properties of compounds.Physical and chemical properties of compounds. Water BalanceWater Balance Soil propertiesSoil properties
Total leaching for source areas to wells or surface water are estimated Total leaching for source areas to wells or surface water are estimated as follows:as follows:
Identify clusters or groups of similar soils and crops (and or rotations).Identify clusters or groups of similar soils and crops (and or rotations). Construct a mechanistic model for each major cluster and run stochastically Construct a mechanistic model for each major cluster and run stochastically
for the range of soil characteristics properties unsaturated-zone for the range of soil characteristics properties unsaturated-zone thicknesses.thicknesses.
Aerially weight the results to estimate mass fluxes for the Morgan Creek Aerially weight the results to estimate mass fluxes for the Morgan Creek watershed.watershed.
Final goal is to construct a mobility index specific to different Final goal is to construct a mobility index specific to different hydroclimatic regions of the country.hydroclimatic regions of the country.
ConclusionsConclusionsThe soils beneath agricultural fields provide a complex The soils beneath agricultural fields provide a complex chromatographic media that can enhance or impede chromatographic media that can enhance or impede the leaching of agricultural chemicals to ground water.the leaching of agricultural chemicals to ground water.
The soil characteristics and thickness of the The soil characteristics and thickness of the unsaturated zone define the properties of the unsaturated zone define the properties of the chromatographic column; The climate and agricultural chromatographic column; The climate and agricultural management practices determine the supply rate and management practices determine the supply rate and residence time of the agricultural chemicals.residence time of the agricultural chemicals.
Conjecture: The spatial and temporal variations of Conjecture: The spatial and temporal variations of agricultural chemicals delivered to ground water may agricultural chemicals delivered to ground water may be reflected in the water pumped from the water-table be reflected in the water pumped from the water-table aquifer.aquifer.
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