Nitrogen Dynamics at a Manured Grass Field Overlying the Sumas-Blaine Aquifer in
Whatcom County
• Barb Carey, LHg, Washington State Department of Ecology • Joe Harrison, Washington State University, Professor Livestock Nutrient
Management Program
Ground Water Protection Council Annual Forum Seattle, Washington
October 6, 2014
Background & Study Location •Transboundary, unconfined glacial outwash aquifer
•High intensity agriculture (dairy and berries)
•Shallow depth to water (mostly < 10 feet); 50 feet thick
•Only feasible water source for ~20,000 people on U.S. side (29% > 10 mg/L Nitrate-N)
•Study site: 22-acre grass field
Purpose • Track N through manure/soil/crop/groundwater system over time • Evaluate effectiveness of land application guidance in dairy nutrient
management plans (DNMPs) in protecting GW quality
• Track nitrogen inputs (manure, fertilizer, irrigation water), outputs (grass crop) and residual (soil, groundwater)
• Evaluate 2 possible indicators of groundwater nitrate impacts o N mass balance o Post-harvest soil nitrate
Objectives
Marine Climate = Wet winters: Precipitation mainly in fall and winter: 2.5 - 3.2 feet (2004-2008)
Relatively mild winter temperatures—rarely stay below freezing
Nitrogen Mass Balance
• Inputs • Outputs • Manure • Fertilizer • Irrigation water • Atmospheric input • Soil organic matter
• Grass crop • Volatilization • Denitrification
• Residual
• Soil nitrate • Groundwater nitrate
Dairyman tilled and replanted grass field in April 2004 --just before start of study (NOT part of study plan)
Groundwater Monitoring
•6 shallow wells (12 feet deep)
•Silt loam soil, w/substantial clay in some drilling samples •Sampled monthly, 4.5 years
•Nitrogen (NH4-N, NO2+NO3-N, total N), chloride, organic carbon and related constituents.
Average nitrate-N concentration and N loading in 6 monitoring wells
0
100
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600
700
800
0
5
10
15
20
25
30
35Gr
ound
wat
erni
trat
e-N
(mg/
L)
Nitrate MCL
2004 2005 2006 2007 2008 2009
Monthly mean groundwater nitrate-N
Totalnitrogen applied annually (lb/acre)
No Data
Total N applied annuallyTillage/ Replant
Results • GW nitrate concentrations highest 1st winter (max 45
mg/L-N) after high manure N loading • GW nitrate decreased with lower manure N loading—
below the MCL (2006-2007)
• Spike in GW nitrate late 2006 after late-season application, and again in 2009 after annual manure loading 2 times the mass removed
• Crop removal consistent despite large variation in N application
1) N Mass balance
(Inputs) - (Outputs) = Excess N to GW 2) Intensive post-harvest soil nitrate testing
(amount of nitrate in soil at the end of growing season)
2 methods for estimating manure nitrogen loading effects on GW
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-100
-50
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200
2004 2005 2006 2007 2008
Exce
ssni
trog
en, N
Exce
ss(lb
/acr
e) Groundwater nitrate-N
(mg/L)
MCL/GW Standard
Estimated excessnitrogen
87
-79 -78
146
No loadingdata
Groundwater Nitrate-N(Nov-Dec mean of6 shallow wells)
1) Estimated annual excess N vs. early winter groundwater nitrate-N
Estimated excess nitrogen NExcess (bars) calculated using N mass balance vs. annual mean early winter (November-December) groundwater nitrate-N concentrations in 6 monitoring wells (line).
2) Soil nitrate results (1 foot depth) • WSU guidance: Post-
harvest soil nitrate 15 ppm and below (1 foot depth), no management changes needed (30 ppm recommended by WA Dept of Agriculture)
• Soil dynamic system (high variability, high recharge fall/winter), post-harvest soil nitrate tests inherently underestimate the amount leaching to GW (Kowalenko, 1987; Kuipers et al, 2014).
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Soi
l nitr
ate
(lb/a
cre)
Soil Nitrate
Fall Target(15 mg/kg or47 lb/acre)
2004 2005 2006 2007 2008
September-Octoberfall sampling period
Soil nitrate (m
g/kg)
Fall Target(15 mg/kg or47 lb/acre)
2004 2005 2006 2007 2008
September-Octoberfall sampling period
Soil nitrate (m
g/kg)
Each measurement = mean of duplicate samples
0
5
10
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25
30
0
20
40
60
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160
2004 2005 2006 2007 2008
Nitr
ate
avai
labl
e fo
r le
achi
ng in
the
fall
(lb/a
cre)
Mean w
inter groundwater nitrate-N
(mg/L)
Winter GW nitrate-Nmean in 6 shallowwells
MCL/GW Standard
Maximum of weeklyfall soil nitrateMean of
weekly fall soil nitrate
2) Fall soil nitrate (1 foot depth) vs. groundwater nitrate-N (early winter)
Simple spreadsheet model to estimate mass of nitrate leaching
•Model back-calculates amount of NO3-N needed to produce GW concentrations observed •We know the final concentration and volume and incoming concentration and volume, as well as site conditions, and volume of recharge
Groundwater inflow entering
from upgradient, with a given flow rate and nitrate-N concentration
Groundwater outflow leaving the site, with a given flow rate and
nitrate-N concentration
Recharge at the site over the fall/winter with unknown nitrate-N concentration
Q,CGW Inflow
CLeachate NO3
C GW Outflow NO3 b
dL
W
dH
Recharge (R, CR → NO3R )
Model 1 NO3-LEACHATE
Model 2 GWNO3-FORECAST
1-2 feet (DDF)
dH dL
= iH
Soil sample horizon (0-1 ft) (ρb; CSoil NO3 0-1)
KH ; AFSZ
Vadose zone (AFVZ)
Total residual soluble nitrate-N (NO3TotResidual)
Model 1 Method A
(NO3TotResidual = NO3Residual 0-2 + NO3R + NO3Supp)
Model 1 Method B (NO3TotResidual = NO3MB Residual + NO3Supp)
Saturated mixing zone
NO3Residual 0-2
Groundwater flow direction
Recharge from precipitation and/or irrigation
LInfiltration ,QLeachate
Not to scale
Pitz, 2014--https://fortress.wa.gov/ecy/publications/publications/1403018.pdf
Backcast N loading estimate to GW
• Average 115 lb/acre nitrate leached to GW during the fall/winter period (Range: 42-230 lb/acre) – Average of 33 mg/L nitrate-N leaching
• 40% of leaching occurred after the fall flush
(January – March)—probably freshly mineralized nitrate from soil organic matter
Comparison of #1 and #2 with model results
• Method 1 estimate of soil organic matter contribution large unknown—not predictive
• Soil nitrate very dynamic all year (including winter), only sampled top foot—not predictive
• Backcast model integrates leaching over the season and spatially—based on field measurements
Take home messages • Field N mass balance not correlated with GW
nitrate (soil organic N contribution = wild card)
• Intensive soil nitrate data (1-foot) not correlated with GW nitrate (high variability, fast leaching in winter)
• Backcast model using GW nitrate provided useful estimate of N mass leaching
• Applying manure too late in the season resulted in groundwater nitrate increases
• Nitrate continued to leach in the late winter/early spring
• Groundwater monitoring was the only reliable way to assess nitrate impacts
Acknowledgements Lynden, Washington dairy producer/landowner Whatcom Conservation District Washington Department of Agriculture Washington State University, Puyallup, staff Washington Department of Ecology staff
Information: Barb Carey, Washington State Department of Ecology [email protected]
Presentation based on: Carey, B.M. and J.H. Harrison, 2014. Nitrogen dynamics at a manured grass field overlying the Sumas-Blaine Aquifer in Whatcom County. Washington State Department of Ecology, Olympia, WA. Publication No. 14-03-001. https://fortress.wa.gov/ecy/publications/SummaryPages/1403001.html
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