The Perfect Storm: Nitrate Concentrations - Frey

High Nitrate Concentrations in Midwestern Streams in 2013 Following the 2012

Drought

Was it really the “perfect storm” we thought it was?

MSQA

Soil and Water Conservation SocietyGreensboro, NC 7-28-2015

Midwest Stream- Quality Assessment

• USEPA National Rivers and Streams Assessment • Temperate Plains Ecoregion• Midwest Cornbelt

100 MSQA Sites• Match our sampling to

NRSA’s first 50 random sites• Fill out stressor gradients

with 50 targeted sites:• Reference – 21 sites• Urban – 12 sites• Trend and other

high-priority sites – 17 sites

West

South

East

Blue – randomGreen – refYellow – other agRed -- urban

100 MSQA sites

Des Moines Water WorksNews ReleaseHistoric Nitrate Levels in Des Moines Water Works’ SourceWaterTuesday, May 28, 2013Nitrate concentrations continue to remain exceptionally high in both the Raccoon and Des Moines Rivers. Through extensive and expensive water treatment, Des Moines Water Works’ finished drinking water is currently under the Environmental Protection Agency’s (EPA) maximum contaminant level (MCL) of 10 milligrams per liter (mg/l) and is safe for drinking.

High nitrate concentrations in Iowa caused expensive and unintended consequences

2012 was a drought followed by a wet spring

Iowa

NAWQA Trend and MSQA site

Eastern study area: 2012 was a drought but wetter earlier

Indiana

NAWQA Trend and MSQA site

2013 - Spatially the highest nitrate concentrations were in IA and MN

2013 - Spatially the highest nitrate concentrations were in IA and MN

Long-term data Comparison• Northeast-Midwest Institute• NWIS/STORET - existing• Searched out additional state, local datasets• Included only data from May-July 1990-2012 (~81,000

NO2+NO3 samples)• Need to quality assure data• Removed sites outside MSQA study area (OK)• All high values were checked with appropriate agency and

removed if needed• Removed all data with “tile”, “Lake”, “inlet”, “outlet” in

name• Final number of NO2+NO3 samples (~35,300)

Historically, Median Nitrate Concentrations were similar or lower in 2013 except for western states: IA, MN, SD

Compared to long term sampling in Iowa the May-July concentrations were the highest ever

NAWQA Trend and MSQA site

Compared to long term sampling in Indiana the May-July concentrations were the 6th highest ever

NAWQA Trend and MSQA site

High nitrate related to high spring streamflow and fall drought conditions

MSQA site

Discharge

Nitrate

At the Indiana site antecedent conditions were much lower than in Iowa

MSQA site

Discharge

Nitrate

Theory: Shallow Groundwater Nitrate Concentrations are higher in IA and southern MN

From Schilling and Zhang, 2004

Many of the highest mean stream NO3 concentrations were found in high BFI watersheds but not all

Acres in Continuous corn

Theory: The amount of corn and continuous corn reflects the MSQA sites with the highest NO3 concentrations

Acres in Corn

Nitrate concentrations were much higher in Row Crop Agriculturally dominated watersheds

Ag Grassland : Ag Row Crops <40% and Ag Grasslands >20%Ag Row Crops : Ag Row Crops >50%

Nitrate concentrations increased as the amount of Row Crop Agriculture increased in watersheds

Ag Grassland : Ag Row Crops <40% and Ag Grasslands >20%Ag Row Crops : Ag Row Crops >50%

Predicting nitrate in unmonitored streams using MSQA data

• Final Model• CornCDL12 – explains 64% of the variability• Forest percent – explains 7%• Irrigation2007 – explains 3%• PPTMJJ13 – explains 3%

Summary

• High concentrations of nitrate were found at many sites in a wet year following a drought• There was a regional pattern to the high concentrations:

West to East, IA and southern MN the highest• Residual N probably contributed to the higher than normal

NO3 concentrations• Historical nitrate concentrations show IA and southern MN

are 3-5 mg/L higher then other states• Many of the highest concentrations of nitrate were found

at streams with a higher connectivity to the groundwater• The amount of the corn was the biggest predictor of stream

nitrate – explains 64% of the variability• Thoughts?

Information and Contacts

NAWQA – USGS http://water.usgs.gov/nawqa Jeff Frey(317) 600-2751 [email protected]

Pete Van Metre (512) 927-3506 [email protected]