Nitrate Distribution, Fate and Transport in Helena Area Waters

James Swierc, PGLewis & Clark Water Quality Protection DistrictLewis & Clark County, MT

April 22, 2014

Background• Helena Valley – SW Montana

– “Typical” Western Montana Valley• Alluvial Aquifer in Valley bounded by Bedrock/Tertiary

Basin Fill• River/Stream – Aquifer connections

– Losing Streams at Upgradient Valley Margins– Gaining Streams at Downgradient Margins

• Irrigation Canals

– Local Water Issues – Nitrate Related• Ground (Drinking) Water Quality

– Public Water Supplies vs Private Wells

• Wastewater treatment/management– Sewered Areas vs Septic Systems

• Concerns over Nitrate Contamination– USGS Studies 1973, 1980, 1992, 2000– County – Septic Maintenance District

• Recent Studies: TMDL 319, MBMG– Water Quality Map (Major Ion Types)– Isotopes (Nitrogen & Oxygen of Nitrate)– Drain Assessment & Cl/Br Ratios

• LCWQPD Monitoring Well Data (2009-2012)

• Residential Areas with Reported Problems (limited data)

• “Background”– Streams

• Recharge Areas < 1 mg/L

– Ground Water • Undefined, < 2 mg/L

• Sources?– Agriculture vs Septics– Differentiation Methods

• Nitrate Isotopes• Cl/Br Ratios

Nitrate in Ground Water

Helena Valley Hydrogeology• Aquifer - Unconsolidated Alluvium

– Clay lenses towards central valley– Vertical upward gradient

• Ground Water Flow– Shallow - Towards Lake Helena– Unconfined at valley margins– Upwelling in central valley

• Artesian flowing wells• Vertical Upward Gradient

– Gaining streams in central valley• Recharge

– Stream loss at valley margins– precipitation, irrigation loss– Bedrock Aquifer discharge in

subsurface to Alluvial Aquifer• Helena Valley Irrigation Canal

– Missouri River Water into Basin– Distribution Laterals/Drains

• Lake Helena– Discharge point for surface and

ground waterWater Surface Map from Briar & Madison (1992)

Recharge to Aquifer from Stream Loss

Recharge to Aquifer from Precipitation

Recharge from Irrigation Waters

Nitrate Isotopes• Nitrate Isotopes– Nitrogen– Oxygen

• Tracking Nitrate Recharge Sources

• Denitrification Pathway– “Lighter” atoms

denitrify first– Microbially

mediated

From Kendall (1998)

All Helena Area Nitrate Isotope Data

Nitrate Isotope Sampling Locations• Datasets– USGS, 2000

(Not Shown)• Bedrock Locations

– MBMG GWIP Studies (2011)

– LCWQPD Studies (2011, 2012)

Nitrate Isotope Sampling Results• Datasets– LCWQPD Studies

(2011, 2012)– No long decay series…

Nitrate Isotope Sampling Results• Sources

– “Old” Problem near Helena; Fairgrounds

– Fertilizer in Central Valley– Remaining Data

“clustered”

Chloride/Bromide Ratio

• Cl/Br ratio – premise, Cl from anthropogenic uses discharged to septic system drainfield

• Bromine stable/constant for “background”• Increase in nitrates associated with increase in

chloride– Nitrate can degrade, leaving elevated chloride

• Problem for Helena Valley – different recharge sources/ hydrologic areas

Ground Water Cl/Br Results• Poor Correlation

at Valley Scale– Inconclusive

Results• Look at Drains

Drain System – Central Valley• Irrigation

System– Main HVID

Canal– Distribution

Laterals– Ditches/Return

Flow Laterals– Tile Drains

• Samples Ground Water from top of water table

Drain Data – West Valley• D2-4

– Drains Residential and Agricultural Areas

– Nitrate decreases in summer, increase after irrigation season

– Cl/Br Ratio varies, 164-500

Drain Data – Southwest• D3-2– Drains Residential and

Agricultural Areas– Nitrate “stable”– Cl/Br Ratio varies, 205-

517

Drain Data – Northwest

• D2.2-1– Drains Residential and Agricultural

Areas– Nitrate increases in summer– Cl/Br Ratio varies, 182-420

• D2.2-5– Drains Agriculture Area– Nitrate increases post-irrigation– Cl/Br Ratio varies, 111-535

Drain Data – North

• D1-1– Drains Residential and Agricultural

Areas– Nitrate increases in summer– Cl/Br Ratio varies, 146-543

• D0-1– Drains Agriculture Area– Nitrate increases post-irrigation– Cl/Br Ratio varies, 170-792

Drain Data – East Valley• D7-1– Drains Primarily

Agricultural Areas– Nitrate low, “stable”– Cl/Br Ratio varies, 145-375

Drain Data – Observations• Nitrates– Some Variability pre/post

irrigation season• Chloride / Bromide – Chloride – generally stable– Bromide – widely variable

• Low/Trace Levels – Lab quantification issues• Different Sources for Br

• Need to characterize recharge sources as baseline for comparison

Chloride (& Bromide) Sources?• From De-icing Solutions for Roads (MgCl, NaCl, etc.)– Highest Chloride concentrations adjacent to roads

• From Fertilizers?– Saffigna & Keeney, 1977 Ground Water, V. 15, No. 2• Wisconsin Agricultural Areas, NO3/CL Ratio constant• Noted FERTILIZERs as source for Nitrate & Chloride

– Potassium Chloride used for Potassium source

– See Nitrate-Chloride Linear Relationship at valley residential wells with Nitrate Problems, by agricultural fields

Fate & TransportVertical Upward Gradient

• East Helena area well cluster (trio)– Well 1, TD 103’– Well 2, TD 128’– Well 3, TD 169’

• 4 feet vertical head upward over 60 feet

Horizontal vs Vertical flow rates• Vertical gradient order of magnitude

higher than horizontal gradient– Hydrograph shows similar response pattern

so affected by same surficial recharge sources– Deep water recharge source, vertical flow

retarded by finer-grained layers• Result – keeps surface contaminants near

surface, even if drawn down by well pumping

Gradients are real, aquifer hydraulic properties are estimated to illustrate concept

High K

High K

Lower K Aquitard

flowpath

flowpath

What does clustering ofIsotope data here mean?

Mixing of different sources?Rapid Denitrification?

Fate and Transport Summary• Nitrates released to top of aquifer from surface– Fertilizer AND Septic System sources

• Central Valley – area near Drains– Denitrification occurs, low Nitrate and low

dissolved oxygen levels in shallow wells– Drains flush top of water table into ditches

• Vertical upward gradient– Dilution of nitrate waters

• High Chloride, Low NO3

– Denitrification, dilution– Added chloride

Questions/Discussion?

Helena Valley from Divide (view East)

Note – work is ongoing, drain study continues through 2014James Swierc, PG jswierc@lccountymt.gov Project Report(s), Poster(s) and more information available at http://www.lccountymt.gov/health/water.html