Shingle Creek Chloride TMDL

Abby Morrisette and Josh Kuhn9/10/11

Shingle Creek

Why is Chloride a Problem?

• TMDL focuses on Chloride

• Inhibits osmo-regulatory processes– “Pickles” organism

• Evidence of decreased invertebrate IBI

Chloride Limit

• Shingle Creek is Class 2 water for protection of aquatic life

• Limits:– Acute: 860 mg/L for one hour duration– Chronic: 230 mg/L over four day average

Major Sources:• Road Deicing• Private Industrial and Residential Deicing• Salt piles• Railway and Airport Deicing

Minor Sources/Transport Mechanisms:• Groundwater Discharge

• From Infiltration• Natural Sources• Water Softeners and Septic Systems• Landfills• Fertilizer

Sources of Chloride

The TMDL Study

Sampling

• Sampled from Dec 2002-August 2003• Assessed historical data– Flows at USGS Queen Avenue Bridge station from

May 1996 to December 1998– Groundwater chloride concentrations from a 1996

USGS study

• Conductivity and flow recorded every 15 minutes• Chloride samples collected biweekly and during runoff

events.

Quality Control• Conductivity loggers calibrated 3x/year

– measurements within 10% of conductivity standards.• Duplicate samples demonstrated <10% difference.

Data Filling• Gaps in data due to frozen conditions and broken data loggers• Gaps filled using regression equations relating the site with

the USGS Queen Avenue station• Summer and fall data used to estimate winter discharge.

– Spring equations run separately.

GIS

• Area of roads calculated using GIS.• Salt applications recorded by Municipality

plow drivers used to calculate total salt applications.

Salt Piles

• Evaluated for runoff and salt composition.

• All point sources are de minimis thus not assigned a waste load allocation.

Conclusions from the Study

Source Allocation

87% Road Salt!!!!

“MPCA believes using the 71% target is a conservative assumption that overestimates the chloride reduction needed to achieve WQSs.”

TMDL critiques

Places where we thought assumptions and methods were

unsound

Chloride-Conductivity Correlation

• This is a questionable relationship, with R=0.80

• The graphical fit appears to underestimate high chloride

• R values are lower in winter

Lack of Groundwater Analysis

“…groundwater interactions with surface waters in the Shingle Creek watershed have not been thoroughly studied.”

~Page 6.10 of TMDL report

Lack of Groundwater Analysis

• Load duration curves indicate that groundwater chloride is at standard

Lack of Groundwater Analysis

• USGS study of shallow wells indicates temporally variable chloride concentrations from 4.3-370 mg/L

• Relationship between surface chloride and groundwater chloride is assumed linear– No data or calculations

Sampling Gaps

• No data collected from September to November

• For broken data loggers, flows were interpolated from Queen Avenue station, assuming linear regression

• Almost all winter flows interpolated, not measured

Queen Avenue Station Analysis

• Flow from Queen Ave Station does not account for ~6 mi2 of the watershed– Exclusion accounts

for highly developed areas

– Flow at Queen Ave may not be linearly related to other flows in the watershed, as assumed

TMDL Summary

Shingle Creek is seasonally impaired for Chloride

Recommended Chloride Reduction

• 71% reduction of chloride, allocated according to source analysis– Over-estimate

Is this realistic?

Big Task

• 71% reduction will require significant resources

• Reducing road salt could be a public safety hazard– Alternatives salts could

be more hazardous than chloride

Questionable Data?

• We need a better understanding of groundwater transport in the watershed

• A lot of major assumptions and interpolations were made

• Link between IBI and chloride is weak

Questions?