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Road Salt: USGS monitoring Shingle Creek

Source: University of Minnesota Extension Service (January 11, 2005)

The USGS is doing real-time monitoring on Shingle Creek, one of the three creeks in the metro area that is developing a TMDL plan (water pollution reduction plan) for chloride (salt = chloride). You can view this monitoring data, plus other stream monitoring (new window) that the USGS is doing in Minnesota. snowplow

Of interest is that baseflow (groundwater seepage) contains chloride concentrations over the chronic criteria (see below). It is thought that decades of deicing salt use in this flat sandy watershed has resulted in the groundwater becoming high in chloride.

When on the site, scroll down for Shingle Creek (Mississippi River Basin), and when viewing Shingle Creek data, click the Specific cond at 25C (what DuluthStreams calls EC25) option on the Available Parameters selection box (it is the last option) for specific conductance. As you will remember from chemistry/physics class, dissolving salt in water increases its electrical conductivity. Specific conductance is measured in microsiemens per centimeter (uS/cm) and then related to an associated chloride level. When viewing the data, keep these state water quality standards in mind:

Chronic criteria (harmful over long exposure) for trout

230 mg/L chloride, associated with a specific conductance of about 1,400 uS/cm

Acute criteria (harmful over short exposure)

960 mg/L chloride, associated with a specific conductance of about 4,000 uS/cm

Final acute value (harmful right now)

1,720 mg/L chloride, associated with a specific conductance of about 6,200 uS/cm

Note: Relation between specific conductance and chloride concentration has been studied, but results are not yet published, so the relation between specific conductance and chloride concentration given above is provisional.

Recent peaks in Shingle Creek specific conductivity that are related to chloride concentrations that exceed the chronic criteria and approach the acute criteria are:

uS/cm
Date
2,830
3/7/04
3,270
3/10/04
2,820
3/14/04
3,150
12/30/04
2,950
1/3/05

Data from 1996-97 show Nine Mile Creek ringing the bell (near or over the Final Acute Value) with over 6,000 uS/cm on 1/21/97 and 2/6/97, and 8,000 uS/cm on 1/31/97.

Thanks to James Fallon, Supervisory Hydrologist, USGS, for this data and analysis and Ron Struss, Extension Educator with the University of Minnesota Extension Service/Water Resources Center and the Minnesota Board of Water and Soil Resources (BWSR) for disseminating the information.