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Conductivity measures the ability of a wire or other medium to carry electrical current. When metals or dissolved solids from road runoff pollute a stream they increase the conductivity. Measuring conductivity is a good indicator of road salt contamination. Kentucky state and local transportation officials apply tons of road salt to the roads and the road salt is contaminating streams and killing stream life. 

A leading peer reviewed journal, Environmental Science and Technology, carried a well researched article on road salt that observed:

“[Monitoring] demonstrate a substantial effect from road salt on streamwater quality and aquatic life. Bioassay results from runoff events confirm that the observed high concentrations of road salt caused acute and chronic toxicity to aquatic organisms. In addition, continuous specific conductance results indicate that elevated levels of road salt were present multiple times per year each year of monitoring. Populations of aquatic organisms in these streams and others with such road-salt influence are likely limited to salt-tolerant species.”

“Road-salt runoff poses an increasing threat to aquatic

ecosystems that are influenced by urban land use and

transportation corridors. Four broad issues suggest that roadsalt runoff is a serious and increasing threat to the nation’s receiving waters.

First, there is a multitude of historical evidence documenting detrimental effects of road salt on water chemistry and aquatic life. This issue was recognized

at least as early as the 1960s (1). Studies have continued each decade since then, with more comprehensive evidence of water-quality impacts from road salt. A small sampling of some representative topics studied includes specific water quality impacts such as increased chloride and sodium concentrations, seasonality, climatic and land-use influence, vertical density gradients, and influence on sediment pore water, mixing and alteration of turnover in lakes (2-5), and aquatic toxicity impacts (2, 6, 7).

Second, road salt usage in the United States has increased steadily beginning in the 1940s through the current decade (8, 9). Average annual salt sales in the United States for deicing purposes by decade beginning in 1940 were 0.28 (1940s), 1.1 (1950s), 4.1 (1960s), 8.7 (1970s), 8.8 (1980s), 13.0 (1990s), and 16.0 (2000-2008)

million metric tons per year.

Third, urban development is increasing each year (10), which increases the amount of impervious area on which winter deicing operations are conducted. This collective information suggests that the increasing road-salt usage trends of the previous seven decades will likely continue under current management conditions.

Fourth, chloride, and to a large degree sodium, the two primary ions in road salt, remain in solution, making it difficult with present-day technology to design effective management practices for reduction of road-salt loadings to receiving waters after application. Currently, reduction in usage appears to be the only effective road-salt-runoff

management strategy.”

Steven R. Corsi et al,

A Fresh Look at Road Salt: Aquatic Toxicity and Water-Quality Impacts on Local, Regional, and National Scales


Chloride concentrations exceeded U.S. Environmental Protection Agency (USEPA) acute (860 mg/L) and chronic (230 mg/L) water-quality criteria

USGS Streamflow monitoring station at

Old Cannons Lane on Middle Fork of

Beargrass Creek. The creek is flanked by I-64

and this location is downstream of the Watterson

Expressway,  St Matthews and Oxmoor Mall parking lots.

“The analysis of historical chloride data from urban areas around the country indicated potential for considerable and widespread impact from road salt on surface water quality and aquatic life.

Despite the limitation that sample results from these selected areas were from numerous studies not necessarily designed to capture periods of road-salt runoff, the influence of road salt was clear.

Streams with urban influence throughout the country in areas where road salt is applied are at risk for substantial contamination and

detrimental effect on aquatic life.”

The Beargrass Creek stream bed is covered with dirty algea. This is a highly impacted urban stream that is substantially dead due to urban runoff contamination.

See what Road salt does to roads and bridges:

Corrosion HERE 

See what Road salt does to roads and bridges:

Sherman Minton   HERE



Highway runoff levels in Louisville Kentucky deliver conductivity readings above 2500 microsiemens per centimeter yet watergroups petition to EPA to mandate state level numerical conductivity limits is targeted at mountaintop removal and seeks a numerical limit of 300 microsiemens per centimenter. Look at the graphs obtained from USGS below. They go a long way to explain the absence of stream diversity in urban impacted waters where automobile metal scrapings --nickel, copper, zinc, iron, manganese and road deicing chloride, cause spikes in conductivity levels.

In 2011, EPA published two peer-reviewed scientific reports documenting the harm caused by conductivity and mountaintop removal mining valley fills.  This research showed that a significant percent of aquatic life is extirpated when conductivity reaches 300 microsiemens per centimeter (μS/cm). While EPA’s 2011 guidance based on that research has been nullified by a district court for procedural reasons, that case is on appeal, and the court did not question the underlying science which remains valid.

July 2011 Guidance at 16 (citing EPA Office of Research & Development Final Report: A Field-based Aquatic Life Benchmark for Conductivity in Central Appalachian Streams

(May 27, 2011)).

"Extirpation concentrations of specific conductance were estimated

from the presence and absence of benthic invertebrate genera from 2,210 stream samples in West Virginia. The extirpation concentration is the 95th percentile of the distribution of the probability of occurrence of a genus with respect to specific

conductance. In a region with a background of 116 μS/cm, the 5th percentile of the species sensitivity distribution of extirpation concentrations for 163 genera is 300 μS/cm. Because the benchmark is not protective of all genera and protects

against extirpation rather than reduction in abundance, this level may not fully protect sensitive species or higher-quality, exceptional waters."

"Using the same water quality data used by EPA, but a different statistical method for analyzing that data, they independently derived a threshold of 308 μS/cm for biological impairment related to increased conductivity. That value is essentially the same as the 300 μS/cm lower value in the range cited in EPA’s 2011 guidance and derived in 2012 by Cormier et al." 

UPDATED to show

300 microsiemens macroinvertebrates