Case Study - Acute Chloride Toxicity Monitoring

LEWAS Lab Civil Engineering Team, Last edited on 08/15/2013

Chloride toxicity in natural water bodies is becoming a major concern across many states in the US. Efforts have been taken in a number of states including Arizona, New Hampshire and Pennsylvania to better regulate chloride toxicity in receiving natural waters. In this particular case study, the effects of road de-icing on chloride toxicity in a small urban stream (Stroubles creek) in Blacksburg, Virginia has been discussed.


Acute chloride toxicity refers to the potential exceedance of species tolerance caused by a one-time, sudden, high exposure of chloride. Chloride may get into surface water from several sources including wastewater from industries and municipalities, effluent wastewater from water softening, road salting, agricultural runoff, and produced water from oil and gas wells. Because chloride ions are persistent and are trapped in the hydrological cycle, all chloride ions applied to roadways as road salts ultimately reach surface water, thus the primary contributor in urban watersheds is road de-icing runoff where road salts are applied either as a solid application or solution. In addition to this, due to the lack of proper BMPs and absence of natural processes by which chloride ions are broken down, metabolized, taken up or removed, the only solution to reducing toxicity is dilution.

Figure 1. Road salt application methods

National recommended water quality criteria under the 304(a) Clean Water Act puts the maximum chronic Cl- exposure level at 230 mg/L over a 4-day period and maximum acute Cl- exposure level at 860 mg/L over a 1-hour period. In order to easily gauge the health of receiving waters, the indicator species commonly used are the Fathead Minnow (Acute Toxicity Limit : 860- 2790 mg/L) and the Water Flea (Acute Toxicity Limit: 507 – 1677 mg/L). These species have high tolerance to chloride ions levels and their death in receiving streams indicates severe impairment.

Figure 2. Fathead Minnow

Figure 3. Water Flea

Effects of Chloride toxicity:

Chloride is toxic to aquatic life and impacts vegetation and wildlife. Aquatic species of concern include fish, macroinvertebrates, insects, and amphibians. Elevated chloride levels can threaten the health of many aquatic species and pose a risk to aquatic species survival, growth, and/or reproduction. Many species of trees, shrubs and ground covers are extremely sensitive to high soil chloride levels, and may be killed, dieback or fail to germinate. Particularly high concentrations of sodium and chloride can be found in snow melt, which many animals drink to relieve thirst and potentially can cause salt toxicity including dehydration, confusion and weakness, among other symptoms.

Figure 4. Infographic showing effects of Cl- toxicity

Analysis of March 24th winter storm event in Webb Branch Watershed:

The winter storm event resulted in precipitation after road deicing. Resultant runoff generated large spikes in specific conductivity at the LEWAS site in Stroubles creek. It was noticed that the highest peaks were many times the base flow values. After determining that the road salt used was primarily Sodium Chloride (about 99% NaCl), a suitable regression equation was adopted to relate specific conductivity to chloride ion concentrations.

Figure 5. Winter storm event on March 24, 2013 in Webb Watershed in Blacksburg, Virginia

On plotting the resultant data as a 1-hour moving average (Fig. 6), it was found that the concentration of chloride in the stream exceeded the EPA acute toxicity levels multiple times over the storm period, but quickly re-established itself to base flow levels in a matter of a few hours. This clearly indicates an advantage to measuring water quality data more frequently since an acute event such as this would have been easily missed if not for the real-time capabilities of the instruments at the LEWAS site.

Figure 6. 1-hour moving average of Chloride concentration in Stroubles creek on March 24, 2013