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CONCRETE WASH-WATER: Characteristics

Graphic of a magnifying glassThe following table summarizes the findings of a study of wash-water produced from the cleaning of concrete delivery trucks. Two main issues are highlighted by these results:

Delivery Truck Wash-off Water:  Characteristics

Average volume of water used = 25.6 L

Average pH of water: 12 pH units1

Average turbidity2 = 27,000 NTU3

Average TSS4 = 79,000 ppm5

1 the Canadian Water Quality Guidelines for the Protection of Aquatic Life state that pH should be between 6.5 and 9.0.

turbidity measures the cloudiness of water

3 NTU = nephelometric turbidity units

4 TSS = total suspended solids

5 ppm = parts per million.  The Land Development Guidelines state that the maximum acceptable level for TSS is 25 ppm above background during dry weather and 75 ppm above background during wet weather.

CONCRETE WASH-WATER HAS A HIGH pH

Image of exerpt from 1868 Fisheries ActPortland cement (the active ingredient of concrete, as well as mortar and tile grout) is the ingredient that kills fish.  When it dissolves in water it forms calcium hydroxide (Ca(OH)2), a highly alkaline substance, and as a result produces a very high pH (~12 pH units at 25°C) liquid.  This is why concrete wash-water has a pH that is deadly to fish.

It is interesting to see that the 1868 Fisheries Act (part of which is shown to the right), enacted shortly after the confederation of Canada, specifically named lime, another name for the main ingredient in Portland cement, as a substance that may not be deposited into fish-bearing water ways.

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What does "pH" mean?

pH is how we quantify the content of acid in a sample. The pH scale goes from 0 to 14, where 7 is neutral. A low pH value means the sample is acidic, while a high pH value means that the sample is basic or alkaline.

The pH of some common substances.

Thumbnail of graph showing pH of common substances .gif (16616 bytes)The figure to the right shows the wide range of pH values that different substances have. To view the full-sized version of this graph click on the thumbnail, then to return to this page, use your browser's "Back" button. 

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The safe pH range for freshwater aquatic life is

6.5-9.0 pH units

Sudden pH "shocks," or changes, even within this range can also harm fish and should be avoided.

Effects of high pH on fish.

The effects of high pH on fish may include: DEATH; damage to outer surfaces like gills, eyes, and skin; and an inability to dispose of metabolic wastes. 

High pH may also increase the toxicity of other substances. For example, the toxicity of ammonia is ten times more severe at a pH of 8 than it is at pH 7.

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How can I measure pH?

pH measurements may be made with:

  • pH test strips with adequate range (an aquarium test kit will not be able to measure high pH values); or

  • portable pH meters (also known as a pH "pen").

Photograph of pH test stripsThe pH test strips are easy and inexpensive to use. All you have to do is dip the test strip into the liquid and the change in color on the strip will correspond with the pH (see photo to right).

A portable pH meter is more expensive, however, it will provide a finer resolution in the pH measurement, if the person operating it is knowledgeable in its use. The use of a pH meter also requires additional equipment and supplies such as calibration standards.

Both pH test strips and pH meters are available from scientific and chemical supply companies (check the yellow pages for companies serving your area).

Is there any way to treat high pH?

A liquid with a high pH can be "neutralized" or returned to the safe range for fish in a number of ways. For example, an acid can be added. However, great care must be taken when using acids because it is very easy to "overshoot" the neutral range and change the pH to a very low pH which can also kill fish (see The pH of some common substances). Alternatively, one can bubble gaseous carbon dioxide through the liquid or put "dry ice" (frozen carbon dioxide) into the high pH material.

The active treatment of high pH wastes is more appropriate for fixed facilities such as concrete ready-mix plants or large construction sites where reliable control equipment can be installed. Fisheries and Oceans does not advocate the active treatment of wastes at job sites. In those circumstances, the most reliable way to prevent environmental harm is to control the waste waters by collecting them and treating them off-site.

It is important to note that just filtering concrete wash-water or putting it into a settling pond (such as might happen at a construction site) WILL NOT treat the high pH. The components that cause the high pH are still there in dissolved form, and the pH will still be high enough to kill fish in a matter of minutes.

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CONCRETE WASH-WATER IS VERY CLOUDY

Photograph of stream substrate covered in solids from tile grout washed into storm drainPortland cement is made up of very fine particles. Concrete wash-water is basically a slurry of these fine particles in water. If a concrete delivery truck or exposed aggregate driveway is washed down into a street drain, a large load of these fine particles, also called suspended sediments when in water, can be delivered into a fish-bearing waterway. The cloudiness is the result of these particles scattering the light as the light passes through the mix.

The picture at the right shows a deposit of fine sediments into a creek in Delta, B.C., from tile grout wash-water dumped into a street drain.  Fish gills were coated with the fine particles and the gravel substrate was clogged (see below).

Fine sediments can:

Clog fish gills

Fish get oxygen from the water that passes over their gills. If the gills get covered with fine sediments, the oxygen cannot pass from the water into the gill tissues and the fish will suffocate and die.

Smother habitat

If the spaces between gravel particles are filled in, incubating eggs will not get enough oxygen and the insects that fish eat will be smothered and their living space obliterated.

Impair feeding ability

 


Salmon are visual predators. If the water they are feeding in is cloudy, the fish will not be able to see their prey.

Thumbnail of graph showing effect of turbidity on prey capture success by salmon .gif (40285 bytes)The graph to the right shows the results of a study of the ability of salmonids (a group of fish that includes salmon and trout) to capture prey in varying degrees of cloudy water. To view the full-sized version of this graphic click on the thumbnail, then to return to this page, use your browser's "Back" button.

In clear water (turbidity = 0 NTU*) the fish were able to capture 100% of the prey, while in cloudy water (turbidity = 60 NTU), the fish were able to capture only 5% of the prey in the test tank.  (*NTU = nephelometric turbidity units)

(Graph adapted from: Berg, L. 1982. The effect of exposure to short-term pulses of suspended sediment on the behavior of juvenile salmonids. In: G. Hartman (ed.). Proceedings of the Carnation Creek workshop, a 10 year review. Feb. 24-26, 1982, Malaspina College, Nanaimo, B.C.)

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Send any comments or questions about this page to:  wernickb@pac.dfo-mpo.gc.ca

 

 
 

http://www-heb.pac.dfo-mpo.gc.ca/water_quality/fish_and_pollution/conc_char_e.htm

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Updated: 2006-05-26