Detergent and fish

I was curious about the toxicity of common detergents (surfactants) and fish.

Foam generation at dam spillways is very visible at certain times of the year and most people assume it is due to detergent somehow entering the waterway.

Some people claim that glyphosate kills of fish might be more related to the surfactants used to improve the product sticking to leaves than to the glyphosate. One brand of glyphosate is "Round-up".

A particularly stupid social media "challenge" is to dump several bottles of dish detergent into a stream above a waterfall and then video yourself jumping into the resulting bubble-bath. This HAS been documented as causing fish kills.

For my part, I have washed my hair in lake-water. Did I kill many fish?

Simplifying the problem

Commercial shampoos and dish detergents are blends of many surfactants, conditioners, fragrance, dyes and thickeners.

Based on some Material Data Safety Sheets, it looks like sodium laureth sulphate is the surfactant used in the greatest quantity in many products.

Furthermore, if you were to combine all surfactants it would amount to about 20% active ingredient in dish-detergent. The rest is mostly water. Products like shampoo have less surfactant/detergent than dish-soap while concentration of detergent in body-wash is much, much lower. Body-wash is mostly water and thickeners.

LC50

LC50 is the concentration that is lethal to 50% of a population of a given species of organism. Different species of fish, for instance, will demonstrate different degrees of sensitivity.

According to this document, the LC50 (96 hours) for sodium laureth sulphate ranges from 0.39 mg/liter to 450 mg/liter. That is a difference of three orders-of-magnitude!!!

Another source lists the LC50 for fathead minnows as 1.4 mg/liter. Minnows are pretty tough so we will run with 0.50mg/liter as our proxy for "fatal concentration".

ERJ washing his hair

Picture your hero standing in water up to mid-thigh. He squirts a nickel-sized dab of dish-soap into his hand and proceeds to wash his hair.

Measurements indicate that 2 ml produces a nickel-sized dab. 2 ml times 20% equals 0.4 grams or 400mg. 400mg is enough to contaminate 800 liters of water to LD50.

A cube 10cm on a side is a liter. Cheerfully committing the sin of mixing units, 800 liters is a cylinder roughly 28 inches tall by 48 inches in diameter.

That is more water than I expected. Still not a big deal if I am the only person who does it in a 100 acre lake because the currents and wind will quickly dilute it.

Somebody dumps an economy-sized bottle of dish-detergent

I just looked at the economy-sized bottle of Ajax detergent next to our sink. It holds 1.5 liters of detergent. That has enough sodium laureth sulphate to kill half of the most sensitive species in a shallow pond (28" deep) that is about a quarter-acre in area.

You can quibble either way. You can make the case that some of the Sodium laureth sulphate will biodegrade in that 96 hours. You can make the case that deaths will continue to occur after the 96 hours. You can make the case that some components are more toxic than Sodium laureth sulphate. You can make the case that some are less toxic.

The take-home

Incidental quantities of dish detergent are unlikely to cause fish-kills under most circumstances.

That does not give you a get-out-of-jail card to use a full bottle of detergent as a fetch-toy for your retriever.

And since detergent attacks the fish's gills, even incidental quantities should be avoided when dissolved oxygen levels are low. That would be when water temperatures are high or when the water contains large amounts of sediment and organic material.

Hat-tip Lucas Machias