Effect of one dog's Feces

Objective: calculate the amount of enterococcus in the feces of one dog. Calculate the volume of water at a small beach. Determine whether one dog's feces significantly affects water quality.

First consider the concentrations of bacteria in the feces of different warm-blooded animals. The concentration of Enterococcus bacteria in dog feces is 40,000,000 entero/gram. Cat feces is even worse, and human feces is comparatively clean.

We know that a lot of dog feces can shut down a beach, but what about just one dog? This question occured to me one day at Asilomar Beach in California. I saw a lady covering her dog's feces with seaweed; she thought it would just wash into the ocean and not be a problem. Let's find out if she was right. Remember that the beach will shut down if enteroccous bacteria are found at a concentration of 104 enterococcus / 100 milli-liter sample, or about 1 entero/ml.

A small dog might poop about 100 grams. The number of enterococcus in its feces is thus
Enterococcus: E = 40,000,000 entero/gram x 100 gram = 4,000,000,000 bacteria.

Asilomar's beautiful beach is in a small cove between two rocky headlands. I estimate that it is about 200 m long. Figure that the bacteria in the feces will be mixed throughout the surf zone, which extends about 20m offshore, and is about 2 m deep at the seaward end. Calculate the volume of water at that beach, in cubic meters, then convert it to milliliters.
Volume of a prism: V = 1/2 x 2m x 20m x 200m = 4,000 m3 water
Convert to milliliters: V = 400 m3 x (100 cm/m)3 = 4,000,000,000 ml water

and there you have it -- there are exactly as many enterococcus bacteria in the dog's feces as there are milliliters of water in the beach. One dog can shut down the beach. Remember that.

Back to Follow Up Studies Huntington Beach Cleanliness of your local beach