What concentration of free chlorine is required to inactivate Giardia in a pool if fecal incident involves formed stool?

To understand why all the options provided are considered valid for inactivating Giardia in a pool following a fecal incident involving formed stool, it's important to review the effectiveness of free chlorine at various concentrations and exposure times.

Giardia is a resilient protozoan parasite that can survive in chlorinated water if the concentration of free chlorine or the contact time is insufficient. Research and guidelines from health authorities indicate specific levels of chlorine concentration and exposure time required for effective disinfection.

Each of the choices specifies a different combination of free chlorine concentration and contact time. The effectiveness of these combinations is supported by guidelines from organizations like the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO), which stipulate that certain levels of chlorine can effectively kill Giardia under varying conditions.

1. When a concentration of 1.0 ppm is maintained for 45 minutes, studies suggest this is sufficient to provide an adequate margin for Giardia inactivation.

2. Similarly, a concentration of 2.0 ppm for 35 minutes is also backed by research showing that this threshold can effectively inactivate the pathogen, providing both a higher level of chlorine and suitably reduced contact time.

3. Finally, at a concentration of 3.0 ppm, the