What is usually used to disinfect the gravel packing of a groundwater well?

The disinfection of gravel packing in a groundwater well is typically done using calcium hypochlorite tablet powder. This compound is effective due to its strong disinfecting properties and stability, especially in solid form. When used in tablet form, it allows for controlled and measured application, making it easier to manage the disinfection process without introducing excessive amounts of chlorine into the well system.

Sodium hypochlorite, while also effective as a disinfectant, is typically used in liquid form and may not be as practical or stable for the specific task of disinfecting gravel packing. Hydrogen peroxide can serve as a disinfectant but is less common for this application compared to calcium hypochlorite. Chlorine gas is effective but is much more hazardous to handle and typically reserved for larger-scale disinfection processes rather than the localized treatment of gravel packing in a well. Therefore, calcium hypochlorite tablet powder is the ideal choice for this purpose, making it the correct answer.