GeoPress® and UltraPress® cartridges filter and purify most freshwater sources. Avoid using them with salty or brackish water. When treating freshwater, start with the cleanest water available, such as streams and rivers.
Grayl's activated carbon effectively adsorbs many chemicals, pesticides, herbicides, heavy metals, etc. However, it is not designed for industrial disasters, coal ash spills, high lead levels, mercury, nuclear disasters, high tannin concentrations, or water with toxic blue-green algae (e.g. cyanobacteria). Find another water source in those situations.
Grayl cartridges are designed to remove low levels of chemicals, pesticides, herbicides, heavy metals, etc., but are NOT designed to protect against industrial disasters, coal ash spills, very high lead levels, mercury, nuclear disasters, high tannin concentrations or water that contains toxic blue-green algae (cyanobacteria). In these conditions, it is very important to find another water source.