Grayl cartridges can filter and purify virtually any freshwater source. Grayl filtration and purification technologies cannot desalinate water, so stay away from salty or brackish water. When treating freshwater, always begin with the cleanest (least silty, least contaminated) water available. For instance, streams and rivers are generally better than lakes and bogs, which often have higher levels of sediment or tannins.
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.