What is a major advantage of gas-operated bladder pumps?

Gas-operated bladder pumps offer a significant advantage in minimizing problems of sample aeration. This is because these pumps use a gas (often nitrogen) to pressurize a bladder, which then pushes the water sample to the surface. The design helps to prevent the introduction of air into the water, ensuring that the sample is collected in a manner that accurately reflects the subsurface conditions and chemistry. This is important in environmental monitoring and water quality assessments, where aeration could lead to inaccurate results, such as the alteration of dissolved gas concentrations or the oxidation of certain analytes.

While the other options present characteristics of pumps, they do not relate to the primary benefits of gas-operated bladder pumps in a way that aligns with the question regarding their major advantage. Specifically, saying they can only be used in shallow wells limits their application, which isn't accurate since these pumps can be designed for different well depths. The statement regarding materials does not pertain to their operational effectiveness, and while some pumps may require additional equipment, gas-operated bladder pumps can function effectively with minimal extra apparatus compared to some other systems.