Which acronym stands for Groundwater Under the Direct Influence of Surface Water?

• A. BAT
• B. NSPS
• C. PWS
• D. GUI

Answer: (D)

Groundwater Under the Direct Influence of Surface Water is a term used to flag a groundwater source that is affected by surface water and thus carries similar contamination risks as surface water. Because such influence means higher potential for pathogens and organic matter typical of surface water, regulators treat it like surface water for treatment and monitoring purposes. The acronym GUI (sometimes also seen as GUDI) precisely denotes this condition, signaling that the water source may need surface-water-type treatment, such as filtration, in addition to disinfection, and ongoing monitoring under drinking water rules. The other terms refer to different regulatory concepts—Best Available Technology, New Source Performance Standards, and Public Water System—so they don’t describe whether groundwater is influenced by surface water.