In a site investigation, MTBE is present in the unconfined aquifer under the site. What is the best way to determine the likely source location for MTBE?

• A. GIS
• B. Groundwater sampling
• C. Tracer tests
• D. Aerial surveys

Answer: (A)

Locating a likely MTBE source relies on integrating spatial data about the plume with the site's hydrogeology. MTBE plumes in an unconfined aquifer move with groundwater flows and reflect where inputs occurred, so mapping concentrations, flow directions, and potential source areas together helps pinpoint the origin.

A geographic information system excels here because it lets you layer groundwater sampling data (plume extent and concentration), hydraulic gradients and flow directions, well locations, and possible sources such as storage tanks, fueling stations, or release sites. By overlaying these layers, you can see where groundwater flow paths converge toward higher concentrations, and you can identify the area most consistent with where a source would be located given the aquifer's properties and recharge patterns. GIS also supports spatial analyses, such as proximity to potential sources, plume shape, and uncertainty visualization, which aids in making a defensible inference about the source location.

Groundwater sampling provides essential data on where the plume is and how it’s changing, but on its own it doesn’t readily reveal the source without a broader spatial interpretation. Tracer tests can map actual flow paths and directions, but they require time, access, and resources and may not always be practical. Aerial surveys can help with surface indicators but MTBE sources are often subsurface and not directly detectable from the air. In this context, GIS is the most efficient and informative way to synthesize available data and identify the most likely source area.