Quoted from: Stempvoort, Dale Van, Lee Ewert, and Leonard Wassenaar. "Aquifer vulnerability index: a GIS-compatible method for groundwater vulnerability mapping." Canadian Water Resources Journal 18, no. 1 (1993): 25-37.  https://doi.org/10.4296/cwrj1801025 

The Aquifer Vulnerability Index (AVI) method for mapping the vulnerability of groundwater is based on two key parameters, 1) thickness of each sedimentary unit above the uppermost aquifer, and 2) estimated hydraulic conductivity of each of these layers. Using water well records, these parameters are combined to obtain a reasonable estimate of the hydraulic resistance of the protective aquitard cover, if present, at each site. AVI regions of isovulnerability (extremely low, low, moderate, high and extremely high) are then contoured using the geographic information system (GIS) SPANS. The AVI method was used to map a 3100 km2 area along the Saskatchewan-Alberta boundary. AVI maps can be used to help define groundwater protection zones, or for screening sites for land use selection. Within SPANS, AVI maps can be compared or merged with other GIS-referenced information, such as land use and water quality.