Quoted from: Machdar, I., T. Zulfikar, W. Rinaldi, and Y. Alfiansyah. "Assessment of groundwater vulnerability using DRASTIC Model and GIS: A case study of two sub-districts in Banda Aceh city, Indonesia." In IOP Conference Series: Materials Science and Engineering, vol. 334, no. 1, p. 012032. IOP Publishing, 2018. https://iopscience.iop.org/article/10.1088/1757-899X/334/1/012032/meta 

The DRASTIC is an approach developed by the US Environmental Protection Agency (USEPA) to provide a standardized evaluation of the potential for groundwater contamination [11]. The DRASTIC index or vulnerability rating present numerical ranking collective description of all the primary hydrological and geological considerations that influence groundwater flow through the vertical profiles of an area. In this approach, spatial collective data on depth to groundwater (D), recharge (R), aquifer type (A), soil properties (S), topography (T), impact of the vadose zone (I), and the hydraulic conductivity (C) are collected. For a defined area being investigated, each criterion is assigned a typical range and a rating relative value scale (r) from 1 to 10. In this scale, the higher values represent more sensitive area for contamination. These index criteria further are combined in an additive equation. Moreover, each criterion is decided a weight factor (w), from 1 to 5, reveal the relative importance of each factor. In the end, the linear equation of the total impact criterion score as the DRASTIC index or vulnerability rating has the following formula:

DRASTIC index (vulnerability rating) = D_rD_w + R_rR_w + A_rA_w + S_rS_w + T_rT_w + I_rI_w + C_rC_w 

where w is weight and r is rank.

In this formula, it applies four assumptions, i.e. (i) the contaminant is introduced at the ground surface; (ii) the contaminant is leached into the groundwater by precipitation; (iii) the contaminant is soluble; and (iv) the evaluated area is 40 ha or larger [11]. Obtained data is simplified by a GIS model interface. The GIS software was used in this study (ArcGIS) to prepare seven maps represent the seven criteria of DRASTIC. Individual map was classified and designated ratings and weighted according to DRASTIC standards [13] as shown in Table 1 and Eq. (1) was rewritten as follows:  

DRASTIC index (vulnerability rating) = 5D_r+ 4R_r + 3A_r + 2S_r + T_rT_w + 5I_r + 3C_r 

Table 1. DRASTIC standard weight.