Colin Gutcher, Jason Krompart, & Peter Nowell
Objective 2: Identify the factors relating to groundwater contamination
model was developed to quantify groundwater vulnerability
at a regional scale, and is meant to qualitatively
characterise risk into three categories, low, medium, and
high (Aller et al. 1987). The DRASTIC model works by
taking into account the factors which influence
groundwater vulnerability. Each variable, or factor, in
the DRASTIC model was multiplied by a weight factor, which
is denoted by the subscript w, as seen in Equation 2:
DI = Dw D + Rw R + Aw A + Sw S + Tw T + Iw I + Cw C (2)
The following is a detailed description of the factors influencing the potential for groundwater contamination and, in turn, constitute the DRASTIC model:
D) Depth to groundwater
The depth to groundwater can also be thought of as the depth to the water table. The deeper the water table, the longer it takes for water or contaminants to reach sensitive aquifers below (Lampman 1995). At groundwater depths greater than 30 m one can assume that some potential pollutants, such as nitrate will not be present in the aquifer (Aller et al. 1987). For shallower water tables the concentrations of agrochemicals, if present on the soil surface, generally decrease with increasing depth (Lampman 1995).
R) Natural recharge rates
R is the recharge rate of the aquifer, which can be calculated from Equation 3 (Babiker et al. 2005).
Recharge = (Precipitation - Evapotranspiration)*Recharge Rate (3)
Recharge is assumed to be the remaining water after evapotranspiration and runoff are accounted for and thus refers to the total amount of water entering an aquifer per year. The higher the recharge rate the greater the associated contaminant transport rate (Aller et al. 1987).
A) Aquifer Media
Aquifer media refers to the composition of the different geological layers that make up the aquifer. Aquifer media affects contaminant travel by various means, namely, how quickly contaminants travel through the aquifer. A more permeable media such as fractured limestone will have a higher permeability and allow contaminants to travel farther and more quickly through the aquifer (Al-Zabet 2002).
S) Soil Media
The effect of soil on the infiltration rate of contaminants is based on the soil's permeability. A soil which is high in clay will impede infiltration, causing greater runoff, than a similar soil with lower clay content and a higher sand content (Aller et al. 1987).
T) Topographic Aspect
Topographic aspect refers to the gradient of the topography. Topography influences how much precipitation, and attached pollutants, percolates downward versus how much becomes runoff. With gentle slopes, the contaminant is more likely to pool and travel downward through soil media, whereas on steep slopes, contaminants are more likely to be transported with runoff (Aller et al. 1987).
I) Impact of vadose zone media
Vadose zone media quantifies the ability of the materials located below the soil surface but above the water table to transport or adsorb contaminants. In this study the vadose zone media was based on the surficial geology of the area. The greater the permeability of the media, the greater the contaminant transport rate will be (Aller et al. 1987).
C) Hydraulic Conductivity
Hydraulic conductivity describes how easily water can move through a material, thus affecting the extent and degree of contaminant travel. The hydraulic conductivity is based on the flux, and hydraulic gradient, which can be determined from an understanding of the aquifer media. The greater the hydraulic conductivity, the greater the risk will be of groundwater contamination (Aller et al. 1987).