Groundwater Contamination is of concern to Hornby Island residents as it is their only source of drinking water on the island. In order to protect this precious natural resource, a groundwater vulnerability assessment using a Geographical Information System (GIS) to map the vulnerable areas is needed. We hope to provide a visual representation of the locations on the island which are more susceptible to contamination by applying the DRASTIC model to Hornby Island.

DRASTIC is an aquifer vulnerability model that has been used in previous studies by the NSW Department of Land and Water Conservation in Australia [Piscopo], which was modified from the original devised by the US EPA. This model will look at the slope variations of the island, the soil type, geology, recharge rate, impact of the vadose zone and precipitation to determine the aquifer vulnerability for Hornby Island. The following section is a brief outline of the model.

For mapping aquifer vulnerability for Hornby Island the DRASTIC model requires a number of spatial attributes to be mapped as a combined description of all the major geological and hydrologic factors that affect and control groundwater movement, into, through, and out of an area. DRASTIC is an acronym for the most important mappable features within a hydrogeologic setting which control groundwater pollution.

D: Depth to Water Table
R: Recharge
A: Aquifer media
S: Soil Media
T: Topography (slope)
I: Impact of Vadose Zone media
C: (Aquifer Hydraulic) Conductivity

For each letter of the DRASTIC acronym a numerical ranking is used. There are three significant parts; weights, ranges and ratings. Firstly, a weight is given to each DRASTIC feature relative to one another in order of importance from 1-5. The most important feature that affects aquifer vulnerability will be assigned a five while the least important feature will be assigned a one. Then the determination of the features upper and lower limits of the media types were devised on the basis of their impact on pollution potential. Lastly, ratings ranging from 1-10 are assigned to each DRASTIC feature which make possible the ranking of the ranges found in each DRASTIC feature map.

Depth to Water Table
The depth to water table (DTWT) provides us with a measurement of how deep the unsaturated material is that the contaminant must travel through before reaching the water table (or the saturated zone). This is important because the depth of the water table increases with greater depth of the unsaturated zone therefore it will take longer for the contaminants to reach the water table. The DTWT ranges and rating are provided in this table.

Recharge
Recharge is the amount of water that infiltrates the ground surface and travels through the unsaturated zone to the water table. The net recharge is important because it transports potential contaminants vertically downwards to the water table and horizontally within the aquifer. The Recharge also controls the amount of water that is available for the transportation of contaminant and the dilution of contaminants in the vadose and saturated zone. The greater the recharge the greater the potential will be for groundwater pollution. Recharge incorporates three variables; precipitation, slope and soil permeability. Their ranges and rating are in the following four tables (precipitation, soil, slope, recharge).

Aquifer media
The aquifer media controls the groundwater flow system within the aquifer and in turn the contaminants in the aquifer. The path length and the porosity in the aquifer media have a large impact on the flow of the contaminant. The path length that the groundwater takes determines the time available for several processes such as sorption, reactivity, and dispersion. In addition, the porosity of the different aquifer media influences the amount of contact between the contaminant and the aquifer media. The rating of the aquifer media is in the following table.

Soil Media
The soil media has a significant impact on the amount of recharge water that can infiltrate to the water table whether it is fine grained or course grained. In turn, the soil media itself has great impact on contaminant transport. Fine grained materials, like clays and silts, decrease relative soil permeability and restrict contaminant migration. In addition, thick and thin soil layers also have an effect on the attenuation of contaminants. This table provides the final factor assigned to the soil media based on how easily contaminants may move through it.

Topography (slope)
Topography is considered as the slope, and slope variability of the land surface. When the slope is steep there tends to be more potential for pollutant runoff and therefore little pollutant retention and in turn little infiltration of contaminants. On the other hand, shallow slopes have more potential for pollutant retention and in turn infiltration of contaminants. This table provides the range and rating assigned to the slope when it is considered a feature on its own.

Impact of the Vadose Zone
The vadose zone is the layer of soils above the water table which is unsaturated or intermittently saturated. The type of material that is present in the vadose zone determines the attenuation characteristics, length, path, the time available for attenuation and quantity of material that is able to come in contact with. The impact of the Vadose Zone incorporates two different features, the depth to water table and the soil permeability. The following three tables provide the ranges and ratings of these features (Depth to Water Table, Soil Permeability, Impact of Vadose Zone).

Conductivity
Hydraulic conductivity is a controlling factor of the rate at which water can move through permeable media under a given hydraulic gradient; it is controlled by the amount of pore space present in the soil media. Groundwater velocities also determine the rate at which water enters the aquifer. This table provides the rating of the conductivity.

Once all seven of the DRASTIC features have been given a range and a rating they are then assigned their weight by multiplying all the ratings by the given weight. This process is done so all features can be overlaid in one single map to provide a visual representation of which areas on Hornby Island are more vulnerable to aquifer contamination. This table provides the weight assigned to each feature.

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