Colin Gutcher, Jason Krompart, & Peter Nowell
Objective 4: Apply the GIS-based MCE model to examine the overall contanmination potential in the Fairchild Creek Watershed
| The Integrated MCE model was composed from the USLETrans and
DRASTIC models. These models were created from several layers
which were prepared using a variety of data sources (Data
Utilised). The USLETrans model consists of 6 layers:
rainfall and runoff, soil erodibilty, slope length and gradient, crop
and tillage practices, support practices, and contaminant transport
probability. These layers were garnered from the soil survey, land
cover, DEM, WRIP, and drainage network input layers, as outlined in
Figure 6. Additionally static values for the support practices layer,
and part of the crops and tillage practice layer were adapted from
Deloitte and Touche (1990). The DRASTIC model has a similar design to
the USLE, requiring several layers and data sources. These layers
consist of: depth to groundwater, recharge rates, aquifer media, soil
media, topography, vadose zone media, and hydraulic conductivity layer.
The DRASTIC model also required several input data layers including:
depth to water table, average climate normals, soil survey, overburden
thickness, surficial geology, bedrock geology, and the DEM (Figure 6).
Unfortunately, a data layer for hydraulic conductivity was not found,
this model layer was based on findings from the Lake Erie Region Source
Protection Committee (2010).
Figure 6 (click to enlarge)
With the exception of the model layers mentioned below, model layers were based off of desired field(s) within data layers with no modification. There were a number of model layers which involved more complex procedures which are further outlined, these included the slope length and gradient layer from the USLETrans, and the aquifer media layer from the DRASTIC model. The Slope length and gradient layer was based off the DEM. Slope was easily calculated using the slope function found within ESRI's ArcGIS, while determining the slope length required the identification of high points within the watershed. The straight line distance was then found from the areas of high relief while the DEM was used to determine the appropriate slope endpoints. The slope length and gradient layer was then created using the slope and slope length factors in conjunction with Equation 4. The aquifer media layer was based on either the bedrock or surficial geology layers. The required layer was determined by comparing the depth to groundwater with the overburden thickness to ascertain whether or not the water table was located within the overburden or the underlying bedrock. All layers used in this study were converted to a 25 m cell resolution, based on the cell resolution of the input data from the GRCA, while the extent of the layers was set to the exact bounds of the Fairchild Creek Watershed as delineated from the Water Resources Information Project data layer.
Much of the data analysis was done using the raster calculator in ArcGIS. First, the layers that constitute the DRASTIC and USLETrans models were classified based on parameterisation values (Table 1, Table 2) discussed in Objective 3. The two model layers were then produced by multiplying the respective constituent layers, as outlined in Objective 3. Because the USLETrans and DRASTIC models each employ a different value and weighting scheme, both layers were then standardized on a scale between 0 - 100. A value of 0 was used to represent the least amount of potential risk while 100 was used to represent the greatest amount of potential risk. The standardized USLETrans and DRASTIC layers were then input into the integrated MCE model. The resulting Integrated MCE model shows a continuum of risk ratings from 0 to 10,000. The integrated MCE model was reclassified into 4 categories: no, low, medium, and high risk. Low are values from 0-36, medium ranges from 36-224, and high potential to contamination is anyhing greater than 244, while areas with a value of 0 have no potential risk. These the values assigned to each category were based on values suggested from Stone and Hilborn (2000), for a farm located in southern Ontario, and Al-Zabet (2002) along with Aller et al. (1987) as they suggest that these ranges are the cut off points for both the erosion in the USLE and contamination in the DRASTIC models.