A GIS-Based Model to Assess the Risk of On-Site Wastewater Systems Impacting Groundwater and Surface Water Resources

Figures & data

Figure 1. Generalized GIS-based framework of risk assessment model outlining model steps of data preparation, risk classification, weighting assignment, and risk map preparation.

Table 1.  Modeled risk parameters and associated risk values on a scale of 1 to 5 (low to high) based on their characteristics

Risk Parameters	Risk Values
		LOW		MODERATE			HIGH
		1	2	3	4	5
Soil	Hydrologic Class	Class B		Class C		Class A and Class D
	Soil Drainage	Well		Improper/Poor		Rapid/ Very Poor
Slope	< 6%	612%	1220%	2025%	> 25%
Lot Size	> 0.4 ha		0.20.4 ha	0.10.2 ha	< 0.1 ha
Surface Water	Outside specified buffer areas				30 m buffer from streams/rivers/water bodies 60 m buffer from locally significant wetlands 120 m buffer from provincially significant wetlands
Floodplain	Outside 100 year floodplain				Within 100 year Floodplain Zone
System Age	< 5 yrs	5 to 15 yrs	15 to 25 yrs		25+yrs or no records
Groundwater Intrinsic Susceptibility Index	Low		Medium		High
Recharge Areas	Outside designated recharge areas				Recharge area present
Water Supply	Private	Outside 100 m Buffer		Within 100 m buffer		Within 30 m Buffer
	Municipal Wellhead	Outside 25 Year Time of Travel	25 Year Time of Travel	5 Year Time of Travel	2 Year Time of Travel	Within 100 m of Municipal Well

Table 2. Developed parameter weightings from Ontario-wide survey of on-site system experts across Ontario based on median importance and certainty values.

Factor	Risk Parameters
	Soil Type	G/W ISI	Lot Size	Water Proximity	Water Supply	Recharge Areas	System Age	Floodplain	Slope
Median Importance	5	5	4.5	4	4	4	4	4	3.5
Median Certainty	3	3	4	3	3.5	3	2.3	3	3
Weighting%	20	15	15	10	10	10	5	10	5

Figure 2. The location of Huron-Kinloss Township study area on the east shore of Lake Huron in Southern Ontario [inset].

Figure 3. Individual risk parameter ratings of each of the risk parameters within the model developed for Huron-Kinloss Township.

Figure 4. Developed recharge area risk ratings in Huron-Kinloss Township provided by Maitland Valley and Ausable-Bayfield Conservation Authorities for Huron-Kinloss Township (10 m resolution) based on significant recharge areas.

Figure 5. Final (before validation) relative risk areas based on the developed risk contours for Huron-Kinloss Township risk model.

Figure 6. Proportion of area within Huron-Kinloss Township that fall under different risk categories under the final modeled results.

Figure 7. Contributions of risk parameters to mean risk score for the validated risk model for Huron-Kinloss Township.

Figure 8. Validated model results using adjustments of weighting by local experts within Huron-Kinloss Township.