Source Water Assessment Program

Determining Susceptibility

For the purpose of Wisconsin's Source Water Assessment Program (SWAP), susceptibility is defined as the likelihood that a contaminant or contaminants will enter a public water supply at a level which may result in an adverse human health impact. Contaminants of concern are those which are regulated under the Safe Drinking Water Act. Susceptibility is affected by four factors:

the construction of the well or intake
the location of the well or intake with respect to potential contaminant sources
the geologic or hydrologic sensitivity of the area around the well or intake
the results of water quality monitoring

Susceptibility determination methods differ for groundwater and surface water systems.

Groundwater System Susceptibility Analysis
Wisconsin's pre-existing Vulnerability Assessment program was the basis of this component of the SWAP. The Vulnerability Assessment program was developed in 1992 to eliminate unnecessary costly monitoring and is performed for all community and nontransient noncommunity public water systems every three years.
A vulnerability assessment consists of:

an inventory of potential sources of contamination within the source water area
an assessment of well construction
pesticide susceptibility
industrial chemical use
vulnerability to volatile organic compounds, ethylene dibromide, asbestos and coal tar

For municipal, other-than-municipal community and nontransient noncommunity wells the State expanded the range of factors considered in the vulnerability assessment process for the SWAP susceptibility determination to include the following additional factors:

presence, thickness and continuity of hydrogeological barriers
type of bedrock (not for wells screened in unconsolidated formations)
permeability of soil/surficial deposits
presence of naturally occurring inorganics or radionuclides in geologic units
presence of microbial, nitrate and inorganic contaminant sources
connectedness of a well to surface water
Age of well (i.e. condition of casing)

See the flow chart for the exact susceptibility determination process used for municipal, other-than-municipal community, and nontransient noncommunity systems

A more limited approach is being used to determine the susceptibility of transient noncommunity wells. For these systems, pathogens and nitrate are the focus of the susceptibility analysis. The strategy is based on the premise that for a system not to be susceptible one or more of the following conditions must be present.

Geological barriers
Hydraulic conditions related to pumping, well construction, and aquifer composition
Absence of potential sources of pathogens and nitrate in the recharge area

See the flow chart for the exact susceptibility determination process used for transient non community systems.

Surface Water System Susceptibility Analysis
Due to the small number of surface water systems each system was evaluated for susceptibility on a case-by-case basis. Factors considered during the susceptibility evaluation process include:

Resource characterization
Contaminant source inventory results
Proximity of stream discharges to intakes
Magnitude of stream discharges
Water quality monitoring data
Input from water supply and watershed professionals

The susceptibility analysis has been reported to the owner/operator of each system in a narrative format covering the above topics.

Back to Wisconsin's Source Water Assessment Program

Last Revised: Monday March 26 2007