Chapter 5
Susceptibility

5.1 Overview

The final step in a Source Water Assessment is to conduct a susceptibility analysis. This is an analysis of the susceptibility of a public water system to contamination from potential contaminant sources. Potential contaminants and contaminant sources that may affect the public water system were identified in the contaminant inventory step. Susceptibility must be determined for each well or intake owned by the public water system.

The EPA defines the susceptibility of a public water system as the potential for the well or surface water intake to draw water contaminated at concentrations that would pose concern. The EPA suggests that contaminants may reach the intake or well by infiltration through geologic strata and overlying soil; direct discharge into groundwater; overland flow; or by contamination of upgradient water. Contaminants may also enter the water at the well, intake, or the conveyance. A conveyance is defined as the pipe, canal, or aqueduct between the well or intake and the first form of treatment, or where the water enters the distribution system if there is no treatment.

Susceptibility is related to three factors. The first is the physical integrity of the well, intake, and conveyances. These structures must be well designed, properly constructed, and adequately maintained to prevent contaminant entry into the system. The second factor is the sensitivity of the area over or through which the contaminants must move to reach the well or intake. Physical, geologic, and hydrologic characteristics of the aquifer and/or watershed are considered. The third factor in determining susceptibility is the nature of the contaminants. The type of potential contaminants, the location of the contaminant source relative to the well or intake, and whether or not a release has been confirmed describe the nature of contaminants involved.

The susceptibility analysis was designed to use information that is readily available for all public water systems in the state. The delineated source water assessment areas, the contaminant inventories, sanitary surveys, and both DEQ and State Engineer's Office well or intake permits will provide all the information necessary to complete the analysis. This information can be used objectively to provide consistent estimates of susceptibility for all Wyoming public water systems.

The susceptibility analysis was also designed to be straightforward. A well or intake rating will be determined based on well or intake integrity and sensitivity. Next, an iterative process will be used to develop contaminant ratings for all potential contaminants from all contaminant sources identified in the contaminant inventory. The well or intake rating will then be combined with the contaminant ratings using a simple matrix. The resulting susceptibility ratings will be summarized to provide the public water system with useful information to protect the drinking water source.

5.2 The Integrity Score

The first step in the susceptibility analysis is to determine the well or intake integrity score. The age, design, and maintenance of the well or intake will be ascertained. This information should be readily available from sanitary surveys, permits, or completion records.

The first piece of information needed is the well or intake completion date. Points assigned for completion date are based on DEQ permitting regulations. Prior to 1983, the Ten States Standards were used for the permitting and construction of wells and intakes. In 1983, DEQ regulations specifying well and intake construction standards and permitting requirements were adopted. In 1993, DEQ began requiring as-built construction drawings and Certificates of Completion to be submitted as part of the permitting process. The various points assigned to completion dates reflects DEQ's increasing confidence in the standards applied to the design, construction, and completion of wells and intakes.

Completion date also serves as an indication of well or intake age. Even the most well constructed and maintained wells and intakes will tend to lose structural integrity with age. Cracks, loose joints, and broken or incomplete seals may allow the entry of contaminants into the system.

Wells will then be evaluated for four factors. The most critical of these is the presence of a surface seal which is in good repair. DEQ feels that the surface seal is a good indicator of the overall well condition. A well without a surface seal or with a seal in poor condition is assumed to be poorly constructed and maintained. In addition, the vicinity immediately around the wellhead should be protected from contaminant sources. This is usually accomplished by enclosing the wellhead in a well or pump house, or a fenced off area. Well integrity is also determined by the presence of an annular seal and proper protection from flooding.

Surface water systems will be evaluated for three factors. One of these is the presence of a screened intake. A screened intake will prevent debris from interfering with the water treatment process. Additionally, screened intakes must be inspected and cleared of debris regularly, for example weekly, to remain effective. Similar to wells, access to the area immediately surrounding the intake should be restricted.

The means of transporting water from the well or intake to the treatment facility must be also be evaluated. Conveyances will be scored based on three factors. These are the length, the risk of damage, and the degree to which the transported water is exposed to contaminants. Longer conveyances are more likely to develop problems than shorter, more easily maintained conveyances. DEQ will consider conveyances in excess of one mile in length to be long. Past history of breakage or breaches and location will determine the risk of damage. For example, a pipeline that has experienced breaks due to settling soils or that is located in avalanche or landslide prone areas would be at risk of damage. Lastly, whether the conveyance is open or closed will influence the ability of contaminants to enter the water. Groundwater sources are assumed to have closed conveyances, such as pipelines. Conveyances such as ditches, canals, or aqueducts are considered open. Source water assessment areas may need to be modified if open conveyances intercept surface or groundwater sources between the point of diversion and the water treatment plant.

After being evaluated for the factors listed above, each well or intake will be scored. Both groundwater and surface water sources will receive a score between 1 and 13. If sanitary surveys, permits, or completion records are not available, or do not contain the appropriate information, the maximum number of points will be added for each factor. For example, if the presence of a surface seal cannot be verified, five points will be added. The well and intake integrity scoring procedure is summarized in Table 5.1.

Table 5.1 Determining surface water and groundwater source integrity.

5.3 The Sensitivity Score

The second step in the susceptibility analysis is to determine the well or intake sensitivity score. The inherent sensitivity of the aquifer or watershed will be combined with indicators of current or past contamination. This information will primarily be available as a result of the delineation process and from a database called the Safe Drinking Water Information System.

In groundwater systems, it is first necessary to determine what kind of aquifer the wells are completed in. Some types of aquifers are much more vulnerable to contamination from surface contaminant sources than others. Porous flow confined aquifers are considered to be the least vulnerable type of aquifer. Conversely, aquifers which require conjunctive delineation are highly vulnerable to contamination by surface contaminant sources. Aquifers that may require conjunctive delineation are sources of groundwater under the direct influence of surface water, alluvial aquifers, karst, and fracture-flow aquifers.

A map of uppermost aquifer sensitivity has been developed for the state of Wyoming using a modified DRASTIC model and certain land use factors. The DRASTIC model was developed by the EPA and the National Groundwater Association. This model takes into account depth to water, recharge and overlying soil characteristics, land slope, vadose zone characteristics, and other geohydrologic characteristics. DEQ will use these maps to assign a sensitivity score to all wells drawing from unconfined aquifers. These sensitivity scores will range from high to low. Aquifers too small to appear on the sensitivity map will be assigned a score based on equivalent aquifer types in the vicinity.

Surface water systems are assumed to be highly sensitive to contamination. There are many factors that influence the ability of a contaminant to reach the intake. Examples include the slope of the land surface, the amount of vegetative cover, soil permeability, water volume, the degree of water mixing, and flooding frequency. Little information is available on a state-wide basis for these factors. While sophisticated fate and transport modeling analyses may be able to incorporate these variables for individual systems, such analyses are expensive and require much time. While DEQ feels that this level of detail is impractical for the initial source water assessments, public water systems may use this type of analysis in subsequent assessments.

Both groundwater and surface water sources will be evaluated for indicators of past or present source water contamination. Detection of any chemical contaminant in either raw or treated water indicates that contamination has already occurred. In order for a well or intake to have a chemical detection, a pathway must exist for contamination to reach the source water. Consequently, chemical detections within the last five years will strongly influence the sensitivity score. New wells or intakes may rely on monitoring results from nearby water sources. If this is not feasible, a high sensitivity rating will be assumed until monitoring results for that intake or well are available.

The Safe Drinking Water Information System database contains information about all public water systems. Monitoring and testing results are entered into this database. The Safe Drinking Water Information System database is one of two sources of information that will be used to indicate watershed sensitivity. The Total Maximum Daily Load program has developed a listing of stream segments that do not meet water quality standards based on their intended use. These stream segments may indicate existing factors in the watershed which could increase watershed sensitivity. Only stream segments that have been listed as a result of credible data and are located within Zone 2 will affect the sensitivity score.

After being evaluated for the factors listed above, each well or intake will be scored. Groundwater sources will receive a score between 1 and 10, and surface water intakes will receive a score of 5 or 10. A listing of sensitivity points assigned to wells and intakes can be found in Table 5.2.

Table 5.2 Determining groundwater and surface water source sensitivity.

5.4 Determining The Well Or Intake Rating

Once a score has been obtained for both integrity and sensitivity, the well or intake rating can be determined. The points for well or intake integrity and sensitivity will be combined. The combined scores will range between 2 and 23. This score will then be converted to a low, medium, or high rating. Wells or intakes with a score between 2 and 8 will be low, 9 and 15 medium, and 16 to 23 high. Converting the numeric values to the low, medium, and high values will enable the well or intake rating to be combined with the contaminant ratings.

This rating only needs to be determined once during the susceptibility analysis process. The well or intake rating will provide the public water system with an assessment of the inherent vulnerability of the drinking water source based on integrity and sensitivity. Public water systems which rate as medium or high may wish to address obvious deficiencies. For example, surface seals greatly reduce the integrity score and are easily installed or replaced. If the current drinking water source needs to be replaced, it may be possible to replace it with one less sensitive. Using groundwater instead of surface water or completing the well into a deeper aquifer are possible options to reduce sensitivity.

5.5 The Contaminant Ratings

The next step will be to determine the contaminant ratings for each well or intake. Because there will be numerous contaminant ratings, obtaining the contaminant ratings for each well or intake will be an iterative process. Each potential source of contamination identified in the contaminant inventory will have from one to three types of contamination. Each of these contaminant types will be assigned a contaminant rating. For example, a landfill may contain more than one type of contaminant. This landfill will require one iteration for each type of contaminant present. This process will continue until each contaminant type present at each potential source of contamination has been given a contaminant rating.

Contaminant ratings are based on four critical pieces of information. These are: whether the drinking water source water is surface water or groundwater, the type of contaminant, the location of the potential source of contamination in relation to the well or intake, and whether or not there has been a known contaminant release to the source water. These factors will be described in more detail in the following pages. A matrix of these factors and the resulting contaminant ratings is included as Table 5.3.

Table 5.3 Contaminant rating matrices for surface water and groundwater sources.

¹Maximum Contaminant Level Goal (MCLG) refers to the level of contaminant in drinking water that treatment facilities attempt to maintain if the Maximum Contaminant Levels required by federal regulation are lower than detection limits or treatment capabilities.

The information required to rate contaminants will largely be gathered during the contaminant inventory process. Appendix D includes a table listing contaminants often associated with some common potential sources of contamination. This table can be used if exact lists of contaminants for each potential source of contamination are unavailable.

The first of the four factors used to rate contaminants is the type of water source. Surface water sources or groundwater sources under the influence of surface water are considered to be at higher risk than groundwater sources. Contaminant ratings for surface water sources are consequently more conservative. This results in higher ratings in the matrix table.

The second factor is the contaminant type. The Safe Drinking Water Act grouped drinking water contaminants into two categories, acute or chronic, and developed standards for them. These standards are based on the levels of consumption which are considered safe for humans. Acute contaminants may cause symptoms to appear after a single exposure, usually within a matter of hours or days. Acute contaminants include microorganisms and nutrients like nitrates. Chronic contaminants will cause symptoms if the individual is exposed to them over several to many years. There are two kinds of chronic contaminants, those that are considered to be carcinogenic and those that are non-cancer causing. Carcinogenic contaminants are identified in the federal drinking water standards by having Maximum Contaminant Level Goals (MCLG=0) of zero.

For the purposes of the Wyoming susceptibility analysis, contaminants will be grouped into two categories: Serious Contaminants and Other Contaminants. Acute contaminants and carcinogens will be considered Serious Contaminants. DEQ considers carcinogens Serious Contaminants because even single or short term exposures to these contaminants may be a significant health hazard. The Other Contaminants category includes the remainder of the chronic contaminants as listed in the federal drinking water standards. The Contaminants of Concern table in Appendix D indicates whether a contaminant is classified as acute, carcinogenic, or chronic.

The third piece of information which is needed to rate contaminants is the location of the potential source of contamination in relation to the well or intake. The contaminant inventory zone in which the potential source of contamination is located has a strong influence on the contaminant rating. Contaminant inventory zones were defined in Chapter 4. For example, potential sources of contamination located within Zone 1 will yield high contaminant ratings, regardless of contaminant type and detection status. In Zone 2, Serious Contaminants will receive high contaminant ratings regardless of detection status. This is a result of the serious nature of the contaminants and the close proximity to the well or waterways which flow towards the intake. Ratings for contaminants in Zone 3 are less strongly affected by proximity to the well or intake.

The fourth factor in determining a contaminant rating is the contaminant release status. This factor is an indication of whether a potential source of contamination has released contaminants into the source water. Documented releases are typically found with potential sources of contamination like facilities with permitted discharges, groundwater pollution control sites, and leaking storage tanks. These sites will be identified in the databases queried during the contaminant inventory. In the matrix provided in Table 5.3, release status often does not affect the contaminant rating. However, this information will become important in the susceptibility analysis summary.

5.6 Determining The Susceptibility Ratings

After the well or intake rating and all the contaminant ratings have been determined, the final susceptibility ratings can be determined. The simple matrix table developed to combine these two variables appears as Table 5.4. Each of the susceptibility ratings will be analyzed further during the susceptibility analysis summary step.

DEQ believes it is important to combine the contaminant and well or intake ratings in a way that the contribution of each to the susceptibility rating is balanced. Both ratings are equally important in understanding the threat to the public water system. For example, a public water source with a high well or intake rating, based on well or intake integrity and sensitivity, should be concerned about all contaminant sources, even if contaminant ratings are low. Conversely, a public water system with a low well or intake rating will not need to be as concerned with low contaminant ratings.

Table 5.4. Well or intake and contaminant rating combination matrix to determine susceptibility ratings.

5.7 Summarizing The Susceptibility Analysis

The goal of this final step in the susceptibility analysis is to provide the public water system with a summary of the susceptibility results. Public water systems must be able to use this information to manage and protect the drinking water source. To accomplish this, they need to know why their well or intake is susceptible to contamination and to which potential contaminants or contaminant sources it is susceptible. This will allow the public water system to manage the current situation as well as make appropriate plans for the future.

Summary Tables

Each susceptibility rating will be entered into two tables. One of these tables will group the susceptibility ratings based on contaminant source type. The other will group susceptibility ratings by contaminant type.

The table grouping susceptibility ratings by contaminant source type should include all potential sources of contamination found in the source water area. Each well or intake will then have a table listing all contaminant sources by type and the level of threat that each poses to the drinking water source. Table 5.5 lists some commonly occurring types of contaminant sources.

Table 5.5 Typical potential sources of contamination grouped by general category.

The second table will group the susceptibility ratings based on contaminant type. Public water systems may choose to further separate contaminant types into subcategories. This may assist the public water systems in defining the specific types of contamination that their drinking water system is susceptible to. Contaminants can be broken down into nutrients, microorganisms, carcinogens, and primary or secondary chronic contaminants, as shown in Table 5.6 below.

Table 5.6 Susceptibility ratings grouped by contaminant type.

Contaminant Releases

Contaminant release status was determined in the Contaminant Rating step. Contaminant releases will be entered into the appropriate column in the summary tables. This will allow the public water system to compare the number of releases to the number of contaminant ratings in each category. This may help the public water system gauge the immediacy or severity of the threat to the water supply.

Narrative Summary

In addition to the summary tables above, a narrative summary of the susceptibility analysis will be written. A discussion of the summary tables will be included in the narrative portion. Additional information about the potential sources of contamination can also be discussed. For example, potential sources just outside of Zones 1 or 2 may be mentioned. Statements about trends in contaminant source types, cumulative impacts, or contaminant quantity issues may also be made.

The narrative summary is also an opportunity to describe the monitoring and compliance history, current treatment capabilities, and planned future improvements of the public water system. What is currently being done to address problems identified by the source water assessment are also important items to discuss. Limitations to future management options would also be appropriate topics for the narrative summary. For example, the size, ownership, and land use characteristics of a watershed or aquifer recharge area will have a large effect on the type of management options which may be pursued.

5.8 Inter-system Ranking

The EPA suggested that each state develop a method to rank public water systems against each other based on the susceptibility analysis findings. At the present time, DEQ does not see the need for ranking public water systems in this manner. DEQ may, however, develop an inter-system ranking process in the future should a need arise.

Chapters
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Glossary | Acronyms
Appendices
A | B | C | D
Wyoming's Source Water Assessment Guidance Document Contents