Source Water Assessment
for the Cache la Poudre River

An EPA National Pilot

Step 3: Susceptability Analysis

Introduction

The Project used the 'susceptibility analysis' method that the Colorado Department of Public Health and Environment (CDPHE) developed for consistent application to all water systems in the state. Chapter 5 and Appendix F, State of Colorado Source Water Assessment and Protection Program Plan (CoSWAP) develops the 'susceptibility analysis' concept and state method for developing the analysis.

CoSWAP states: "The susceptibility analysis for a public water system (PWS) consists of determining the level of threat that potential sources of contamination (PSOCs) within the delineated source water assessment area (SWAA) pose to the water supply." CoSWAP continues: "The susceptibility of a PWS will be determined by the possibility for a PWS to draw water that potentially could be contaminated at concentrations that may pose a concern to consumers of the water. This susceptibility analysis will determine the relative susceptibility of a PWS to different PSOCs and different classes of contaminants that may be present."

The CDPHE susceptibility analysis method:

• Uses "… an iterative approach";
• Applies a "non-numerical technique" CDPHE calls "matrix combination"; and
• Gives priority to "… evaluating those contaminants that have established drinking water protection standards (i.e. contaminants of greatest concern)".

The CDPHE "non-numerical", "matrix combination" method uses a series of matrices and an eight-step process to rate susceptibility.

For a surface system like Fort Collins' Cache la Poudre (CLP) water source, the CDPHE process considers these factors for each PSOC:

• 'Hazard' posed by the contaminant(s)" and "its … impact on human health";
• 'Likelihood of release';
• 'Structural integrity' of the (water system) intake or well";
• 'Sensitivity', accounting for the distance of the PSOC to surface water and the water system intake.

Ultimately, the CDPHE method produces a rating for each identified PSOC as a "HIGH", "MODERATE", or "LOW" threat to the water source.

CoSWAP indicates that the rating system is designed to produce a "conservative analyses that will provide considerably more than adequate protection for the water source". In taking this approach, CDPHE hopes to attract "broad public involvement in the analysis process".

Threat Identification

CoSWAP defines 'threat' as "indication of impending danger or harm (Source: American Heritage Dictionary). The CoSWAP method develops a 'threat' rating a PSOC and its associated contaminants pose to a public water system by assessing the:

• 'Hazard', i.e., "possible source of danger", posed by the contaminants; and
• 'Likelihood' that these contaminants will be released from the source.

Each factor is assigned a rating.

The 'hazard' rating is based on consideration of:

• Specific chemicals present;
• Potential health implications of exposure to each; and
• Relative quantity of each present at the site.

The 'likelihood of release' rating is based on consideration of information including:

• Construction and condition of the facility;
• Current management practices; and
• Historical record of past contamination incidents.

Results: Threat Identification

Ideally, each rating would be based on evaluation of detailed information about each specific PSOC. However, because this analysis is essentially a 'first cut' and the Project was unable to develop detailed information for each PSOC, it assigned the default ratings. CoSWAP Table E1 presents the 'hazard' default ratings. The 'likelihood of release' default rating for each identified PSOC is 'unknown'.

A 'threat' rating for each PSOC combining the two ratings is assigned using CoSWAP 'threat rating' Table 5.3. The Threat Identification 1 Spreadsheet presents results of this analysis for PSOCs in the CLP source water assessment area.

Following the CoSWAP method produces a 'High' threat rating for every identified PSOC.

Risk Identification

CoSWAP defines 'risk' as "possibility of suffering harm or loss (Source: American Heritage Dictionary)." The CoSWAP method requires a rating of the 'risk' a PSOC and its associated contaminants pose to a public water system to be developed by assessing and rating the:

• 'Integrity of the system', i.e., " "; and
• 'Setting sensitivity'.

CoSWAP defines 'integrity of the system' as the "structural soundness and maintenance of the intake and the connections between the intake and the distribution system up to the first form of treatment". A structurally sound intake or well should help to reduce the likelihood of contaminants entering the system.

For a water system drawing from a surface water source, the 'integrity' rating is based on consideration of:

• Intake structure age and integrity; and
• Exposure of source water to the environment as it flows from the diversion point to the treatment plant, for example in open ditches or channels.

According to CoSWAP:

Setting sensitivity attempts to assess, in very general terms, the risk posed by the contaminant transport differences within the SWAA. The theory is that if the potential movement of contaminants within the watershed or aquifer is rapid, potential impacts and disruptions to the PWS could be realized quickly if the PSOC is located in close proximity to the PWS.

For a water system drawing on surface sources, the CoSWAP method for developing a 'setting sensitivity' rating generally is based on consideration of the distance between the PSOC and:

• Surface water; and
• Intake.

The CoSWAP method requires identifying buffer zones at increasing distances from surface waters, and identifying 'near' and 'far' zones defined by a fifteen-mile radius from the water intake. A sensitivity rating for a PSOC is based on accounting for the buffer, and 'near' or 'far' zones in which it is located.

CoSWAP directs the 'setting sensitivity' rating to be increased by one rating if "any stream or segment of a stream in the SWAA has been identified on the 303(d) list in the 305(b) report, or has had a Total Maximum Daily Load (TMDL) developed for it …".

CoSWAP Chapter 5 elaborates on how sensitivity ratings are to be developed ideally.

Results: Risk Identification

System Integrity

A 'system integrity' rating of 'OK' is assigned, because Fort Collins water system structures are sound and well maintained. Fort Collins replaced the intake structure completely in 1987, designing it to withstand forces greater than those produced by a one hundred year flood. After flowing through the intake, the water is piped to an enclosed presedimentation unit to enable sand to settle. Then the water is piped to the water treatment facility.

Setting Sensitivity

Distance Between PSOC and Surface Water: Every PSOC is assigned a 'Zone 1' rating, to account for the 'distance between PSOC and surface water' component of the sensitivity rating. 'Zone 1' is the most conservative rating possible. The Project, with the concurrence of the Fort Collins water system operator, and CDPHE and Environmental Protection Agency supporters, assigned this rating because:

• Environmental characteristics of the SWAA (for example, steep slopes, shallow or no soils, water table at or near the surface, fractured bedrock) would promote relatively rapid transport of released contaminants to surface waters;
• Most, if not all, PSOCs are located on the valley floor;
• Local stakeholders are perceived as valuing highly, preservation of existing high quality water and relatively pristine environment; and
• Pragmatic considerations (e.g., involving the effort and resources that would have been required to develop site specific information for PSOCs and maps of a large enough scale to depict buffer zones in an appropriate and meaningful way; the fact that most facets of the Project were determined prior to CoSWAP proposing and finalizing its buffer requirement).

Distance Between PSOC and Water Intake: Each PSOC is assigned a 'near' or 'far' rating based on where it is located on the appropriate contaminant inventory, or in some cases, other maps. Where the location of a PSOC has not been determined (e.g., where a PSOC is identified in a database, but can not be observed on one of the reviewed maps), it holds the more conservative 'near' rating. Please note that additional research (e.g., site verification, review of larger scale maps) could reveal that the rating of PSOCs is inaccurate, particularly for sites depicted as located on or near the 'near/far' or SWAA boundaries on the small-scale maps presented on this web page.

No streams or stream segments in the SWAA are identified on the 303(d) list in the 305(b) report, or have developed Total Maximum Daily Load (TMDL) standards.

Setting Sensitivity Assigned: Each PSOC is assigned a 'sensitivity' rating based on CoSWAP Table 5.5.

Risk Rating Assigned

A 'risk' rating for each PSOC combining the 'system integrity' and 'sensitivity' ratings is assigned using CoSWAP 'risk rating' Table 5.7.

The Risk Identification Spreadsheet presents results of this analysis for PSOCs in the CLP source water assessment area.

Following the CoSWAP method produces a 'High' 'Risk' rating for every PSOC located in the 'Near' zone and a 'Moderate' 'Risk' rating for every PSOC located in the 'Far' zone.

Vulnerability Determination

According to CoSWAP, the "vulnerability of a surface … system to a PSOC is determined by combining the threat and risk ratings to derive the vulnerability rating". Thus, CoSWAP appears to intend that the vulnerability rating encompass each specific factor for which the matrices account up to this point.

Results: Vulnerability Determination

A 'vulnerability' rating for each PSOC combining the 'threat' and 'risk' ratings is assigned using CoSWAP 'vulnerability rating' Table 5.9.

The Vulnerability Identification Spreadsheet presents results of this analysis for PSOCs in the CLP source water assessment area.

Following the CoSWAP method produces a 'High' 'Vulnerability' rating for every PSOC located in the SWAA.

Susceptibility Determination

CoSWAP states that:

The relative susceptibility of a system to different classes of PSOCs and, therefore, to different classes of contaminants is determined by the outcome of the vulnerability assessment. As a result, the outcome will be summarized in two ways. First the outcome will be summarized by tabulating the number of vulnerability ratings for each contaminant hazard rating given to a PSOC, such as the example given in Table 5.10. This table will summarize the number of LOW, MODERATE, and HIGH vulnerability ratings that Class A, B, and C PSOCs in the SWAA received during the susceptibility analysis. The State will also tabulate the number of vulnerability ratings by general PSOC classification (e.g., agricultural, industrial, etc.), such as the example given in Table 5.11. This table will summarize the number of LOW, MODERATE, and HIGH vulnerability ratings received by the different classes of PSOCs (e.g. agricultural, industrial, etc.).

In addition to the summary tables, CoSWAP directs that the susceptibility analysis "be narratively (sic) summarized in a general and concise manner". CoSWAP directs the "narrative report to include brief discussions on the factors that impacted the analysis (i.e., contaminant hazards, likelihood of release, structural integrity of the PWS, and setting sensitivity) and the outcome of the susceptibility analysis." CoSWAP also requires summary maps "showing the distribution of PSOCs within the SWAA that received a HIGH or MODERATE vulnerability rating in the susceptibility analysis". The Project website, including the previous pages, text, and maps, meet this to the degree feasible within the constraints imposed on conduct of the Project.

Results: Susceptibility Determination

Table 1 presents a tally of the vulnerabilities by contaminant class derived by following the required CoSWAP susceptibility determination method. The approach identifies one hundred twelve instances of Class A and sixty instances of Class B, for a total of one hundred seventy two vulnerability tallies within the Fort Collins Cache la Poudre
SWAA.

TABLE 1: SUMMARY OF VULNERABILITIES BY CONTAMINANT CLASS

Table 2 presents a tally of vulnerabilities by contaminant source class. Following the CoSWAP method identifies one hundred seventy high vulnerability ratings. The footnotes to this table note some concerns about if the CoSWAP method for summarizing vulnerabilities results in an apparent understatement of water system susceptibility to potential nonpoint sources of contamination.

TABLE 2: SUMMARY OF VULNERABILITIES BY CONTAMINANT SOURCE CLASS

1. Equating this non-point source entry with a single PSOC of another type as per the Project staff's interpretation of the CoSWAP method may understate the relative potential threat posed by roads. Hundreds of linear miles of roads are located in the SWAA.
2. Equating this entry with a single PSOC of another type as per the Project staff's interpretation of the CoSWAP method may understate the relative potential threat posed by ISDS. Thousands of ISDS are located in the SWAA.
3. Equating this entry with a single PSOC of another type as per the Project staff's interpretation of the CoSWAP method may understate the relative potential threat posed by wells. Identifying individual wells was well beyond the resources available to this work. A reasonable proxy for wells would be each improved parcel of any type (e.g., residential, commercial, public, etc.), as represented approximately by the maps of septic occurrence.
4. The value entered understates the potential threat from public/institutional buildings. Identifying every public building was beyond the resources available to this project.
5. Equating this non-point source entry with a single PSOC of another type as per the Project staff's interpretation of the CoSWAP method may understate the relative potential threat posed by irrigated agriculture.
6. Equating this non-point source entry with a single PSOC of another type as per the Project staff's interpretation of the CoSWAP method may understate the relative potential threat posed by grazing.

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