17.1 Contamination Sources and Susceptibility
Key Takeaways
- A potential contaminant source is an activity or condition that could release a hazard; its presence does not prove that source water is contaminated.
- Susceptibility connects the nature and severity of a threat with the likelihood that it can reach the intake or well through a credible pathway.
- Distance alone does not rank risk: release potential, travel path, hydrogeology, intake or well construction, time, and existing controls all matter.
- An operator should verify observations, preserve the source condition, follow the approved notification and sampling path, and document evidence before changing treatment.
Think source, pathway, and receptor
The 2025 WPI Class I outline expects an operator to identify potential sources of source-water contamination. A source is an activity, facility, natural condition, or event that could introduce a biological, chemical, or physical hazard. It is not the same as a contaminant detected in raw water. Susceptibility describes the potential for the public-water source to be affected by an identified threat. In the U.S. Environmental Protection Agency (EPA) source-water-assessment framework, it connects the nature and severity of the threat with the likelihood that the threat can contaminate the source.
Use a three-part model:
| Part | Operator question | Examples of evidence |
|---|---|---|
| Source or hazard | What could be released, in what amount, and how often? | Materials stored, land use, spill history, waste practice, natural mineral occurrence |
| Pathway | How could it move toward the source? | Runoff, drainage, fractures, permeable soil, recharge, stream-aquifer connection, damaged casing |
| Receptor | What well, spring, reservoir, river, or intake could receive it? | Contributing area, intake depth, well screen, pumping pattern, travel time |
If no credible pathway exists under the assessed conditions, a nearby activity may rank lower than a more distant activity connected by drainage, a permeable aquifer, or a fast conduit. Conversely, absence of a mapped source does not prove zero risk; inventories can be incomplete and conditions change.
Build an inventory without declaring contamination
Common point sources have an identifiable location, such as a leaking tank, chemical storage area, wastewater lagoon, landfill, spill site, mine opening, or failing septic system. Nonpoint sources are dispersed, such as fertilizer, pesticide, manure, road salt, urban runoff, erosion, or wildfire effects across a broad area. Natural geology can contribute arsenic, radionuclides, fluoride, salinity, iron, manganese, or other constituents without a human release. These categories organize an investigation; they do not predict the exact contaminant or concentration by themselves.
Surface water and groundwater expose different pathways. A surface-water intake can receive upstream spills, storm runoff, wastewater discharges, wildfire ash, algal material, or bank erosion. Timing may be rapid and strongly tied to flow and weather. A well can be affected by recharge through soil, fractures or karst conduits, leaking infrastructure, a poorly sealed annulus, flooding at the wellhead, or induced movement from a connected stream. Groundwater can be slower, but karst or fractured settings can transmit some changes quickly. Source type alone never justifies a fixed travel time.
Contaminant behavior also changes the pathway analysis. Solubility, density, sorption, persistence, decay, and reaction with soil or water can affect transport, while pumping can alter groundwater direction and capture. These properties require qualified assessment; an operator should not infer arrival time from a chemical name. Monitoring locations and frequency should follow the approved assessment, event response, and authority requirements.
A source-water assessment commonly includes a delineated protection or contributing area, an inventory of known and potential sources, and a susceptibility determination. Delineation asks where water reaching the source may come from. Inventory asks what threats occur there. Susceptibility analysis ranks their potential impact. A compliance sample asks a different question: what was measured at a stated place and time? Do not call an assessment result a positive sample, and do not call one negative sample proof that an identified threat is absent.
Rank threats with evidence
A useful ranking considers several dimensions together:
- Hazard: toxicity, persistence, mobility, microbial character, quantity, and release likelihood.
- Connection: drainage direction, recharge area, geology, soil, well or intake construction, and surface-water/groundwater interaction.
- Time and condition: season, storm flow, pumping rate, source level, operating status, and estimated travel time where a validated model exists.
- Controls and evidence: containment, inspections, monitoring points, spill prevention, sanitary barriers, historical detections, and data quality.
- Consequence: the population and treatment barriers that could be affected and how quickly the system could respond.
A susceptibility score is specific to the method and authority that produced it. WPI does not supply a universal scoring scale or distance that makes a source safe. The operator uses the approved assessment and source-protection plan, then reports changed land use, new facilities, spills, floods, security concerns, or unusual raw-water trends so responsible staff can update decisions.
Scenario: two fuel-related observations
A small properly contained fuel tank is close to a well, while a larger facility farther away has a documented release and lies upgradient within the modeled contributing area. Choosing solely by distance is weak. Review the release evidence, material, containment, groundwater gradient, geology, well construction, pumping conditions, monitoring results, and assessment method. The farther facility may rank higher, but the operator should not announce contamination until appropriate sampling and investigation support that conclusion.
If an operator sees sheen near a surface-water intake or learns of a spill upstream, the first actions come from the emergency and source-water plans: protect people, notify the designated roles, verify location and movement, preserve representative observations or samples as authorized, and consider approved intake or operating options. Do not taste or directly contact unknown material, improvise an unapproved chemical response, or erase the original trend after a repeat result. Record time, weather, source condition, instrument or sample checks, notifications, decisions, and outcomes.
Official source trail
A chemical-storage site appears on a source-water inventory. What does that fact establish by itself?
Which comparison best supports a susceptibility ranking between two potential sources?
An operator is told that a spill occurred upstream of a surface-water intake. What is the strongest initial response?