Water Safety Concerns at Surface Levels: Potential Risks to Potable Water Supplies
The Hazard Ranking System (HRS) is a crucial tool used to evaluate the potential harm posed by hazardous waste to drinking water sources. This system assesses specific hazardous waste properties such as ignitability, corrosivity, reactivity, and toxicity.
To calculate the HRS waste characteristics score, one must first determine if the waste is hazardous based on the presence of any of the four key hazardous characteristics. For each characteristic present, a corresponding numeric hazard value is assigned according to regulatory guidelines. The overall waste characteristics score is then computed, reflecting the severity of the threat posed to the drinking water.
Toxicity and persistence values for substances can be found in the Superfund Chemical Data Matrix (SCDM). The toxicity values were derived as described in Section 2.4.1.1, pages 51589-51590 of the HRS rule, while the persistence values were derived as per Section 4.1.2.2.1.2, page 51612.
Any target not subjected to actual contamination but within the Target Detection Limit (TDL) is subject to potential contamination. Targets subject to potential contamination are evaluated using dilution weights based on the flow rates of surface water bodies. The formula for calculating the potential contamination factor value is PC = (Wi * i) / 10, where Wi is the population served by the intake and i is the dilution weight.
The quantity value from HRS table 2-6 is based on the sum of the quantities for all sources that have a non-zero containment value against either overland flow or flood. There are two persistence values for each substance in SCDM: one for lakes and the other for rivers, oceans, coastal tidal waters, and the Great Lakes.
The highest product of toxicity and persistence for any substance available to the surface water pathway is determined by the highest product of toxicity and persistence values found in the SCDM. The assigned values for "River" or "Lake" are based on half-life calculations that include hydrolysis, biodegradation, photolysis, and volatilization as loss mechanisms.
When calculating the score for waste characteristics, multiply the value for toxicity/persistence by the value for hazardous waste quantity, cap the product at 10, and find the score for waste characteristics from HRS table 2-7. For example, the toxicity value for PCBs in rivers is 10,000, and the persistence value for PCBs in lakes is 1.
The HRS target value for an intake subject to potential contamination is determined by population served, dilution weight, and potential points (if any). An intake is considered subject to potential contamination if it lies beyond the zone of actual contamination but within the TDL. The best data for average annual stream flow is from USGS gauging stations which are frequently located at dams, bridges, and intakes.
In rivers, oceans, coastal tidal waters, and the Great Lakes, flows and tides are assumed to carry a substance to the target distance limit more quickly than in lakes. If a sampling point meets observed release criteria, including attribution to the site, all drinking water and environmental targets between that point and the Protection Priority Exclusion (PPE) are considered subject to actual contamination. An intake is subject to actual contamination if it lies within the zone of actual contamination defined by the observed release sample.
The principles of including residents, workers, and students and of assessing blended supply systems and standby intakes are the same as they were for the ground water pathway. State designation of usability under Section 305(a) of the Clean Water Act is an easy-to-document resource use that may be overlooked.
For the most accurate and detailed calculation method, refer directly to the EPA's HRS documentation or technical guidance on the Hazard Ranking System, as it provides formulas and values used for scoring waste characteristics for drinking water threats. The cited sources mostly cover hazardous waste determination principles rather than the full score mechanics.
[1] Title 22 CCR 66262.11, which guides hazardous waste determinations by regulators like the DTSC (Department of Toxic Substances Control)
- The HRS target value for an intake subject to potential contamination is determined by the population served, dilution weight, and potential points (if any).
- Any target not subjected to actual contamination but within the Target Detection Limit (TDL) is subject to potential contamination.
- For the most accurate and detailed calculation method, refer directly to the EPA's HRS documentation or technical guidance on the Hazard Ranking System.
- The highest product of toxicity and persistence for any substance available to the surface water pathway is determined by the highest product of toxicity and persistence values found in the SCDM.
- The quantity value from HRS table 2-6 is based on the sum of the quantities for all sources that have a non-zero containment value against either overland flow or flood.
