An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »


Section 5: Waste Characteristics

Waste Characteristics Factor Category

5.1 Calculating Substance - Specific Factors

Substance-specific factors are the chemical characteristics of the hazardous substances found at a site that are available to migrate from the site sources. These chemical characteristics include toxicity (human or environmental), mobility, persistence, and bioaccumulation potential. The product of these factors is referred to as the "combined factor value," which will allow the scorer to select the single most potentially hazardous substance at the site, which will in turn be used in scoring Waste Characteristics. The following formulas show how the product of substance-specific factors are calculated in each pathway:

Ground Water

  • Toxicity  x  Ground Water Mobility

Surface Water1 Toxicity x Persistence

  • Drinking Water:    Toxicity  x  Persistence
  • Human Food Chain:    Toxicity  x  Persistence  x   Bioaccumulation (FC)
  • Environmental:    EcoToxicity  x  Persistence  x  Bioaccumulation (EN)
  • Ground Water to Surface Water:    Multiply Ground Water Mobility to the above

Soil Exposure

  • Toxicity


  • Toxicity  x  Air Mobility

1 If surface water is threatened, check the possibility of polyaromatic hydrocarbons, heavy metals, and pesticides. They tend to have high bioaccumulation values.

The Hazard Ranking System (HRS) Rule, Section 2.4.1, page 51589, contains a rather complex method for determining the toxicity factor value for a substance. Pre-calculated values for toxicity, as well as mobility, persistence, and bioaccumulation factors, can be obtained from the Superfund Chemical Data Matrix (SCDM). SCDM has calculated factors for roughly 300 of the most commonly evaluated substances, significantly reducing the time it takes to evaluate hazardous substance characteristics. Even so, additional, non-substance-specific information may be needed to identify the appropriate value from SCDM. For example, ground water mobility values are presented for "karst" and "non-karst" aquifers for substances in a "liquid" or "non-liquid" state. As each pathway is reviewed in this training material, obtaining the correct factor values from SCDM will be discussed. If a chemical value is not in SCDM, contact EPA. Do not try to work out the value yourself from the detailed instructions in the HRS unless EPA first approves.

5.2 Calculating Hazardous Waste Quantity

Hazardous Waste Quantity (HWQ) estimates the total quantity of waste containing hazardous substances at the site. However, wastes associated with a source with a containment factor of zero for the pathways are not included in the HWQ factor evaluation for that pathway. Thus, HWQ values may vary across pathways because a source with non-zero containment for one pathway (ground water, for example) may have zero containment for another pathway (air, for example). In addition, the HWQ is almost always different for the soil exposure pathway because additional evaluation restrictions apply to this pathway.

Information covered in the section includes:

The HWQ factor evaluation is complex due to the limitations in the data available on waste quantities at most sites and the resulting need to maximize use of surrogate measures. The factor is designed to use whatever information is available, with preference for higher-quality information, if complete. The first step in determining the HWQ factor is the hierarchical evaluation of the information available on a site.

The evaluation of the HWQ factor for the migration pathways begins with the assignment of hazardous substances and hazardous wastestreams to the sources at the site based on the available information. The HRS provides for a surrogate "unallocated source" for purposes of evaluating this factor to computationally account for material that cannot be allocated to identified sources. In evaluating the soil exposure pathway, only those hazardous substances and wastestreams containing such substances that meet the criteria for observed contamination are included in the HWQ calculation. In all pathways, the unallocated source is considered to have a containment value greater than zero.

The Four Tiers

For each pathway, the source hazardous waste quantity value for each source having a non-zero containment value is evaluated using four hierarchical tiers of information. First, Tier A (Hazardous Constituent Quantity) information is used. If the hazardous constituent quantity is either not available or not adequately determined, then Tier B (Hazardous Wastestream Quantity) information is used. Similarly, if the hazardous wastestream quantity is either not available or not adequately determined and the source in question is not the unallocated source, then Tier C (Volume) information is used, whenever available. Tier D (Area) information is used if, and only if Tier C information is not available. In other words, in the HRS evaluation of a source, move down through tiers, assigning values based on the available information, stopping as soon as the information is adequately determined (Tiers A and B) or available (Tier C).

Once the source hazardous waste quantity value has been evaluated for each source or area of observed contamination (including as applicable, the unallocated source), the hazardous waste quantity value is calculated as the rounded sum of the source (or area of observed contamination) values and assigned a factor value using Table 2-6.

Tier A, Hazardous Constituent Quantity, is the first tier of information for a source to be evaluated by the scorer. This tier represents the amount of CERCLA hazardous substances (as defined in CERCLA Section 101(14), as amended) allocated to the source (or area of observed contamination) subject to certain restrictions for RCRA hazardous wastes (see HRS Section Tier B, Hazardous Wastestream Quantity, is the second tier (Tier B) of information for a source to be evaluated by the scorer. This tier represents the amount of material containing CERCLA hazardous substances allocated to the source, together with the amount of any other CERCLA pollutants and contaminants that are allocated to the source (or area of observed contamination). The total mass of the wastes subject to the RCRA restrictions for Tier A are included in the Tier B evaluation. Tier C, Volume, is based on the actual volume (or capacity) of the source as differentiated from the volume of material placed in the source or currently contained in the source. Tier D, Area, is based on the area of the source (or area of observed contamination).

The following is an example of the hierarchical process of evaluating a source hazardous waste quantity:

  • Consider a lagoon that is known to have received wastewater from a distant plant consisting solely of 100,000 gallons of water containing 153 pounds of 1,2-dichloroethylene (no other hazardous substances). This Tier A information (i.e., 153 pounds of 1,2-DCE) adequately determines the hazardous constituent quantity and would fully determine the source hazardous waste quantity value (as evaluated using Table 2-5). It would not be appropriate to proceed further through Tiers B, C, and D.
  • If, however, the quantity of 1,2-DCE were not precisely known (e.g., the material contained at least 153 pounds of 1,2-DCE) then hazardous wastestream quantity (Tier B) would be evaluated, as well as Tier A, based on the 100,000 gallons of material (wastewater) containing a CERCLA hazardous substance (1,2-DCE). Since, in this case, the hazardous wastestream quantity is adequately determined, the source hazardous waste quantity value would be determined based on the maximum of the Tier A and Tier B values (evaluated using Table 2-5).
  • If neither the quantity of 1,2-DCE nor the quantity of wastewater were adequately determined, then Tier C or Tier D would be evaluated based on the information available on the dimensions of the lagoon. If length, width, and height estimates are available (such as from on-site measurements) then the source volume (Tier C) would be calculated and the source hazardous waste quantity value would be evaluated as the highest of the Tier A, Tier B, and Tier C values (evaluated using Table 2-5). If the height estimates were lacking, making the calculation of a volume infeasible, (as would be the case if aerial photographs were used to estimate the dimensions) then the source area (Tier D) would be calculated and the source hazardous waste quantity value would be evaluated as the maximum of the Tier A, Tier B, and Tier D.

The information in each of the four tiers, however, are measured in different units. Tier A information is in terms of pounds of CERCLA hazardous substances; Tier B information is in terms of pounds of wastestream mass (a wastestream contains more than just the CERCLA hazardous substance); Tiers C and D are measured in terms of volume (cubic yards or gallons) and area (square feet), respectively.

It is also important to clearly understand how Tier B information differs from Tier C information. Waste information is frequently expressed in volumetric units, for example, gallons of liquid waste, cubic yards of smelter wastes, or number of drums. Such information is Tier B information, not Tier C. HRS Table 2-5 provides default conversion factors to be used when site-specific data are unavailable in order to convert Tier B volume information into Tier B mass information: 1 ton=2,000 pounds=1 cubic yard=4 drums=200 gallons. To re-emphasize, Tier B information includes waste volume that can be converted to mass units, Tier C information is restricted to the volume of the source (not the volume of the wastes placed in the source).

Because the HWQ factor is evaluated in terms of the total quantity of hazardous substances at the site, it is necessary to convert the information in the tiers into consistent units. HRS Rule, Table 2-5, page 51591 performs the conversion using divisors that are specific to each tier and source type. The divisors are based on a number of waste sites that have been studied in detail for total mass of pure CERCLA hazardous substances. For example, a study was conducted and it was estimated that the average concentration of a hazardous substance in a wastestream for CERCLA sources was 200 ppm. So, for every 5,000 lbs. of wastestream, there is 1 lb. of hazardous substances. Thus a divisor of 5,000 is used in Tier B.

(The soil exposure pathway uses an equivalent to Table 2-5 - Table 5-2 of the HRS Rule (page 51647). This is discussed in further detail in Section 18 on the soil exposure pathway.)

Top of Page

Review of Tier A:    Hazardous Constituent Quantity

Tier A evaluations directly employ estimates of the quantity of CERCLA hazardous substances in a source (within the restrictions discussed in HRS Section for RCRA hazardous wastes). As such, they are the most appropriate basis for use in determining HWQ. Unfortunately, Tier A data are frequently incomplete, and sufficient information to "adequately determine" Tier A is rarely available. Examples of Tier A estimates include:

  • spills of known quantity:    a spill of 1,000 lbs. of benzene
  • spills of known volume and composition:    a spill of 100 liters of benzene can be assigned a Tier A value of about 193 pounds (100 liters * 876.5 g-benzene/l-benzene ÷ 454 grams/pound)
  • concentration-based estimates:    a lagoon known to have received 100,000 liters of wastewater containing an average concentration of benzene of 100 mg/l can be assigned a Tier A estimate of about 22 pounds (100,000 liters * 100 mg-benzene/l-wastewater * 10-3 g/mg ÷ 454 grams/pound)

In the third case, concentration data quality considerations (e.g., representativeness, accuracy, and precision) become very important, as does the restriction regarding the constituents of RCRA hazardous wastes. EPA should be contacted prior to using concentration data to estimate Tier A values.

As stated in SHRS Rule, Section, page 51590, if the available information allows the hazardous constituent quantity (Tier A) to be adequately determined, a value of zero is assigned to Tiers B-D. Recall that if higher quality information is complete and accurate, the hierarchical evaluation process stops because there is no reason to evaluate lesser quality information if better information can be used. However, if Tier A cannot be adequately determined, a Tier A value is assigned based on available information (because information is available, does not necessarily imply that it can be adequately determined, e.g., it may be incomplete), and the evaluation process then proceeds to Tier B. An explicit definition of "adequately determined" (for purposes of Tier A only) is provided in the HRS Guidance Manual, Section 6.2, page 91. Section 6.2 of the HRSGM also provides general guidance on evaluating Tier A.

Top of Page

Review of Tier B:    Hazardous Wastestream Quantity

Tier B information, which is used when Tier A data are not adequately determined, represents the quantity of the wastestreams that went into the source. The wastestreams include not only CERCLA hazardous substances but also CERCLA pollutants and contaminants and the waste material that contains them.

As seen in the HRS Rule, Table 2-5, page 51591, the Tier B divisor is equal to 5,000. This number is based on investigations of the average concentration of hazardous substances in many wastestreams sampled at NPL sites.

The following is an example of an evaluation using Tier B:

  • Available records indicate that a surface impoundment received 200 gallons of electroplating waste per hour, 40 hours per week, 50 weeks per year over a period of 10 years. The average concentration of hazardous substances in the waste cannot be determined from the records and the available sampling data are not considered sufficiently representative to calculate an average. Thus, no Tier A evaluation can be performed.
  • A Tier B estimate can be developed as follows:   200 gallons/hour * 40 hours/week * 50 weeks/year * 10 years = 4 million gallons. Since a site-specific density is not available, use the conversion factor provided in HRS Rule, Table 2-5, page 51591 to calculate the hazardous wastestream quantity as 40 million pounds (4 million gallons * 2000 pounds/ton ÷ 200 gallons/ton). Tier B is then assigned a value of 8,000 (40,000,000/5,000) using the Table 2-5 equation.
  • If the records are found to be reasonably complete and accurate, then Tier B would be "adequately determined" and Tiers C and D assigned values of zero. Otherwise, Tier C (and possibly D) would be evaluated.

Just like for Tier A, an explicit definition of "adequately determined" (for purposes of Tier B only) is provided in the HRS Guidance Manual, Section 6.3, page 99. Section 6.3 of the HRSGM also provides general guidance on evaluating Tier B.

Top of Page

Review of Tier C:    Volume

Tier C information represents the volume of a source. In other words, it is based on the capacity, not the actual contents of the source. Because it is a lower quality source of information for measuring HWQ, Tier C is evaluated only if Tiers A and B data are not adequately determined.

Volume is measured in units of cubic yards, except for drums which are measured in terms of gallons. In estimating the volume, use the dimensions of the source. If the dimension of the source are unavailable, the engineered capacity of certain types of sources (such as landfills or surface impoundments) can be used when such information is available. For source types that are not specifically engineered (such as piles and quarries), use the maximum volume unless there is evidence that the source was never completely filled. In this instance, the scorer should attempt to estimate the level at which the waste accumulated, and then calculate volume based on this level. After the volume is determined, assign a value for the source from the HRS Rule, Table 2-5, page 51591.

If the volume measure can be calculated, assign a volume measure and assign a value of zero for Tier D. If it cannot be calculated, assign a value of zero for Tier C and move on to evaluate Tier D.

Section 6.4 of the HRS Guidance Manual provides guidance in evaluating Tier C.

Top of Page

Review of Tier D:    Area

Tier D evaluates the area of a source. It is used only if the source was not assigned a Tier C measure and if Tiers A and B were not adequately determined. Because Tier D measures surface area only, it implicitly assumes a default depth for each source in the Table 2.5 equations. Do not evaluate Tier D if the actual depth of the source can be estimated. Evaluate Tier C instead.

Area is measured in units of square feet for all source types. In estimating the area, use the aerial (surface area) dimensions of the source. This area is also known as the footprint of the source. After the area is determined (expressed in square feet), assign a value for the source from the HRS Rule, Table 2-5, page 51591.

When trying to obtain records of source areas, the HRS Guidance Manual, Section 6.5, Subsection D1 provides several suggestions on the types of information that may provide accounts of the areas.

Top of Page


The following example illustrates a calculation employing all four tiers.

A chemical plant pipe ruptured spilling at least 1,500 gallons of process fluids over 200 square feet of soil contaminating it to a depth of 10 feet. Records indicate that the process fluid included one pound of ethylene glycol and an unknown quantity of other glycol ethers.

  • Tier A:    Based on the presence of 1 lb. of a CERCLA hazardous substance, a hazardous constituent quantity value of 1 is assigned. The presence of other glycol ethers of unknown quantity indicates that Tier A is not "adequately determined," Tier B should be evaluated.
  • Tier B:    At least 1,500 gallons of material containing CERCLA hazardous substances was spilled. Using the conversion factors from HRS Table 2-5, this is equivalent to 15,000 lbs. of material, yielding a hazardous wastestream quantity value of 3 (15,000/5,000). Because the actual volume of the spilled material is not known, Tier B is also not "adequately determined", Tier C or D should be evaluated.
  • Tier C:    The source in question is a contaminated soil source. The dimensions of the contaminated soil are 200 ft2 x 10 ft, yielding a volume of 2,000 ft3or about 74 yd3. Using Table 2-5, the source volume value would be about 0.03 (74/2,500). Since a Tier C value could be estimated, no Tier D calculation is required.
  • Tier D:    If the depth of the contaminated soil was not known, then a Tier D value of 0.006 (200/34,000) would be assigned based on the area of contaminated soil.

After evaluating all appropriate tiers, make a list of the value(s) for each tier. Select the highest value among all the tiers. This value is the source hazardous waste quantity value. The HRS Guidance Manual, Section 6.6, Highlight 6-9 provides a typical calculation for the source hazardous waste quantity value.

To arrive at the HWQ for the migration pathway, sum the applicable source HWQs for that pathway, rounding to the nearest integer. However, if the sum is greater than 0, but less than 1, round it to 1. Based on this value, select a hazardous waste quantity value for the migration pathway using the HRS Rule, Table 2-6, page 51591.

Table 2-6 of the HRS Rule has very broad ranges, into which the sum of the source HWQs will usually fit comfortably. If this is the case, there is no need to labor over the precision of data. However, if a reasonable error in measurement could move the sum of the source HWQs across a breakpoint, extreme care should be taken in calculating and documenting the HWQ.

Top of Page

Minimum Factor Values

If Tier A is not adequately determined for all sources, then the pathway HWQ may receive a misleadingly low value for quantity from Table 2-6 simply because of the lack of information about what was deposited at the site. To mitigate this situation, a minimum factor value is applied to compensate for a low hazardous waste quantity value due to a lack of waste information about the sources. Since Tier A information is rare at CERCLA sites, the minimum factor value almost always applies.

The HRS Rule, Section, page 51591, provides the rules for calculating the minimum factor values. They are summarized as follows:

If Tier A is not adequately determined for any source at a site, then a minimum HWQ value of 100 is assigned for each migration pathway if:

  • A target in the pathway is subject to actual contamination; or
  • A qualifying removal has taken place and the quantity before the removal was 100 or greater.

Otherwise, assign a minimum factor value of 10 to the pathway. If Tier A is not adequately determined for any area of observed contamination, then a minimum HWQ value of 10 is assigned in the soil exposure pathway.

5.3 Calculating Waste Characteristics

When the most hazardous substance at the site is determined, its combined factor value is calculated, and the HWQ factor value calculated, the Waste Characteristic (WC) factor is calculated using the HRS Rule, Section 2.4.3, page 51592.

The combined factor value for the most hazardous substance at the site is multiplied by the HWQ factor value. Based on this product, a Waste Characteristics factor category value is assigned to the pathway using Table 2-7. The assignment structure of Table 2-7 prevents the Waste Characteristics factor category from dominating the HRS score which could otherwise occur due to the wide range of hazardous substance toxicities, the large degree of uncertainty about waste quantities at a site, and the extreme variation in waste quantity between sites. Table 2-7 compresses the WC factor category value so that it does not range over several orders of magnitude. In addition, the WC factor category value is subject to a maximum of 100, except when the bioaccumulation potential factor value (BPFV) is considered in two of the surface water pathway threats. In this case, the procedure for determining the WC factor value is a bit more complex and the maximum value is 1,000.

Here is an example using the Ground Water pathway:

  1. It has been determined that the most hazardous substance at the site for the ground water pathway is arsenic (from SCDM, Toxicity = 10,000 and Mobility = 1).
  2. Multiply Toxicity x Mobility = 10,000. This is the combined factor value.
  3. It has also been determined that the HWQ factor value for the pathway is 100 (from Table 2-6).
  4. Multiply the combined factor value (10,000) x HWQ factor value (100) = 1,000,000.
  5. Waste characteristics factor value = 32 (from Table 2-7).

5.4 Questions and Answers

How is it decided which tiers to use to evaluate the hazardous waste quantity for a source?

Start at Tier A and determine if there is enough good quality data to evaluate it. If so, determine if the data accounts for all hazardous substances in the source. If they do, stop there. If they do not, continue through the other tiers until one is "adequately determined" or Tier C or D are evaluated.

What if the most hazardous substance at a site is in the smallest source?

The most hazardous substance at a site does not have to exist in every source.

What do you do if the hazardous waste quantity at your site falls close to one of the breakpoints on Table 2-6?

If the site hazardous waste quantity is close to a breakpoint, determine if a small change will make any difference in the site score. If so, be especially careful of the data quality and documentation and determine if the minimum values apply.

Navigate to another section of the course:

        Table of Contents      
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20