Consideraciones sobre los efectos de la crisis económica

CCW risk assessment results at the 90th percentile suggest that the management of CCW in unlined or clay-lined WMUs result in risks greater than 1 in 100,000 for excess cancer risk to humans or an HQ greater than 1 for noncancer effects to both human and ecological receptors. Key risk findings include the following:

ƒ For humans exposed via the groundwater-to-drinking-water pathway, risks from clay- lined units that dispose CCW or CCW comanaged with coal refuse are lower than those for unlined units. However, the 90th percentile risks for clay-lined units are still well within or above the range of concern (10-6 to 10-4) for cancer risk and above an HQ of 1 for noncarcinogens. For example, arsenic III cancer risks in clay-lined units range from 1 in 5,000 for landfills to 9 in 10,000 in surface impoundments. The thallium HQ was as high as 2 for clay-lined landfills, and the clay-lined surface impoundment HQ was as high as 200 for cobalt and 4 for boron.

ƒ Arsenic was the constituent with the highest risk for landfills. Clay-lined landfills presented 90th percentile risks above an excess cancer risk of 1 in 100,000 for arsenic (risks as high as 1 in 5,000) and an HQ of 1 for thallium (HQ of 2). When landfills are unlined, they also present risk above an HQ of 1 for antimony and molybdenum, each with an HQ of 2. Here, arsenic cancer risks were as high as 1 in 2,000. Clay-lined FBC landfills also presented 90th percentile risks above and HQ of 1 for antimony (HQ = 3) and thallium (HQ = 4) and showed excess cancer risks of 3 in 50,000 for arsenic.

However, unlined FBC landfills differed in that they only exceeded a 1 in 100,000 excess cancer risk for arsenic.14 At the 50th percentile, arsenic III from CCW codisposed with

coal refuse unlined landfills showed an excess cancer risk of 1 in 50,000: all noncarcinogenic constituents were well below an HQ of 1.

ƒ Arsenic and cobalt were the constituents with the highest risks for surface

impoundments, with risks as high as 1 in 50 and an HQ of 500, respectively, for unlined units. Clay-lined surface impoundments presented 90th percentile cancer risks above 1 in 100,000 for arsenic (7 in 1,000 cancer risk), HQs above 1 for boron (HQs as high as 4), cadmium (HQ as high as 3), cobalt (HQ as high as 200), molybdenum (HQ as high as 5), and nitrate (an MCL-based HQ as high as 10). When surface impoundments are unlined, they also show risk above an HQ of 1 for lead (HQ of 9) and selenium (HQ of 2). Here, arsenic cancer risks are as high as 1 in 50, and cobalt had HQs as high as 500. The only 50th percentile surface impoundment results that exceeded the risk range or HQ criterion were arsenicand cobalt. Here, unlined units had arsenic cancer risks as high as 6 in 10,000 while clay-lined units had arsenic cancer risks as high as 1 in 5,000. Cobalt HQs were as high as 20 and 6 for unlined and clay-lined surface impoundments, respectively. ƒ For the groundwater-to-drinking-water pathway, composite liners, as modeled in this

assessment, effectively reduce risks from all constituents to below a cancer risk of 1 in 100,000 and an HQ of 1 for both landfills and surface impoundments at the 90th and 50th percentiles.

ƒ For the groundwater-to-drinking-water pathway, arrival times of the peak concentrations at a receptor well are much longer for landfills (hundreds or thousands of years) than for surface impoundments (most less than 100 years).

ƒ For humans exposed via the groundwater-to-surface-water (fish consumption) pathway, unlined and clay-lined surface impoundments posed risks above an excess cancer risk of 1 in 100,000 and an HQ of 1 at the 90th percentile. For CCW managed alone in surface impoundments, these exceedences came from selenium (HQs of 3 and 2), while for CCW comanaged with coal refuse these exceedences came from arsenic (3 in 100,000 and 2 in 100,000 excess cancer risks for unlined and clay-lined units). All 50th percentile surface impoundment risks are below a cancer risk of 1 in 100,000 and an HQ of 1. No

constituents pose risks above these risk levels for landfills (including FBC landfills) at the 90th or 50th percentile for the fish consumption pathway.

ƒ Waste type has a much larger effect when wastes are managed in surface impoundments than when they are managed in landfills. In the case of surface impoundments, some constituents (boron, molybdenum, nitrate, and selenium) presented higher risks from CCW managed alone. However, others (arsenic, cadmium, cobalt, and lead) presented higher risks when CCW is comanaged with coal refuse, because of their association with the sulfide minerals concentrated in the refuse.

ƒ The higher risks for surface impoundments than landfills are likely due to higher waste leachate concentrations and the higher hydraulic head from the impounded liquid waste. This is consistent with damage cases reporting wet handling as a factor that can increase risks from CCW management.

ƒ For ecological receptors exposed via surface water, risks for landfills exceed an HQ of 1 for boron (HQ of 281 for unlined and 78 for clay-lined), lead (HQ of 8 for unlined), and selenium, arsenic, and barium (HQs of 2) at the 90th percentile, but 50th percentile HQs are well below 1. For surface impoundments, 90th percentile risks for several

constituents (boron, lead, arsenic, selenium, cobalt, and barium) exceed an HQ of 1, with boron showing the highest risks (HQ over 2,000). Only boron exceeds an HQ of 1 at the 50th percentile (HQ = 7 for unlined surface impoundments). The HQs over 1 for boron and selenium are consistent with reported ecological damage cases, which include impacts to waterbodies through the groundwater-to-surface-water pathway.

ƒ For ecological receptors exposed via sediment, 90th percentile risks exceed an HQ of 1 for both landfills and surface impoundments because certain CCW constituents strongly sorb to sediments in the waterbody. Here, the 90th percentile HQ for lead was 58 for unlined landfills and clay-lined surface impoundments, and 311 for unlined surface impoundments. For arsenic, HQs were 11 and 3 for unlined and clay-lined landfills, and 127 and 55 for unlined and clay-lined surface impoundments. Cadmium had HQs of 5 for unlined landfills, and 30 and 9 for unlined and clay-lined surface impoundments.

Antimony had an HQ of 2 for unlined landfills. Composite lined surface impoundments also had risks above an HQ of 1 for lead (HQ of 4), arsenic (HQ of 31), and cadmium (HQ of 2). The 50th percentile risks are an order of magnitude or more below an HQ of 1 for ecological receptors exposed via sediments.

Sensitivity analysis results indicate that for most of the scenarios evaluated (over 70 percent), the risk assessment model was most sensitive to parameters related to the contaminant source and groundwater flow and transport: WMU infiltration rate, leachate concentration, and aquifer hydraulic conductivity and gradient. For strongly sorbing contaminants (such as lead and cadmium), variables related to sorption and travel time (adsorption coefficient, depth to

groundwater, receptor well distance) are also important.

One of the most sensitive parameters in the risk assessment (infiltration rate) is greatly influenced by whether and how a WMU is lined. The 1994–2004 DOE/EPA survey results (U.S. DOE, 2006) do not include information on how many unlined facilities are still operating today, but do indicate that more facilities are lined today than were in the 1995 EPRI survey data set on which this risk assessment was based. This suggests that the risks from future CCW disposal facilities are likely to be lower than the results presented in this report.

There are uncertainties associated with the CCW risk assessment, but scenario uncertainty (i.e., uncertainty about the environmental setting around the plant) has been minimized by basing the risk assessment on conditions around existing U.S. coal-fired power plants around the United States. Uncertainty in environmental setting parameters has been incorporated into the risk assessment by varying these inputs within reasonable ranges when the exact value is not known. Uncertainty in human exposure factors (such as exposure duration, body weight, and intake rates) has also been addressed through the use of national distributions.

Some uncertainties not addressed explicitly in the risk assessment have been addressed through comparisons with other studies and data sources.

ƒ Appropriateness of CCW leachate data. Data on another highly sensitive parameter, leachate (porewater) constituent concentration, were available and used for CCW surface impoundments. However, available data for landfills were mainly TCLP analyses, which may not be representative of actual CCW leachate. Comparisons with recent (2006 and 2008) studies of coal ash leaching processes show very good agreement for arsenic. However, although the selenium CCW data are within the range of the 2006 and 2008 data, some of the higher concentrations in both Vanderbilt data sets are not represented by the TCLP data, and U.S. EPA (2008c) show similar trends for barium and

molybdenum. This suggests that risks for these metals may be underestimated, which is consistent with selenium as a common driver of the damage cases.

ƒ Impacts of mercury rules (CAIR and CAMR). While CAIR and CAMR will reduce emissions of mercury and other metals from coal-fired power plants, mercury and other more volatile metals will be transferred from the flue gas to fly ash and other air pollution control residues, including the sludge from wet scrubbers. EPA ORD has research

underway to evaluate changes to CCW characteristics and leaching of mercury and other metals from CAIR and CAMR. Data from the first report (U.S. EPA, 2006c) suggest that although total mercury will increase in CCW from the use of sorbents as mercury

controls, the leachability of mercury may be reduced. Data from U.S. EPA (2008c) add to this assessment by supporting similar findings.

ƒ Mercury and nondetect analyses. Because of a high proportion of nondetect values and a limited number of measurements, the risks from mercury in CCW could not be

evaluated for either landfills or surface impoundments and for antimony and thallium in surface impoundments. The 2006 leaching study data suggest that mercury levels are fairly low in fly ash from coal combustion, a conclusion generally confirmed by the 2008 study report (U.S. EPA, 2008c), although that study did find higher mercury leachate concentrations from scrubber sludge than other coal wastes and found that blending fly ash and lime can increase mercury leaching from scrubber sludge.

Uncertainties that are more difficult to evaluate with respect to CCW risk results include the following:

ƒ Well distance. Nearest well distances were taken from a survey of MSW landfills, as data were not available from CCW sites. EPA believes that this is a protective

assumption because MSW landfills generally tend to be in more populated areas, but there are little data available to test this hypothesis.

ƒ Liner conditions. Liner design and performance for CCW WMUs were based on data and assumptions EPA developed to be appropriate for nonhazardous industrial waste landfills. EPA believes that CCW landfills should have similar performance

characteristics, but does not have the quantitative data to verify that.

ƒ Data gaps for ecological receptors. Insufficient data were available to develop screening levels and quantitative risk estimates for terrestrial amphibians, but EPA acknowledges that damage cases indicate risk to terrestrial amphibian and plant communities through exposure to selenium and boron.

ƒ Ecosystems and receptors at risk. Certain critical assessment endpoints were not evaluated in this analysis, including impacts on managed lands, critical habitats, and threatened and endangered species.

ƒ Synergistic risk. The impact of exposures of multiple contaminants to human and ecological risks was not evaluated in this analysis. EPA recognizes that a single- constituent analysis may underestimate risks associated with multiple chemical exposures.

These are potentially the more significant uncertainties associated with the CCW risk assessment. Other uncertainties are discussed in Section 4.4.

Given the results and characterization above, composite liners, as modeled in this risk assessment, effectively reduce risks from all pathways and constituents to levels below an excess cancer risk of 1 in 100,000 or an HQ of 1 for both landfills and surface impoundments. The CCW risk assessment suggests that the management of CCW in unlined landfills and unlined surface impoundments may present risks to human health and the environment. From the perspective of what is known about toxic effects in humans, arsenic, nitrates, cadmium, and selenium appear to be among the constituents that may present risks of concern depending on the specific waste management practices employed. From the perspective of what is known about toxic effects in ecological receptors, arsenic, boron, lead, and selenium emerge as having documented adverse effects on ecological receptors.

The estimated human health arsenic risks from clay-lined units are lower than the risks of unlined units, but are still above a 1 in 100,000 excess cancer risk or an HQ of 1. In addition, surface impoundments typically showed higher risks than landfills, regardless of liner type. These risk results are largely consistent with damage cases compiled by EPA (U.S. EPA, 2000, 2003e, 2007) and others (Lang and Schlictmann, 2004; Zillmer and Fauble, 2004; Carlson and Adriano, 1993; Rowe et al., 2002; Hopkins et al., 2006). These results suggest that with a higher prevalence of composite liners in new CCW disposal facilities, future national risks from onsite CCW disposal are likely to be lower than those presented in this risk assessment (which is based on 1995 CCW WMUs).