Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationcoal refuse unlined landfills showed an excess cancer risk of 1 in 50,000: allnoncarcinogenic constituents were well below an HQ of 1.• Arsenic and cobalt were the constituents with the highest risks for surfaceimpoundments, with risks as high as 1 in 50 and an HQ of 500, respectively, for unlinedunits. Clay-lined surface impoundments presented 90th percentile cancer risks above 1 in100,000 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 only50th percentile surface impoundment results that exceeded the risk range or HQ criterionwere arsenicand cobalt. Here, unlined units had arsenic cancer risks as high as 6 in10,000 while clay-lined units had arsenic cancer risks as high as 1 in 5,000. Cobalt HQswere as high as 20 and 6 for unlined and clay-lined surface impoundments, respectively.• For the groundwater-to-drinking-water pathway, composite liners, as modeled in thisassessment, effectively reduce risks from all constituents to below a cancer risk of 1 in100,000 and an HQ of 1 for both landfills and surface impoundments at the 90th and 50thpercentiles.• For the groundwater-to-drinking-water pathway, arrival times of the peak concentrationsat a receptor well are much longer for landfills (hundreds or thousands of years) than forsurface impoundments (most less than 100 years).• 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 of1 in 100,000 and an HQ of 1 at the 90th percentile. For CCW managed alone in surfaceimpoundments, these exceedences came from selenium (HQs of 3 and 2), while for CCWcomanaged with coal refuse these exceedences came from arsenic (3 in 100,000 and 2 in100,000 excess cancer risks for unlined and clay-lined units). All 50th percentile surfaceimpoundment risks are below a cancer risk of 1 in 100,000 and an HQ of 1. Noconstituents pose risks above these risk levels for landfills (including FBC landfills) atthe 90th or 50th percentile for the fish consumption pathway.• Waste type has a much larger effect when wastes are managed in surface impoundmentsthan when they are managed in landfills. In the case of surface impoundments, someconstituents (boron, molybdenum, nitrate, and selenium) presented higher risks fromCCW managed alone. However, others (arsenic, cadmium, cobalt, and lead) presentedhigher risks when CCW is comanaged with coal refuse, because of their association withthe sulfide minerals concentrated in the refuse.• The higher risks for surface impoundments than landfills are likely due to higher wasteleachate 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 increaserisks from CCW management.April 2010–Draft EPA document. 4-58