Human and Ecological Risk Assessment - Earthjustice

Appendix CSite DataParameterArea aTable C-8. Model Lake Used in CCW Risk AssessmentWater column depth (dwc) aHydraulic residence time (HRT)Annual flow mixing volume13 acres9 feeta Source: Shipman City Lake, IL (Jones et al., 1998).ValueRandom, triangular distribution:Minimum = 1 monthMean = 6 monthsMaximum = 24 months= (Area × dwc) / HRTC.6.3 Water Quality DataSurface water temperature, total suspended solids (TSS), and pH data were collected byUSGS hydrologic region from the STORET database. EPA’s STORET system is the largestsingle source of water quality data in the country. The Legacy STORET database contains over275 million analyses performed on more than 45 million samples collected from 800,000 stationsacross the United States for the period 1960 through 1998. STORET can be accessed from theWeb at http://www.epa.gov/STORET.STORET water quality data are notoriously “noisy” because they are influenced byhydrology, point sources, nonpoint sources, stream/lake morphology, and varying data quality.The following issues in using STORET data must be considered before using the data:• Not all of the data have undergone rigorous QA/QC.• STORET site locations can be biased, especially to known “problem” waters.• The sample times are often at critical periods, such as summer low flows.Statistical analysis techniques were employed taking into account the above issues(including coordination with gage statistical analysis and Reach Files, the use of median valuesto avoid bias in central tendency estimates, and specification of a minimum number ofmeasurements to estimate median values). As a result of these techniques, which can be thoughtof as extracting the underlying “signal” of water quality from the inherent “noise” of waterquality data, the above issues were manageable.Surface water temperature data were collected as median values for each hydrologicregion. These data are shown in Table C-9 along with the number of the modeled CCW plants ineach region.April 2010–Draft EPA document. C-17