2004-01-29_REVISION - M1987113 (2)

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Final OU 11 Combined Phases RFI/RI environmental media. The PCOCs aze taken from Tier 1 of the ecological contaminant of concern (ECOC) screening methodology and are derived from the "Gilbert" approach as employed for the human health risk assessment (Gilbert and Simpson, 1992). Subsequent steps of the EPA methodology are more detailed and are aimed at refining risk estimates and determining site-specific cleanup goals. These steps aze performed only if the screening-level risk assessment indicates significant risk. If none of the PCOCs is present at ecotoxic concentration, the site is considered to present de minimis risk and a more detailed evaluation of the toxicity and exposure or a quantitative risk assessment is not warranted (EPA, 1994). 6.2.1 Step 1 -Preliminary Problem Formulation and Ecological Effects Characterization The preliminary problem formulation focuses on defining the following elements: • environmental setting • nature and extent of contamination • potential receptors and exposure pathways 6.2.1.1 Environmental Setting The environmental setting for OU 11 is described in Section 1.2 and in the ERAM Technical Memorandum No. 2 - Sitewide Conceptual Model (EG&G, 1995d). OU 11 (IHSS 168) covers approximately 105 acres west of the main industrial area of RFETS (Figure 1.2-1). As described in Section 1.2, water from SEPs 207B North and 207B Center was sprayed on sections of OU 11 to reduce volume through evaporation. Water was sprayed primazily bn three areas totaling approximately 14 acres. Water from the SEPs contained organic compounds, metals, and radionuclides that may have been deposited in spray areas of OU 11. The biota in OU 11 exhibited no sign of stress associated with the spray areas or azeas where sprayed water had pooled. As presented in Appendix E, which summarizes results of the OU I1 ecological data collection and analysis, no vegetative stress was evident from casual observation or from vegetation community data collected in 1993. Thus, the motivation for the OU 11 ERA was "source-driven" with a possible contaminant source (the spray areas) but no obvious ecological effects upon which to focus-the investigation. The entire site is occupied by xeric and mesic mixed grassland typical of Rocky Flats (EG&G, 1995d). There aze no persistent water bodies, wetlands, or riparian azeas within IHSS 168. A narrow riparian zone exists along the North Walnut Creek drainage just north of the IHSS (EG&G, 1995d). Flows in the creek bed of this section tp~2509072~sec6.doc 6-3 6/8/95