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r C) FORMER AMERICAN FOREST PRODUCTS 5.0 Modeling to estimate Impact of Soil Closure Summary Report Contamination� on Groundwater{ URS Project 17322736 July 2009 Page 5-1 5.0 MODELING TO ESTIMATE IMPACT OF SOrL CONTAMINATION ON GROUNDWATER Model simulations using VLEACH software(CH2M Hill, 1990)were used to estimate the impact of remaining soil contamination on groundwater quality "—"a-�-� p' the 1� 9 y(Dames&Moore, 1994);impact,estxmated_by ma_deiing was�negligible.VLEACH modeling was based on nn phthalene the most mobile constituent of TPHd, and conservatively neglected the effects of biodegradation. A period of 25 years using three groundwater level scenarios was modeled.Depths to groundwater of 35 feet bgs and 70 feet bgs were considered the extreme ranges, and an average depth of 54 feet bgs was considered the most likely long- j F0termaverage water level depth. The model predicted that after 25 years of water levels at an average --- depth of 35 feet bgs,the naphthalene concentration in groundwater would reach 36.5 micrograms per liter (gg/L), while at the more likely average water-level elevation of 54 feet bgs, a naphthalene concentration of 2.98 gg/L would be reached after 25 years. This concentration is below the proposed water quality goal j for the site of 20 gg/L, which is the EPA Suggested No Adverse Response Level(SNARL) for naphthalene. Based on that modeling result, soils remaining at the site would not constitute a designated waste,provided average water levels remained at an average depth of about 54 feet bgs. Water levels measured during the June 2009 groundwater monitoring event ranged from about 46 to 49 feet bgs. The j average depth to groundwater in DMW-1 for the period of record from August 1990 through June 2009 is ' r— 46 feet bgs(Figure 5). I. r b K ftrocmU273Mmericaa ForMs Closur�S�ary-Report-RevOl.doc y }