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SDMS Document ^ m ciaren ctrC, Inc. 115591 May 16, 2000 Ms. Caroline Kwan Remedial Project Manager U. S. Environmental Protection Agency - Region II Emergency Remedial Response Division 290 Broadway, 20* Floor New York, NY 10007-1866 RE: VIRGIN ISLAND CHEMICAL SITE ST. CROIX, U.S. VIRGIN ISLANDS REVISIONS TO CALCULATION OF SITE-SPECIFIC DILUTION ATTENUATION FACTOR Dear Ms. Kwan: On behalf of Berlex Laboratories, Inc. (Berlex) and Pharmacia & Upjohn (P&U[), McLaren/Hart, Inc. (McLaren/Hart) is submitting this letter in response to comments on the calculation of a Site-specific Dilution Attenuation Factor (DAF) for the Virgin Island Chemical Site located in St. Croix, U.S. Virgin Islands. McLaren/Hart submitted correspondence on January 19, 2000 that presented the technical basis and input parameters used in the calculation of a Site-specific DAF. This value is to be used in conjunction with the U.S. Environmental Protection Agency's (EPA's) Generic Soil Screening Levels (SSLs) in the derivafion of soil cleanup criteria for the Site. EPA forwarded comments to this submittal on January 27, 2000. Specifically, EPA raised concerns with the technical approach of accounting for an impermeable surface cover in the calculation of an infiltration rate for the Site. Therefore, this input parameter has been revised from the January 19, 2000 submittal as follows: • Infiltration Rate (I): The infiltration rate on St. Croix varies from 1% to 6% (Robison 1972) of the total rainfall, which averages 44 inches/year. Based on empirical data for the island, one of Robison's principal findings presented in the 1972 Ground Water in Central St. Croix, is that groundwater recharge is estimated to be 3% of the rainfall. As discussed with and agreed to by CDM Federal Programs Corporation (CDM Federal) during a conference call held in February 2000, a value of 4.5% (mid-point between Robison's value of 3% and the "high end" value of 6% for the island) is being used for the infiltration rate. Therefore, the revised Site-specific infiltration rate is |0.050 meters/year. Based on this revision to the infiltration rate, the Site-specific jrevised mixing zone depth (d) is 18.18 metersi and the [revised DAF is 9.42|. The calculation worksheet is provided as Attachment I. DAF final.doc 25 Independence Boulevard, Warren, New Jersey 07059 908.647.8111 • 908.647.8162 fax • www.mclaren-hart.com 305321 o recycled paper Ms. Kwan May 16, 2000 As you are aware, McLaren/Hart collected additional Site-specific hydrogeologic information during the implementation of the soil vapor extraction (SVE)/air sparging (AS) pilot-scale study. Specifically, slug testing was conducted at shallow monitoring wells MW-6 and MW-11, and at deep monitoring wells MW-8, MW-12, and MW-15 (replacement well for P-1) to evaluate hydraulic conductivity values across the Site. This information is being submitted as part of the Addendum to the February 2000 Final Remedial Investigation Report for the Site. However, since one of the input parameters to the calculation of a Site-specific DAF is aquifer hydraulic conductivity (K), slug test evaluations for shallow wells MW-6 and MW-11 are provided for reference as Attachment II to this submittal. In the January 19, 2000 submittal, (K) was derived from the slug test conducted at shallow monitoring well MW-2 (3.19 feet/day or 355 meters/year). Using the most recently collected data from other shallow monitoring wells onsite, in conjunction with the data from MW-2, an average (K) can be calculated; thus, a representation of hydraulic conductivity across the Site. The geometric mean of MW-2 (3.19 feet/day), MW-11 (1.83 feet/day), and MW-6 (18.14 feet/day) is 4.73 feet/day => 526 meters/year. Using this average (K), the Site-specific mixing zone depth is calculated as 17.54 meters; the DAF 13.04. The calculation worksheet is provided in Attachment II. To remain conservative in the derivation of soil cleanup criteria for the Site, Berlex and P&U proposes to use a Site-specific DAF of 10 for the Virgin Island Chemical Site. If you have any questions regarding this submittal, please do not hesitate to contact either of us at (908)647-8111. Very truly yours. Kij/T/2^ f^cxM^ M^/jovvvxi^cy/r ^-cO-it -• Perry Katz Norma Eichlin Managing Environmental Scientist Supervising Engineer cc: Terry Grimmer - Berlex Laboratories, Inc. Kevin Walsh - Pharmacia & Upjohn Bill Gierke - Pharmacia & Upjohn Jose Agrelot - ERTEC Syed Syedaii - Virgin Islands Department of Planning and Natural Resources Pam Philip - CDM Federal Programs Corporation DAF final.doc 305322 ATTACHMENT I •05323 Revised Calculation of Site-Specific Dilution Attenuation Factor Virgin Island Chemical Site St. Croix, U.S. Virgin Islands Parameter Value Used in Calculation Units Mixing Zone Depth Mixing zone depth (d) Source Length (L) Infilitration Rate (I) Aquifer Hydrualic Conductivity (K) Aquifer depth (da) Hydrualic Gradient (ig) ' • 18.18 152 0.050 355 20 0.01 meters meters meters/year meters/year meters meter/meter Dilution Attenuation Factor Dilution Attenuation Factor (DAF) 9.42 umtless DAF_fmal.xls (K=3.19) 305324 AQTESOLV Input Data - Shallow Monitoring Wells Virgin Island Chemical Site St. Croix, U.S. Virgin Islands Well ID: Test Type: Test Number: MW-6 Rising Test 1 MW-11 Rising Test 4 Casing Diameter (in) Borehole Diameter (in) Total Depth (ft)* Static Depth to Water (ft) 4 8 26.7 14.2 4 8 40 24.86 Ho (ft) Re (ft) Rw(ft) D(ft) L(ft) H(ft) 2.265 0.167 0.333 66 9.3 12.5 2.877 0.167 0.333 10 10 15.14 *: Referenced from top of inner casing in: inches ft: feet slug_shallow.xls (Inputdata) 305325 ATTACHMENT II 305326 M c L a r e n / H a r t , Inc Client: I s l a n d C h e m i c a l L o c a l i o n : S t . C r o i X MW-6 Rising Head Slug Test c 01 E o K! "S. 10. 1. 0.1 0.01 I I I I I I I I I I I I I I I I I I I I I I I I I 11 I I I j I I I I I I I 11 I I I I I U o o o o o o o o - Q O O l I I I I I I I I I I I I I 11 I I l \ I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 0 . 1 . 2 2 . 4 3 . 6 4 . 8 6 T i m e ( i n l n ) DATA SET: rTw6. d at 0 5 / 0 6 / 0 0 AQUI FER TYPE: Un c o n f I n e d SOLUTI ON IVIETHOD: Bo u wc r - Ri c e TEST DATE: 2 / 9 / 0 0 OBS. VVELL: MW 6 ESTI IWIATED PARAMETERS: K = 0 . 0 1 2 6 t t / m i n yO = 4 . 2 4 6 f t TEST DATA: HO = 2 . 2 6 5 t t r e = 0 . 1 6 7 f t r w = 0 . 3 3 3 t t L = 6 . 3 f t b = 6 6 . f t H = 1 2 . 5 f t McLaren / H a r t , Inc. 1. 0.9 0.8 0.7 0.6 o lid \ 0.5 K 0.4 0.3 0.2 0.1 0. 0. =—1—1- = 1 1 Dl MW- -L-Lmii 1 M i n i 0.1 11 Rising ] 1 l l l l l l l l 1 1 1 l l l l l l l l 1 1 1. T i m e ( m i n ) Client: I s l a n d C h e m i c a l L o c a l I o n : S t . C r o i X Head Slug Test H l l M I I I l l l l l l l l l I I I l l l l l l I I I I I I l l l l l l l l l 1 I I I I I I l l l l l l l l ff 1 0 . t o o . DATA SET: mwl 1 . d a t 0 5 / 0 8 / 0 0 AQUI FER TYPE: C o n f i n e d SOLUTI ON METHOD: C o o p e r et a 1 . TEST DATE: 2 / 1 0 / 0 0 OBS. WELL: MW 11 ESTI MATED PARAMETERS: T = 0 . 0 1 2 7 1 f t 2 / ml n 8 = 0 . 0 0 0 1 TEST DATA: HO = 2 . 8 7 7 f t r e = 0 . 1 6 7 f t r w = 0 . 3 3 3 f t Summary of Slug Test Data and Results - Shallow Monitoring Wells Virgin Island Chemical Site St. Croix, U.S. Virgin Islands Well ID MW-6 Rising MW-11 Rising Input Parameters * Ho (ft) Re (ft) Rw (ft) D (ft) L (ft) H (ft) 2.265 0.167 0.333 66 9.3 12.5 2.877 0.167 0.333 10 10 15.14 Computed Parameters T(ft^/min) K(ft/min) K (ft/day) K (cm/s) not calculated 1.260E-02 18.1440 6.300E-03 1.271E-02 1.271E-03 1.830 6.355E-04 *: Analytical Program: AQTESOLV, Duffield and Rumbaugh -1989 Ho; Initial drawdown in test well (Note: Yo is recalculated by program based on linearity of graph) Re: Internal radius of test well casing Rw: Effective radius of test well D: Saturated aquifer thickness under static conditions L: Length of test well screen H: Height of water column in test well under static conditions T: Transmissivity as calculated by Aqtesolve program. K: Hydraulic conductivity The value for K was computed by the Aqtesolve program for MW-6. For MW-11, K was calculated as the quotient of transmissivity divided by saturated thickness. Note: The Bouwer-Rice method for unconfmed aquifers was used to calculate parameters for MW-6. The Cooper-Bredehoeft-Papadopoulos method for confmed aquifers was used to calculate parameters for MW-11. slug_shallow.xls (Results) 3 0 5 3 2 9 Calculation of Site-Specific Dilution Attenuation Factor Using Average (K) Value Virgin Island Chemical Site St. Croix, U.S. Virgin Islands Parameter Value Used in Calculation Units Mixing Zone Depth Mixing zone depth (d) Source Length (L) Infilitration Rate (I) Aquifer Hydrualic Conductivity (K) Aquifer depth (da) Hydrualic Gradient (ig) 17.54 152 0.050 526 20 0.01 meters meters meters/year meters/year meters meter/meter Dilution Attenuation Factor Dilution Attenuation Factor (DAF) | 13.04 | umtless K calculated from slug test at MW-2 (3.19 feet/day) and MW-11 (1.83 feet/day) in the FPP Area, and MW-6 (18.14 feet/day) in the AST Area for a Site-specific geometric mean of 4.73 feet/day. DAF_fmal.xls (K=4.73) 305330