SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

� r Mr. Michael Infurna September 16, 1496 Page 2 A feature of Version 2.5 of SESOIL is the option to estimate the change in contaminant concentration in ground water m the saturated zone resulting from movement of contaminant from the vadose zone,above. This option was used to provide an estimate,under the conditions modeled,of resulting benzene concentration in ground water. The;model vindicates that benzene will reach the water table within 3 years and will ultimately increase in.concentration in the ground water to about four times the background or original concentration. This is.represented in the output by an increase in average benzene.concentration in groundwater from 9.9&3 Whil to 4.13E-2 4n-d at the end of year 4: After year 4,the concentrat on clf benzene in ground water Jo4ines until the original background concentration is reached at the end.ofYear 8, By the end of year 23 the concentration of benzene in soil moisture,adsorbed on soil,and iri soil air ih the sublayers of the lower layer. reach nearly stable values,;indicating that;movement ofbenzene ire ft layer above the water table>has greatly diminished. The record of benzene concentration in water from MW-2,the well downgradient of EB4- shows no per eepttble t w4...either up or down. Benzene concentration in water from MW-2,in pct,has varied lather randomly in the last four years from nondetectable concentrations to 120 ppb: The last two times water ft0mMW4.was analyzed it was found not to contain detectable benzene concentrations. It appears that fir some reason:or reasons,benzene concentrations in ground waiter at MW-2 vary from one sits#o the: other of the aiWytiW:detection limit,but do not describe;an increasing trend. Further,the modest increase in benzene coneentration predicte&by the SESOIL model(about 31 ppb)falls within the rauge:ofvauriation of benzene concentrations actuially measured:in water from MW-2. Hence,if the predicted transport of benzene to the water table really occured;it might difficult to detect using benzene concentration...in groundwater as an indicator :We believe that the record of benzene concentrations measuidd in ground. water samples fi`om the:tin site wells,together with the presence of MTBE in the. und;water,is indicative of input of gasoline hydrocarbons:.to the saturated zone at some distance upgradient ofthe.wells. The:only upgrad ent.location.neafby where MTBE could have been input to the ground w## system is at.the. operating ARCO station at 501 West Lodi Avenue: The SESOIL calculations predict no:surface runoff. This has the effect of all precipitation to be considered,as infiltration through the soil surface. Some evapotranspiration is subtracted from the amount of water infiltrating,but the total amount of water reaching the water table is:very likely overestimated. In. real life,at 305 South Hutchins,much of the soil surface is paved with asphalt and concrete which assures some substantial amount of surface runoff that never sees the subsurface anywhere near 305 South Hutchins. The SESOIL calculations indicate that a very nuxlest initial increase in benzene concentration in ground water may result from Teaching of benzene(bund in the vadose zone. It is:questionable whether this predicted increase in concentration that may have been or might be realized could be recognized in the noise of normal ground-water benzene concentration variation. "There is reason to believe that in developed areas SESOIL overestimates the amount of water reaching the water table through the process of infiltration from the surface. This,in combination with the use of a 40 foot water table depth,would tend to provide an overestimate of the amount of benzene leached to the water table.