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TABLE 1 Input Parameters Parameter Initial Value Chemical of Concern Benzene Type of Release Groundwater, 24-hrs Timestep 0 5 years Simulation Time (modeling period) 9 5 years Benzene Concentration(mean of recent samples) 68 ppb Plume Cross-Sectional Area (known) 750 square feet Impacted Aquifer Thickness (estimated) 10 feet Soil Porosity(estimated) 030 Mass Released (computed) 0 01 kilograms Soil Density (estimated) 1,400 kg/m3 (90 lbs/ft) Per Cent Organic Carbon (estimated) 0 002% Hydraulic Conductivity(medium sand) (estimated) 10-2 cm/sec* Hydraulic Gradient (known) 0 004 ft/ft** Benzene Partition Coefficient(Log Koc) (known) 192 Calculated Retardation Factor (Kd) (computed) 170 cm3/gram Biodegredation Half Life (days) (estimated) 6,931 * Conservatively estimated, actual value is probably 10-4 or less **The actual hydraulic gradient was doubled for this exercise to insure that the result would be conservative Actual measured gradient is 0 002 ftlft i Benzene Concentration I One of the principal parameters affecting the model is the initial (input) benzene concentration We estimated this concentration by averaging the benzene concentrations 'that were detected in the most recent groundwater samples from the four impacted wells Two wells (MW-1 and SW-2)were most recently sampled in December 1998, and concentrations,of 1 1 parts per billion (ppb) and 23 ppb were reported The other two wells were sampled in July 1999, and concentrations of 248 ppb and'I ppb were detected I I in MW-2 and SW-1,respectively The mean of these four samples'is 68 ppb I ' Mass Balance ' In,order to determine the total mass of residual benzene within the contaminant plume, it " is necessary to estimate the volume of the plume The volume can be computed from data obtained during drilling The areal extent of the impacted aquifer is approximately 750 I ' I square feet, and it is assumed that benzene is present within the upper 10 feet of the ' 1 aquifer(in actuality, because benzene is lighter than water, it may be limited to the upper few feet of the aquifer, but a more conservative value was selected for safety) The ,'I volume of groundwater within this,impacted aquifer is determined from its porosity, l' " I , 'assuming a porosity of130%, which is a typical'value for,an`'unconsolidated'Holocene lfluvial'sand,l the remaining benzene mass in the'aquifer can be'calculated'as 1 Il' l ' pp r� � I '' { i I l 1,il,r,�I, i�f ll ilia�i,ll Ir '1.1-1 4.1111 kid .Ed Illir+l l�I 111, {I �r it rpI�IV lk l'Ir Idll�l�klll�,'� I'l'l ill rill lhil it E I,kl{IRI I0t, �� i�I'll l 'I�loll 11 ,I'�