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RAU UMWNC CONTAMINANTS IN SUBSURFACE EMMOMMENTti , <br /> (;onfi� r 0 SECTION 8 �23 Y + <br /> of rnn umi aro For grour►dwaters that contain significant dissolved DOC, the con- <br /> ventional retardation equation (Eq 8 4)can be modified to reflect the <br /> � ) sreoer-sioie flow effect that DOC has. N ; <br /> Lo <br /> W <br /> Ln <br /> R = I + I + Kooc (DOC) 10 (BIL) m <br /> , <br /> 7 Z <br /> A <br /> where- R - retardation coefficient (dimensionless) <br /> Kd'0 k = sorption coefficient(L/kg) <br /> Pe - soil bulk density(kg/L) Y <br /> 6 = porosity((imensionless) in <br /> W ., <br /> --� KDcc = sorption constant for compound in DOC(L/ka) i LL <br /> (- ICoc) w ' <br /> Ke=I DOC - dissolved organic carbon concentration(mg/L) L <br /> This equation presumes that the solid organic matter is less than ' <br /> 0.M,so Kd-type constants may replace Koc constants. Higher levels of <br /> n Ka x 10 organic carbon in the solid matter can increase sorption and elevate _ <br /> N retardation effects. <br /> Kss distribution coefficient Figure 8-9 shows that retardation factors are significantly lower at <br /> 0 50 loom Tronsporl Pune =60 yews DOC concentrations of 100 ppm,particularly for high-molecular weight <br /> per p 9 orgaruc compounds The variabilitiy in the retardation factors due to <br /> t+rdr>LtcooMuctlrity.e.6nUft dissolved carbon adds to the inherent variance, Table 8-1 shows that i <br /> So "1°m retardation factors range over two orders of magnitude. The higher <br /> 7at .o sm <br /> 1nop t pine.cep YOM molecular weight aliphatic compounds are most strongly retarded. M ` <br /> Conoenlnikori contours aA CICO■t19,t)7.0 S,Q3,and 01 <br /> I.lpure 0-9 Effect of the dishtbution ooeffWwnt on contarninarit retardation during 8.2.3 Fixation (Detslls In Section 4.Z.3) <br /> transport In a shallow groundwater Now system Source- Freeze and <br /> Cherry, 1979 (Copyright Pisifte-Haq Inc, airs ReprodoW with 8 2 3 f Partitioning onto lmmdWe(Stattonary)Phase <br /> pe"sel") <br /> Sorption onto soil particles (locus no. 4)is a possible means of ruca- <br /> other gasolinc constituents. Indeed, Garrett et al1986 r tion Soil sorption is an important factor in solute retardation,discussed <br /> ( } report that in the preceding section.Sorption is not reliable for preventing transport <br /> MTBE often forms a "halo"around a dissolved gasoline plume, where with the ambient groundwater flow for all compounds. For other com- <br /> MTBE can be detected but other gasoline compounds are not detected. to soil <br /> MTBE also decreases the retardation of other gasoline constituents by pounds, however,pedal]sorption ot molecular-weight, <br /> virtually prevent s,sorption transport special eight compounds} Thus,sorption <br /> mereasing their dissolved concentrations in the groundwater flenzene, would probably not be an effective mechanisrg for permanent fixation <br /> toluene, ethylbenzene, and xylenes (BTEX) are more soluble in ether of all of the contaminant,although a portion or the contaminant may be <br /> than in water. When MT$E is present 1h the gasoline plume, the BTEX irreversibly fixed relative to a specific time scale. i <br /> concentrations in the groundwater arp higher. Because the BTEX coin- Sorption could possibly be an effective mechanism for fixation of r <br /> pounds are more soluble, they are likely IO be more mobile (retarded most of the contaminant in a relatively quiescent groundwater system A t <br /> less) in the groundwater slower groundwater velocity results in s greater amount of sorption and, i <br /> i <br />