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r 0 the contaminant is abnorm ilk thin and is und,-r- ' o8 lain by_a continuous low permeability unit (clays or shales) For this case a better approximation 30 a would be W0' I 4(DCy = Co H erf Y Vim) t ] ]j (20) TY - ox where all parameters are as pre�,iously defined, and 0 0=34 Os 08 10 12 14 16 16 xo [H)s the thickness of the available ,ertical spreading W zone_Clearly, H would have to be very thin ' Fig 5 Error function (perhaps two times Z) to warrant utilization of equation (20) Note that as H approaches Z. the model incorporates horizontal spreading only The From Figure 5, erf 0 12 equals 0 13 and erf 0 62 11mlt5 to horizontal and vertical spreading are equals about 0 61 Hence, in this case the error factors to be determined in the field, and will of function of rhe argument is approximately equal to course constrain the solution given as equation the argument, and (19) Cy = Co(013)(0 61) = 0 08 Co CONCLUDING STATEMENTS That is, over a distance of 150 in (about 500 feet)_ The average value calculations described in from the measurement at the soad waste boundary, this a er may provide a i.seful screening mecha- the concentration decreases from Co to 0 08 Co nism_for evaluating the dilution potential of waste The quantity 1/0 08 gives the dilution factor, sites prior to intensive investigations Factors such ' which in this case is about 12 Stated another way, as potential recharge, available spreading thickness, as long as the concentration at the waste boundary ground-water velocity and chemical retardation, is less than the MCL times 12, the concentration at and the proximity and nature of ground-water the alternate boundary will be less than the MCL discharge into surface-water systems are some of r Some additional examples are given in Table 1 the numerous factors that come into play in such We have made the following assumptions an evaluation These factors have meaning in a E ' screening procedure only if containment of the [I 1 No longitudinal dispersion, only transverse waste is not a viable option and if minimum per- t, No chemical attenuation formance standards are regarded as acceptable in per- Z. protection 3 Recharge and other dilution mechanisms The requirements of screening differ con- art ignored slderably from those of management control that ' 4 Vertical spreading is not impeded by very must be employed by regulatory agencies and low permeability materials owners-operators of waste facilities Clearly, the most important part of any rrasre management ' Of these assumptions, only No 4 gives rise to some scheme is a reliable monitoring system Given such problems when the permeable horizon transmitting a system, the monitoring program and the proposed ' Table 1 Example Calculations for Hypothetical Situations ' Z z X X C Co DT/v y Z X erf erf � meters meters meters meters 2(DTy/Vy)44 2(DTy/Vy)i4 4(DTy/Vy)'h 4(D-ry/Vy)4 Co CY 1 150 3 30 012 0 13 065 064 008 120 ' 1 150 6 60 025 027 1 35 094 0 25 3 9 1 60 3 30 019 021 1 05 080 0 is 5 1 60 6 60 038 041 205 099 041 2 2 150 3 30 008 009 046 048 004 23 0 2 150 6 00 0 18 020 095 032 016 0 1 2 60 3 30 0 13 0 14 074 070 009 10 2 2 60 0 60 028 030 1 45 096 029 ' 5 310