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3.4.2 Buffering Capacity of Soil and Groundwater 1 The ability of soil and groundwater to neutralize either acidic Fe(II) solution or basic Cascade® is illustrated in Figures 14 Figures 1 and 2 show how the pH of groundwater 1 (MW-3) changes upon addition of either acid (H}) or base (OH-), while Figures 3 and 4 show how the pH of soil (SB-10-3) changes Although groundwater can neutralize some acid or base without drastically changing pH, the amount Is relative small. Hence, most of the natural buffering capacity in the sub-surface will be due to the soil is 12 10 � a C CL 6 4 ff-13r,mu;dw—ale,:(MW-L3)D.pJ 2Guater 0 0 02 04 06 08 1 12 14 16 18 2 mmokm OK Added !100 mL Water ' Figure 2. Titration of 100 mL Water using 0.1 N NaOH. Figure 3 shows that addition of acid (up to 100 mmol H+/kg soil) had little long-term effect on the pH Although the pH initially dropped to about pH 5 5, it rebounded to pH 7 8 by Day 14 Thus Information can be used In conjunction with soil porosity and density to estimate the amount of acidified solution that can be added without causing significant long-term change In pH For example, if the buffering capacity of the soil is 100 mmol/kg,then up to 10 L of pH 2 (100 mmol H+/L) solution can be added to each kilogram of soil If the soil porosity is 30% and the density Is 10 g/cm3 (pore volume = 300 mL), this is equal to 33 pore volumes of solution Note that these calculations do not 1 take into account the effect of influx of groundwater on the soils after treatment As untreated groundwater upgradient of the treatment zone flows into the treated area., soil pH will tend to return to pretreatment levels Figure 4 shows that addition of base (up to 10 mmol OH-/kg soil) had little long-term effect on pH Addition of 10 mmol OH-/kg soil initially increased the pH to 10 1, which ' PRIMA Environmental I I Eva[of Reductants January 19,2005 ATC-Diamond Walnut