[[sigma].sub.Happ] = [([[sigma].sub.Happ]).sub.PZNC]) to obtain the PZNPC.
In the second method, the PZNPC was calculated before ([[sigma].sub.Happ]).sub.PZNPC], and was independent of the proton titration data.
When Eqn 9 was used to determine the PZCPC, ([[sigma].sub.Happ]).sub.PZNPC] was found using Eqn 7 to calculate the value of [[sigma].sub.Happ] when X = PZNPC. Comparison of the PZNPC calculated with Eqns 7 and 8 and that calculated with Eqn 9 provides a test of the titration and ion adsorption data as discussed in Anderson and Sposito (1992).
The PZNPC calculated by Eqns 7 and 8 and that calculated by Eqn 9 were very similar, for the Humic Podosol-organic, Ferrosol, and Vertosol soils only (Table 3), and therefore satisfy surface charge balance principles (Anderson and Sposito 1992; Chorover and Sposito 1995; Polubesova et al.
kaolinite) have a PZNPC of <5, and would typically exhibit a net negative surface charge, whereas soils dominated by sesquioxides have a PZNPC of >7 (Table 3) and could still exhibit a net positive surface charge after leachate application.
At pH values above the PZNPC, hydrous Fe and Al oxides undergo a surface charge reversal and carry a net negative charge.
Unfortunately this aspect of the effect of landfill leachate on surface charge characteristics was not investigated here, but it is possible that DOC adsorption could cause a decline in the PZNPC, particularly in those soils dominated by hydrous Fe oxides such as Ferrosols.