where t1/2 is equal to the half-life.

Table 9.12.1 lists the hydrolysis half-lives for several organic chemicals. Half-lives vary from seconds to tens of thousands of years. Certain compounds such as alkyl halides, chlorinated amides, amines, carbamates, esters, epoxides, phosphonic acid esters, phosphoric acid esters, and sulfones are potentially susceptible to hydrolysis (Dragun 1988b). Other compounds such as aldehydes, alkanes, alkenes, alkynes, aliphatic amides, aromatic hydrocarbons and amines, carboxylic acids, and nitro fragments are generally resistant to hydrolysis (Dragun 1988b; Harris 1982).

For organic chemicals undergoing an acid- and base-catalyzed hydrolysis (in the case of acid or alkaline solutions), the total hydrolysis rate constant kT can be expressed (Harris 1982; Mabey and Mill 1978) as kT — kH


T = total hydrolysis rate constant kH = rate constant for acid-catalyzed hydrolysis [H+] = hydrogen ion concentration [OH-] = hydroxyl ion concentration kN = rate constant for neutral hydrolysis ko

= rate constant for base-catalyzed hydrolysis

Several other parameters can affect the rate of hydrolysis including temperature, the pH of the soil particle surfaces, the presence of metals in soils, the adsorption of the organic chemical, and the soil water content (Burkhard and Guth 1981; Konrad and Chesters 1969).

After the hydrolysis rate constant k is estimated, the behavior of a compound can be modeled with a form of the advection-dispersion equation. Equation 9.13(1), that includes a first-order degradation term.

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