The first-order degradation rate constant for the herbicide metribuzin is report

Question

The first-order degradation rate constant for the herbicide metribuzin is reported in the literature to be 5.3 × 10-8 s-1 (Iversen et al. 2008. J Environ. Qual. 37:1909-1917). If the metribuzin concentration in soil solution at the surface is 17 mg L-1 and the groundwater table is at a depth of 10 m, what downward leaching flow velocity is needed to ensure that the concentration of metribuzin is below the EU drinking water standard limit of 0.1 mg L-1 when the percolating water reaches the groundwater table? Assume degradation as quantified by the rate constant is the only process changing the soil solution level of metribuzin.

Explanation / Answer

The first-order degradation rate constant (K) for the herbicide metribuzin K= 5.3 × 10-8 s-1

let Co= metribuzin concentration in soil solution at the surface is 17 mg L-1

C= drinking water standard limit of 0.1 mg L-1

the first order reaction is -dC/dt= KC, when integrated, C/CO= exp(-Kt)

the tme (t) needed to drop to given concentration need to be calculated.

0.1/17= exp(-5.3*10-8*t),

t= 96901857 sec

flow velocity = distance/ time(t)= 1.032*10-7 m/s

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