PM07 Pollutant Chemistry Transport, Cycling and Fate
Exhibit Hall
8:00 AM - Monday
(PM068) Use of depth-dependent soil concentrations in multimedia mass balance models.
Hollander, A1, Hessels, L2, de Voogt, P2, van de Meent, D1, 3, 1 University of Nijmegen, Nijmegen, The Neteherlands2 University of Amsterdam, Amsterdam, The Netherlands3 RIVM, Bilthoven, The Netherlands
ABSTRACT- In multimedia mass balance models, the soil compartment is usually modeled as a single box with uniform properties. Implicitly, uniform concentration in soil is assumed, as if the soil was well mixed. This deviates from reality, where often decrease in concentrations with depth is observed. In mass balance modeling, use of uniform soil concentrations leads to systematic under- or overestimations of volatilization to air, run-off to surface water, leaching to groundwater, and uptake by plants. Theory predicts that at steady state, soil concentrations follow an exponential decrease with depth, characterized by the soil penetration depth, which can be estimated as a function of mobility and degradation rate of the substance in soil. We have used theoretically expected soil concentration profiles to correct mass transfer coefficients in SimpleBox 3.0, and explored the effects of such corrections on model performance, testing nine compounds in four environmental compartments. In case of emissions to soil only, the adapted model yielded different concentrations in plants for substances that do not penetrate deeply into soil. For these compounds, predicted concentrations in surface water and air were different too, but to a lesser extent than for plants. Model changes had little effect for substances with large penetration depths. Effects on concentrations in the soil compartment itself were small for all substances studied. Insufficient data were available for testing if the modifications actually improved the predictions. Comparison of the theoretically expected concentration gradients with soil profiles reported in the literature showed that theory tends to underpredict soil penetration depths. However, since the modifications are mechanistically clear and theoretically sound, and can be easily implemented in standard multimedia mass balance models, we feel that the corrections are useful extensions, worth to be investigated in more detail.
Key words: soil penetration depth, multimedia mass balance model, transport coefficients, volatilization