The impact of agrochemicals on groundwater quality has been the subject of considerable research and public debate. Mathematical models often are used to predict the fate of these chemicals and to develop regulations. In this research, we modified the pesticide degradation component of a management model to estimate soil temperature with depth and time and to incorporate the effect of temperature variation on the pesticide degradation rate. Estimated pesticide mass leaching beyond a depth of 1 m was two or more orders of magnitude greater when the temperature effect was incorporated into the model. Predicted soil temperatures at four different depths using measured surface soil temperatures followed the seasonal temperature variation of observed data with an average deviation <0.3°C. Among the input parameters analyzed, the amount of pesticide leached was most sensitive to uncertainties in activation energy of a degradation reaction, reference half‐life, and annual mean soil temperature. Uncertainty in annual change in surface soil temperature had a moderate impact on the simulated amount of pesticide leached. Uncertainties in damping depth and time lag of annual minimum temperature had little effect. Uncertainties in model parameters can result in differences on the order of one‐ to fourfold in simulation output. Although these are large, they are clearly much less than the differences of 2 to 8 orders of magnitude, which can occur if temperature effect is ignored. We conclude that models used for pesticide risk assessment should incorporate temperature effects on degradation. The algorithm presented here can be incorporated readily into many leaching models.
The content of water and soil in physical intermixture was measured simultaneously and nondestructively by the attenuation of a dual-energy beam of γ rays. The beam, 1 mm by about 3 cm in cross section, was devised by placing a 280-mCi source of 137Cs behind a 389-mCi source of 241Am, with lead collimators suitably aligned in front of each source and the scintillation probe. The probe was connected in parallel to two-separate amplifier-analyzer-scaler systems, one being set in the integral mode to receive all pulses greater than 550 keV (for 137Cs, 662-keV peak), with the other being set in the differential mode to receive all pulses in a band 35–85 keV (for 241Am, 60-keV peak). When related to the count intensity in the high-energy range, the count intensity e caused by 137Cs in the low-energy band was empirically found to be independent of the material in the binary mixture (soil and/or water) placed in the beam for measurement. Also, e could be well expressed by a cubic polynomial that was then used along with a dead-time correction to determine the attenuated count intensity attributable to the 241Am source alone. Calibration of the system was then possible. Over-all measuring accuracy was on the order of ±0.01 cm3/cm3 in water content and ±0.02 g/cm3 in soil content (bulk density) for a counting period of 5 min; these changed to ±0.04 cm3/cm3 or g/cm3 for a counting period of 5 sec.
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