Groundwater Hazard Calc. for Peanuts and Cotton in Oklahoma Introduction, D.L Nofziger
Groundwater Hazard Calculation for Peanuts and Cotton in Oklahoma
D.L. Nofziger and Jinquan Wu
The groundwater hazard used for estimating the impact of different herbicide treatments on groundwater is defined as the ratio of the estimated concentration of the active ingredient in the aquifer to the critical concentration of the active ingredient. Details of this parameter are given in the PEET user's manual.
The CMLS model was used to estimate the quantity of active ingredient leaching past the 1 m soil depth. All chemical passing that depth was assumed to enter the aquifer. The depth of mixing and porosity of the aquifer can be specified by the PEET user in the Options menu. The estimated concentration is then calculated as the estimated mass leached divided by the volume of water in the aquifer mixing depth. Groundwater hazards associated with multiple chemicals was taken as the sum of the individual groundwater hazards.
Soil mapping units in classes 1-4 were simulated. For each soil layer, CMLS requires the depth of the bottom of the layer, bulk density, organic carbon content, and water contents at permanent wilting point, field capacity, and saturation. The SCS curve number is also required to estimate runoff and infiltration. CMLS also requires daily infiltration and evapotranspiration amounts. These were estimated from daily weather data. One weather station was used for each county. The historical weather data was used to obtain the stochastic weather parameters needed in the WGEN weather generator. The generator was then used to simulate 100 weather realizations for each county. The groundwater hazard value exceeded at different probability levels were caculated from the results of 100 or more simulations of each chemical for each soil-management system. The user's manual contains more details on this calculation. The PEET user can specify the probability to be displayed in the Options menu.
Organic carbon partition coefficients, degradation half-life values, and critical concentrations were obtained from Hornsby et al (1996) and personal communication with Dr. Hornsby. Partition coefficients for each soil layer were caclulated as the product of the organic carbon content of that layer and the organic carbon partition coefficient for each chemical. the half-life was assumed to be constant with depth in the upper 1 m of soil.
Hornsby, A. G., R.D. Wauchope, and A.E. Herner. 1996. Pesticide Properties in the Environment. Springer. New York. 227 p.
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Last Modified January 16, 2008.