LATE FALL SITE-SPECIFIC SOIL NITRATE UPPER LIMITS FOR GROUNDWATER PROTECTION PURPOSES

For temperate regions, such as Germany, a simple management model is d eveloped, with which NO3 seepage losses during winter can be estimated , when the amount and distribution of soil NO3 in late fall are known. The semi-analytical, one-dimensional model is developed for homogeneo us soils. The model is derived for steady-state flow conditions at a c onstant soil water content (field capacity). Essential part of the mod el is a solute transport equation based on mixing-cell theory. Sample calculations show that when the height Delta z of the mixing-cells is chosen such that Delta z = 2D/nu, where D is the soil dispersion coeff icient and nu is the pore water flow velocity, mixing-cell model resul ts compare well with results from convective-dispersive models. The ad vantage of mixing-cell models compared with convective-dispersive mode ls is that mixing-cell model solutions frequently appear as simple mat hematical expressions. With the model sample calculations concerning N O3 seepage losses during winter were carried out for a variety of site conditions. The calculations show that NO3 seepage losses vary consid erably, even when the initial amount and the distribution of the soil NO3-N are the same. The calculations also show that if a limit is set for the total amount of tolerable NO3 seepage losses during winter, si te-specific late fall upper limits for soil NO3-N can be derived. A ta ble of such values, for a wide range of site-conditions, is presented.