The 'aquifer-risk map' combines various processes (risk factors) affecting the fate of nitrate as it percolates down from the topsoil to the aquifer. For nitrate (and also for pesticides) these factors are:
Groundwater recharge (B1) has been calculated as precipitation excess less water flow components (surface runoff and interflow). The water flow components have been estimated by taking into account the slope of the land surface, the composition (texture) of the upper soil (infiltration capacity), the air temperature (frost periods) and the aquifer type. This information has been derived from existing databases. The pedology of the upper soil layers and the thickness of the unsaturated zone were taken from the FAO World Soil Map. Determination of the aquifer risk for each grid cell (10 x10 minutes) was carried out by ranking the individual risk factors and adding them using the following equation: Aquifer risk = WFB1 x RB1 + WFB2 x RB2 + WFB3 x RB3 + WFB4 x RB4 where WF is the weighting factor, and R is the value of the risk factor. The weighting and risk factors (Table 5.6) have been selected so as to calculate aquifer risks ranging from no risk to maximum risk. Table 5.6 - Weighting and risk factors used to generate the 'aquifer-risk map' Source: RIVM Risk factor (R) Weighting factor (WF) Value of risk factor (R) B1: Groundwater recharge 3 range: 110 where 10 = groundwater recharge >500 mm/year and 1 = groundwater recharge < 50 mm/year B2: Thickness of unsaturated zone 1 range: 810 where 10 = groundwater < 3 m below surface and 8 = groundwater > 5 m below surface B3: Composition (texture) of layers in unsaturated zone 2 range: 09 where 9 = coarse soils and 0 = histosols (peat and muck soils) B4: Aquifer type 4 range: 110 where 10 = sedimentary aquifer with good porosity and 1 = hardrock with poor porosity
Determination of the aquifer risk for each grid cell (10 x10 minutes) was carried out by ranking the individual risk factors and adding them using the following equation: Aquifer risk = WFB1 x RB1 + WFB2 x RB2 + WFB3 x RB3 + WFB4 x RB4 where WF is the weighting factor, and R is the value of the risk factor. The weighting and risk factors (Table 5.6) have been selected so as to calculate aquifer risks ranging from no risk to maximum risk. Table 5.6 - Weighting and risk factors used to generate the 'aquifer-risk map' Source: RIVM Risk factor (R) Weighting factor (WF) Value of risk factor (R) B1: Groundwater recharge 3 range: 110 where 10 = groundwater recharge >500 mm/year and 1 = groundwater recharge < 50 mm/year B2: Thickness of unsaturated zone 1 range: 810 where 10 = groundwater < 3 m below surface and 8 = groundwater > 5 m below surface B3: Composition (texture) of layers in unsaturated zone 2 range: 09 where 9 = coarse soils and 0 = histosols (peat and muck soils) B4: Aquifer type 4 range: 110 where 10 = sedimentary aquifer with good porosity and 1 = hardrock with poor porosity
Aquifer risk = WFB1 x RB1 + WFB2 x RB2 + WFB3 x RB3 + WFB4 x RB4
where WF is the weighting factor, and R is the value of the risk factor. The weighting and risk factors (Table 5.6) have been selected so as to calculate aquifer risks ranging from no risk to maximum risk. Table 5.6 - Weighting and risk factors used to generate the 'aquifer-risk map' Source: RIVM Risk factor (R) Weighting factor (WF) Value of risk factor (R) B1: Groundwater recharge 3 range: 110 where 10 = groundwater recharge >500 mm/year and 1 = groundwater recharge < 50 mm/year B2: Thickness of unsaturated zone 1 range: 810 where 10 = groundwater < 3 m below surface and 8 = groundwater > 5 m below surface B3: Composition (texture) of layers in unsaturated zone 2 range: 09 where 9 = coarse soils and 0 = histosols (peat and muck soils) B4: Aquifer type 4 range: 110 where 10 = sedimentary aquifer with good porosity and 1 = hardrock with poor porosity
Source: RIVM