Box 5D Estimating aquifer risk to pollution

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:
B1= the groundwater recharge in terms of annual averages
B2= the thickness of the unsaturated zone, ie, the layer between the upper soil and the groundwater table
B3= the composition (texture) of layers in the unsaturated zone
B4= the aquifer type (Map 5.4)

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