Intrinsic and Specific Vulnerability of Groundwater in Central Spain-The Risk of Nitrate Pollution

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Intrinsic and specic vulnerability of groundwater in central Spain: the risk of nitrate pollutionJuan J. Martnez-Bastida & Mercedes Arauzo & Maria Valladolid Abstract The intrinsic vulnerability of groundwater in the Comunidad de Madrid (central Spain) was evaluated using the DRASTIC and GOD indexes. Groundwater vulnerability to nitrate pollution was also assessed using the composite DRASTIC (CD) and nitrate vulnerability (NV) indexes. The utility of these methods was tested by analyzing the spatial distribution of nitrate concentrations in the different aquifers located in the study area: the Tertiary Detrital Aquifer, the Moor Limestone Aquifer, the Cretaceous Limestone Aquifer and the Quaternary Aquifer. Vulnerability maps based on these four indexes showed very similar results, identifying the Quaternary Aquifer and the lower sub-unit of the Moor Limestone Aquifer as deposits subjected to a high risk of nitrate pollution due to intensive agriculture. As far as the spatial distribution of groundwater nitrate concentrations is concerned, the NV index showed the greatest statistical signicance (p254 Rating 1 3 6 8 9

Topography slope (%) Range 02 26 612 1218 >18 Rating 10 9 5 3 1

Hydraulic conductivity (m day1) Range 04.1 4.112.2 12.228.5 28.540.7 40.781.5 Impact of the vadose zone Range Conning layer Silt/clay Shale Limestone Sandstone Bedded limestone, sandstone and shale Sand and gravel with signicant silt and clay Sand and gravel Basalt Karst limestone Rating 1 2 4 6 8

Rating 10 10 9 8 7 5 4 3 2 1

Aquifer media Range Massive shale Metamorphic/igneous Weathered metamorphic/igneous Glacial till Bedded sandstone, limestone and shale sequences Massive sandstone Massive limestone Sand and gravel Basalt Karst limestone

Ratinga 13 (2) 25 (3) 35 (4) 46 (5) 59 (6) 49 (6) 49 (8) 49 (8) 210 (9) 910 (10)

Ratinga 1 26 (3) 26 (3) 25 (3) 27 (6) 48 (6) 48 (6) 48 (8) 210 (9) 810 (10)

Typical ratings in parentheses according to Aller et al. (1987)

The specic vulnerability to nitrate pollution ranges from 28 to 280 according to this index and is calculated using the following equation: CD index Dw Dr Rw Rr Aw Ar Sw Sr Tw Tr Iw Ir Cw Cr Lw Lr Where, Lw Lr Relative weight of the potential risk associated with land use Rating of the potential risk associated with land use 3

vulnerability to nitrate pollution according to the CD index. The vulnerability ranges for the CD index are shown in Table 7.

The rest of the parameters are the same as in Eq. (1). The map of the potential risk associated with land use (CORINE Land Cover 2000: European Environmental Agency 2000) was produced using the same methodology applied to the rest of the DRASTIC index parameters. Table 6 shows the ratings assigned to the potential risk associated with land use (L) in the CD index, according to Secunda et al. (1998). These ratings characterize extensive land uses as potential sources of groundwater pollution. The greatest impacts corresponded to irrigated eld crops and urban areas, with scores twice as high as for nonirrigated eld crops. The lowest impact corresponded to natural areas such as forests. The intrinsic vulnerability map based on the DRASTIC index and the potential risk associated with land use map were combined using ArcGIS 9.2 to generate a new map showing specicHydrogeology Journal (2010) 18: 681698

Nitrate vulnerability index (multiplicative model) This is a new specic vulnerability index which is proposed as another adaptation of the DRASTIC index and has been developed with the objective of achieving greater accuracy in the estimation of specic vulnerability to nitrate pollution, based on the real impact of each land use. The model attempts to integrate the risks of groundwater pollution by nitrate related to land uses (as a potential source of nitrogen). It incorporates both the negative impacts, over time, of some of these uses and also the protective effects that others may have upon aquifer media (uses that do not contribute signicant quantities of nitrate and do not enhance leaching, such as the protected natural areas). This is possible because it is based on a multiplicative model, involving the addition of

Table 4 Vulnerability ranges corresponding to the GOD index (Source: Foster et al. 2002) Vulnerability Negligible Low Moderate High Extreme Ranges (GOD index) 00.1 0.10.3 0.30.5 0.50.7 0.7 DOI 10.1007/s10040-009-0549-5

688 Table 5 Ranges and ratings of the GOD parameters (Source: Foster et al. 2002) Groundwater connement Range Rating Depth to groundwater Range Rating 0.9 0.9 0.8 0.7 0.6 Rating