the quality of the groundwater in the netherlands

10
ELSEVIER Journal of Hydrology 207 (1998) 179-188 Journal of Hydrology The quality of the groundwater in the Netherlands H.F.R. Reijnders*, G. van Drecht, H.F. Prins, L.J.M. Boumans RIVAl, LaboratotTfor Soil and Groundwater; A Van Leeuwenhoekh~an 9, 3720 Bilthoven, The Netherlands Received 25 July 1097: revised 17 February 1998; accepted 18 February 1998 Abstract Information collected as part of the Netherlands National Groundwater quality Monitoring Network and the Provincial Groundwater quality Monitoring Network are presented. A more detailed analysis of nitrate and aluminium is provided to illustrate the major features observed. Groundwater quality per sampling depth, soil type and landuse is expressed as average concentrations as well as percentages of surface area, showing target value exceedances for the year 1992 and presented in graphs. The percentages surface area with target value exceedances per physico-geographical region for the year 1992 are depicted in maps. The change in groundwater quality per sampling depth, soil type, landuse and region in the 1984-1993 period is also investigated. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Ground water; Water quality; Monitoring; Networks; Netherlands; Spatial variations; Standards; Confidance limits; Confidance intervals; Uncertainty 1. Introduction The National Groundwater quality Monitoring Net- work (NGMN), established between 1979 and 1984, is used to: (i) determine the quality of the groundwater per landuse and soil type and to construct maps; and (ii) to establish if changes in the groundwater quality occur. The NGMN comprises about 400 locations. At each location groundwater was sampled annually from 1984 at depths of approximately 10 and 25 m below the ground level. The samples were analysed annually for components such as chloride, nitrate, sul- phate, bi-carbonate, ammonium, potassium, sodium, magnesium, calcium, iron, manganese, total phosphorus, dissolved organic carbon, pH, electrical conductivity (EC) and the inorganic micro-compo- nents: barium, strontium, zinc, aluminium, cadmium, * Corresponding author. Tel: 00 31 030 2742061; Fax: 13(]31 030 2742971. 0022-1694/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0022-1694(98)00132-2 nickel, chromium, copper, arsenic and lead. Pesti- cides, organo (chloro)compounds and lanthanides were analysed incidentally. Results from the Provincial Groundwater quality Monitoring Networks (PGMN), established from 1989, are combined with the data of the national monitoring network at the National Institute of Public Health and Environmental Protection (RIVM). The objectives, arrangement and use of the provincial measuring stations are similar to those of the NGMN. However, the density of the stations of the PGMN is higher than average in areas with potential threats of groundwater contamination, for instance, in the sandy areas. Groundwater samples were taken of about 600 locations of the NGMN and PGMN in 1992. The groundwater quality of the Netherlands was described in an overview by van Drecht et al., 1996. The groundwater quality has been tested using limit values for groundwater, This study used observations

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Page 1: The quality of the groundwater in the Netherlands

E L S E V I E R Journal of Hydrology 207 (1998) 179-188

Journal of Hydrology

The quality of the groundwater in the Netherlands

H.F.R. Reijnders*, G. van Drecht, H.F. Prins, L.J.M. Boumans

RIVAl, LaboratotTfor Soil and Groundwater; A Van Leeuwenhoekh~an 9, 3720 Bilthoven, The Netherlands

Received 25 July 1097: revised 17 February 1998; accepted 18 February 1998

Abstract

Information collected as part of the Netherlands National Groundwater quality Monitoring Network and the Provincial Groundwater quality Monitoring Network are presented. A more detailed analysis of nitrate and aluminium is provided to illustrate the major features observed. Groundwater quality per sampling depth, soil type and landuse is expressed as average concentrations as well as percentages of surface area, showing target value exceedances for the year 1992 and presented in graphs. The percentages surface area with target value exceedances per physico-geographical region for the year 1992 are depicted in maps. The change in groundwater quality per sampling depth, soil type, landuse and region in the 1984-1993 period is also investigated. © 1998 Elsevier Science B.V. All rights reserved.

Keywords: Ground water; Water quality; Monitoring; Networks; Netherlands; Spatial variations; Standards; Confidance limits; Confidance intervals; Uncertainty

1. Introduction

The National Groundwater quality Monitoring Net- work (NGMN), established between 1979 and 1984, is used to: (i) determine the quality of the groundwater per landuse and soil type and to construct maps; and (ii) to establish if changes in the groundwater quality occur. The NGMN comprises about 400 locations. At each location groundwater was sampled annually from 1984 at depths of approximately 10 and 25 m below the ground level. The samples were analysed annually for components such as chloride, nitrate, sul- phate, bi-carbonate, ammonium, potassium, sodium, magnesium, calcium, iron, manganese, total phosphorus, dissolved organic carbon, pH, electrical conductivity (EC) and the inorganic micro-compo- nents: barium, strontium, zinc, aluminium, cadmium,

* Corresponding author. Tel: 00 31 030 2742061; Fax: 13(] 31 030 2742971.

0022-1694/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 1 6 9 4 ( 9 8 ) 0 0 1 3 2 - 2

nickel, chromium, copper, arsenic and lead. Pesti- cides, organo (chloro)compounds and lanthanides were analysed incidentally.

Results from the Provincial Groundwater quality Monitoring Networks (PGMN), established from 1989, are combined with the data of the national monitoring network at the National Institute of Public Health and Environmental Protection (RIVM). The objectives, arrangement and use of the provincial measuring stations are similar to those of the NGMN. However, the density of the stations of the PGMN is higher than average in areas with potential threats of groundwater contamination, for instance, in the sandy areas. Groundwater samples were taken of about 600 locations of the NGMN and PGMN in 1992.

The groundwater quality of the Netherlands was described in an overview by van Drecht et al., 1996. The groundwater quality has been tested using limit values for groundwater, This study used observations

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180 H.F.R. Reijnders et aL/Journal of Hydrology 207 (1998) 179-188

from the NGMN and the PGMNs from the 1984-1993 period. This article is intended to make the results of the study more widely known. Although, the study was done on a great number of substances the discus- sion is limited to nitrate and aluminium because of their relevance to the environment and public health.

1.1. Brief background on nitrate and aluminium concentrations of groundwater

1.1.1. Nitrate The intensification of agricultural landuse during

the last few decades have, in large parts of the Netherlands, led to an increase in the amount of nitro- gen-N as manure from the entire livestock population plus artificial fertilizer on all agricultural land of 180kgha- ]a -l in 1950 to 550kgha-~a -1 in 1986 (Netherlands Central Bureau of Statistics, 1993). Highly productive agricultural landuse is only possi- ble with additional N-fertilization. This leads to a greater washout of nitrate, causing high nitrate con- centrations to be observed in the upper groundwater (Fraters et al., 1997). At the same time the NH3-emis- sion to the atmosphere also increased. The growth in the road traffic raised NOx-emissions. The increased N-emission led to a larger atmospheric N-deposition. The average atmospheric N-deposition in De Bilt (village in the centre of the Netherlands), through precipitation amounted to 2 0 k g h a -] a -I in the 1983-1987 period (KNMI/RIVM, 1989). In areas with intensive animal husbandry the atmospheric N- deposition can increase to more than 50 kg ha < a -1. In forests the atmospheric N-deposition could be 1.5-2 times higher than average (Erisman, 1993).

Under forests, that are situated most in sandy areas, high nitrate concentrations are found in the ground- water (Boumans, 1994), as a consequence of at- mospheric N-deposition and washout. We expect a ranking of landuse to N-load in the order of forests, built-up, agriculture and agriculture with maize crop. The washout will be related to the N-load in this order.

Nitrate seldom occurs in deep groundwater. The nitrate washed out is probably converted by denitrifi- cation to gaseous compounds during the transport to deep layers. Denitrification takes place in the presence of reducing compounds (pyrite, organic compounds, etc.). The uppermost groundwater is more anaerobic when groundwater tables are closer to the surface

(Boumans et al., 1989). Because of the denitrification being connected with the depth of the groundwater table, the level of an area is a prime factor for the occurrence of nitrate. For areas with groundwater tables close to the surface that are heavily manured, nitrate concentrations are expected to be high and to rise, since the nitrogen load in the last few decades increased. At the same time, the groundwater tables have dropped due to increased drainage, thereby redu- cing the denitrification. Areas with deep groundwater tables are found in dunes and coast walls, the sandy areas and the river banks.

1.1.2. Aluminium Aluminium is a main component of the earth's crust

making up 8% of its mass. The solubility of aluminium is determined by the pH. Dissolved alumi- nium rarely occurs at the pH range of 5-8. Due to the atmospheric N-deposition (from manuring and traffic) on nature areas in which pH is not controlled by lim- ing, the pH in the soil decreases and the aluminium concentration increases. In the sandy areas acidifica- tion is neutralised mainly by the weathering of aluminium containing minerals. Therefore, high alu- minium concentrations are expected in shallow groundwater under areas with the landuse nature. In shallow groundwater under agricultural soil low aluminium concentrations are expected because of liming. High concentrations are toxic for flora and fauna (Kloppel et al., 1997).

2. Methods

2.1. Selection of observations

Observations were selected on the basis of year of sampling, filter depth, landuse, soil type and area. Most of the filters were mounted at depths of about 10 and 25 m below ground level and have a length of 2 m. There were two separate depth ranges distin- guished: 5 - 1 5 m below ground level (shallow groundwater) and 15 -30m below ground level (deep groundwater).

The landuse was derived t¥om the Landuse Classi- fication of the Netherlands (Thunnissen et al., 1992) from LANDSAT satellite recordings from the 1986- 1988 period. From 17 landuse types, six groups were

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H.F.R. Reijnders et al./Journal of Hydrolog> 207 (1998) 179....188 181

+ NGMN oPGMN

/ ~ + - . +

, + ,f÷/ .~' 6 . + ccd~ c,(

~ < / f 'I + + + + ~ # ÷ + o+. ~ I C; / , /

/ + +

/< c)+ - l ,,' ~ + 4. ~ - - I

/

J

Fig. I. Selecled monitoring stations of the National (NGMN) and Provincial (PGMN Groundwater Monitoring Networks in 1992.

formed: distinguished into nature, horticulture, crop- lands, grassland, maize and built-up.

The soil type was determined using a geographical data base of the soil map, scale: 1:250000 (Steur et al., 1985). Soil types were distinguished into holocene deposits: low peat, dune sand, cleared peatland, marine clay, river clay and pre-hotocene deposits: cover sand, loam/loess (fine texture aeolic pre-holo- cene deposits), clay, pre-holocene clay.

Nine areas were identified by classification to physico-geographical properties. The map of the classified areas resembles the soil map (van Drecht et al., 1994). The sandy area was split into the south- ern, central, eastern and northern parts. Further differ- entiations were the marine clay and low peat area, the river area and loam/loess area, and the dunes.

2.2. Observation groups

For 1992 a number of 607 measuring stations from the database remained after selection (see Fig. 1). Observations have been spread over the groups on the basis of combinations of landuse, soil type and physico-geographical area as well as depth (shallow and deep groundwater). A group size of minimally 10 was aimed at. In estimating the groundwater quality per group, the accuracy increases in general with the number of observations. The observations in the marine clay and the low-peal: areas were taken together since the groundwater quality in these areas did not differ clearly. Of the available measuring sta- tions, 78% were used for the classification into groups. Close to 66% of the surface area in the

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182 H.F.R. Reijnders et al./Journal of Hydrology 207 (1998) 179-188

NO3-N (g/rrl 3)

35 I~1 deep . . . . . . . . . . . . . . . . . . . . . . ~ ............................

30 . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . ' ' . . . . . . . . . . . . . . . . . . 20

2 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . [ . . . . . . . .

20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15 . . . . . . . . . . . . . . . . . . . . . : ! . . . . . . : .... 106

1 0 . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . .

, , 222 33,3 " ' " f I N • [4 i

- - - - - - W - " - - - - " ' T -

n/ds hlds g/1 g /mcc /mcb /mc g/rc b/rc g/s rnJs c/s f/s b/s <dune&coast wallxlow peat, marine clay areas >< river area x sandy area >

n=natural areas, h-homculture, cncrops, g=grassland, m-maize, b=built-up areas, ds--,dune sand, I-low peat,

mc=marine clay, rc-river clay, s-sandy area

Fig. 2. Average nitrate concentration in groundwater per landuse, soil type in 1992; 95% confidence intervals for mean concentration, number of observations and target value.

Netherlands (2.3 million ha) is represented by these groups.

2.3. Average values

Observations were interpreted as independent mea- surements of the average quality of groundwater in relation to the combination of landuse, soil type and area. The uncertainty in the estimated average groundwater quality is expressed in confidence inter- vals (P = 0.95).

2.4. Percentage surface area above the target value/ standard

For this calculation, the obserations have been interpreted as individuals of a distribution. The realised percentage of observations in an area that exceeds the target value was interpreted as an estima- tion of the percentage surface area with a concentra- tion in the groundwater higher than a limit (PST for nitrate and PSD for aluminium). The limit value for nitrate is taken equal to the target value for nitrate in groundwater (Ministry of Housing Spatial Planning

and Environment, 1994) and for aluminium to the drinking water standard (European Commission, 1980). The percentages of surface per combination of landuse, soil type and region is also estimated. The uncertainty in the percentage is expressed in con- fidence intervals (P = 0.95) and calculated using the cumulative binomial probability distribution.

For this calculation, the probability P of k occur- rences or more (observed number of exceedances of a critical value) in n trials (number of observations) given probability p per trial (actual fraction exceedances) is established (Press et al., 1988b). The probability p is varied until P equals 2.5% (bi- section procedure (Press et al., 1988a)). The p found is interpreted as the lower limit of the 95% confidence interval for p. The upper boundary is established by varying p until P equals 97.5%. The PST value inter- vals were estimated for groups, physico-geographical areas, the four large sandy areas collectively and for the whole of the Netherlands. The PST value intervals were classified as: very low: interval entirely under 10%; low: interval entirely under 20%; not low and not high: interval comprises the 10% and 20%; high: interval entirely above 10%; very high: interval

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H.F.R. Rei]mlers et aL/Journal of Hydroh~gy 207 (1998) 179-188 183

•0 •

[ ] s h a l l o w J 90 [ ] deep _~i . . . . ......... :

80 ': 2s

70 . . . . . . . . . . . . :

60 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " . . . . . . . . . . . . . . . . . . . . . . .,29 ~ .

50 ...... . . i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . ~2 . . . . .

4 0 . . . . . . : ' 1 3 . . . . . . . . . . . . . .... 14

. . . . . . . . . . . . . . . . . . . . . . . . .

- - r - - - - - - - r - - - - - - - r - - - ~ i " - -

n/ds tgds g/1 g /mcc /mc b/mc g/rc b/rc g/s m/s c/s f/s b/s <dune&coast w a l l x l o w peat, marine clay areas >< river areas x sandy area >

n - n a t u r a l areas , h -ho r t i cu l tu re , c - c r o p s , g - g r a s s l a n d , m=rnaize , b - b u i l t - u p areas , ds=dune sand, l - l o w peat ,

m c - m a r i n e c lay, r c - r i v e r clay, s - s a n d y area

Fig. 3. Ni t ra te in g r o u n d w a t e r pe r l anduse , soil t ype in 1992; 9 5 % c o n f i d e n c e in te rva l s fo r the p e r c e n t a g e o f su r f ace a r ea a b o v e the t a rge t v a l u e for n i t ra te N (5 .6 g / m 3) a n d n u m b e r o f obse rva t i ons .

entirely above 20%. Maps have been made of the classified PSTs per area.

2.5. C h a n g e in the g r o u n d w a t e r q u a l i t y w i t h t i m e

The difference between the first (1984) and last observation (1993) was assessed for each filter with a long series of observations. The correlation of the concentration with the time in the 1984-1993 period was also calculated. A filter for which there was no observation at the beginning or end of the period con- sidered was not taken into account.

Limits of determinations are not constant with time. Observations in a series lower than the highest limit of determination were taken as equivalent to the highest limit of determination occurring in the series in question. This was necessary to suppress an apparent correlation due to changes in the limit of determination. If in an area or group the average of the differences and the average of the correlation coefficients are both less than zero or both greater than zero (one sided test where P = 0.95) the concen- tration in the period is concluded to be significantly increased or decreased.

3. Results and discussion

3.1. N i t r a t e

In Fig. 2 confidence intervals are given for the aver- age concentrations per landuse/soil type and depth range. In the west of the country (dune and coast wall, low peat and marine clay areas) the average concentrations are significantly below the target value. Nitrate occurs in the deep groundwater under horticultural areas on dune sand. In contrast to the groundwater under built-up areas on river clay no nitrate was present in the groundwater under grass on river clay.

In shallow groundwater in the sandy areas, the range of nitrate concentrations was larger than else- where. Especially under grass on sand, maize on sand and croplands on sand, high concentrations were found. Only for maize on sand the average concentra- tion in the deep groundwater significantly is above the target value.

Confidence intervals for the PST per landuse/soil type and depth range are given in Fig. 3. The PSTs in both shallow and deep groundwater were low for

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H.F.R. Reiinders et al./Jourmzl of Hydroh)gy 207 (1998) 179-188 184

5-15 m below ground level

~ very high

high f f _ , ~--~

15-30 m below ground level ~

Fig. 4. Nitrate in the groundwater in 1992: percentage of the surface area above the target value per physico-geographical area (1 = loam/loess, 2 = southerly sand, 3 = central sand, 4 = easterly sand, 5 = northerly sand, 6 = dunes and coastal walls, 7 = rivers, 8 = marine clay, 9 = low peat).

grass on peat, grass on marine clay, croplands on marine clay and grass on river clay. A low PST was also found for built-up areas on marine clay in the deep groundwater.

High PST is found in shallow groundwater under grass on sand and croplands on sand and built-up on river clay. Very high PSTs were lound in the shallow groundwater under maize on sand. Low PST were found ~br grass on sand and forest on sand in deep groundwater.

The physico-geographical areas are classified on the basis of the PST (Fig. 4). Low PST were found

in the west of the country and the river area. in the central, eastern and southern sandy areas a high PST was found in the shalh)w groundwater. The PST is also high in the deep groundwater of the central sandy area and the loam/loess area. The results are in agreement with the expectations mentioned in the introduction.

Fig. 5 shows confidence intervals for the average change in the nitrate concentration per group and per area in the 1984-1993 period. For the shallow groundwater in the central sandy area the average change and correlation coefficient are

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H.F.R. Reijnders et al./Journal ~[ Hydrology 207 (1998) 179-188 185

NO3-N (g/(m3.deca))

7.5 .....

5

2.5

0

-2.5 . . . . . . . .

- 5 i r

1 2

i :: : [ [] shallow Im deep

3 4 5 6 7 8 9 10 l l

physico-geographical area

l=loam/loess, 2=southerly sand, 3-central sand, 4-easterly sand, 5=northerly sand, 6,,,dunes and coastal walls,

7-rivers, 8-marine clay, 9-low peat, 10---complete sandy area, 1 l-the whole of the Netherlands

Fig. 5. Nitrate in the groundwater in the 1984-1993 period; 95% confidence intervals for the average concentration change per 10 years (deca) and the number of observations per area.

significant positive. A significant increase of about 2g.m -3 in 10years in the N O 3 - N concentration was established for the shallow groundwater in the central sandy area. Increases in the nitrate concentra- tion in the sandy areas is expected because the groundwater tables have dropped. For the loam/loess area no interals for the change could be calculated because only a few measuring stations are available.

3.2. Aluminium

Fig. 6 shows confidence intervals for the average concentrations per landuse/soil type and per depth class. In the west of the country (dune and coast wall, low peat and marine clay) and in the river clay areas the averages were significantly under the drink- ing water standard. The confidence intervals for the average concentration in the shallow groundwater under grass on sand, maize on sand and forest on sand in the sandy areas were higher than in the west of the country. For maize on sand, the average con- centration was significantly higher than the drinking water standard. The average concentrations in the

deep groundwater under grass on sand and croplands on sand were significantly below the drinking water standard. The broad confidence intervals for most groups in the sandy areas were the effect of spreading in the measured concentrations.

The percentage surface area above the drinking water standard was calculated for aluminium. The confidence intervals for the PSD landuse/soil type is shown in Fig. 7. The PSDs are generally low. High and very high PSDs are found for the shal- low groundwater under maize, croplands and forest on sand.

The southern and northern sandy areas, respec- tively, stand out because of the very high, and high PSD (Fig. 8). Low PSDs are found especially for the river area and the low-peat areas. The PSD in the deep groundwater is low or very low.

As expected, high aluminium concentrations occur in shallow groundwater under sandy areas with nature as landuse. The high percentages of surface area with concentrations above the drinking water standard in the shallow groundwater under maize and croplands were not expected, but these high PSDs cannot be explained. It is speculated that acidification and

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186 H.F.R. Retjnders et al./Journal o/'Hydrology 207 (1998) 179-188

A1

900

800

700

600

500

400

300

200

100

0

(mg/m 3)

[ ] shallow [] d o o p ................ i . . . . . .

drinking water standard

33

| 1 4 ~ 1 4 1 3 : , u ? 7 " ~ , ~ [ . . J Q [ ' 7 1 9 I 9 1 8 1 3 1 2

i E i i i J r r l r r l

n/ds h/ds g/1 g/mc c/mc b/mc g/rc b/rc g/s m/s c/s f/s b/s <dune&coast wal lx low peat, marine clay areas >< river area >< sandy area >

n-natural areas, h-horticulture, e=crops, g-grassland, re=maize, b-built-up areas, ds- dune sand, I- low peat,

me=marine clay, rc=river clay, s=sandy area

Fig. 6. Average aluminium concentration in groundwater per landuse, soil type in 1992; 95% confidence intervals for mean concentration, number of observations and drinking water standard.

% [ ] sha l low I : :

90 [ ] deep j . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8O

70 . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . 20 . . . . . . . . . L

60 . . . . . . . . . . . ! . . . . . . . . 2 S ~ . . . . . . . . . I

4050 . . . . .

m;' N" . . . . . . : i~ 3O f i r e

20 ~ 262s ~3 R U :

10 t " i . . . . - . . . . - ,o, 0 • . i . . i D.i . . . . . . .

r i i ~ i i i i i u i i u

n/ds h/ds g/1 g / m c c / m c b / m c g/rc b/rc g/s m/s c/s f/s b/s <dune&coast wall><low peat, marine clay areas >< river area x sandy area >

n-natural areas, h-horticulture, c-crops, g-grassland, m-maize, b-built-up areas, dr,.- dune sand, l=low peat,

mc=marine clay, rc-river clay, ~ sandy area

Fig. 7. Aluminium in the groundwater per landuse, soil type in 1992; 95% confidence intervals for percentage surface area above the drinking water standard (200 mg/m 3) and number of observations.

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H.F.R. Reijnders et al./Journal of Hydrology 207 (1998) 179-188 187

lUn°thigh °

i 15-30 m below ground level

Fig. 8. Aluminium in the groundwater in 1992; percentage surface area above the drinkingwater standard per physico-geographical area (I =

loam/loess, 2 = southerly sand, 3 = central sand, 4 = easterly sand, 5 = northerly sand, 6 = dunes and coastal walls, 7 = rivers, 8 = marine clay, 9 = low peat).

aluminium mobilisation is caused by fertilization of croplands with ammonium sulphate in former times.

The very low PSD for the easterly sandy area is contrary to expectation. The reason for these low PSDs may be the geochemical situation in this area. The change in the concentrations in the long term has not been established because aluminium has only been determined for 4years at all locations.

4. Conclusions

4.1. Nitrate

Average nitrate concentrations in groundwater are found to be under the target value in the west of the country and in the river areas constituted by clay and peat. The average concentrations in the sandy areas in the cast of the country are higher than in the west of the Netherlands, while the average concentrations in

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188 14. F.R. Reijnders et al./Journal of Hydrology 207 (1998) 179-188

shallow groundwater under maize are significantly higher than the target value.

High percentages of surface area above the target value in the shallow groundwater are found under grassland and croplands on sand, while very high per- centages are found for maize on sand. At regional level the central, eastern and southern sandy areas are striking because of high percentages of surface area above the target value. In the deep groundwater these percentages are low.

In the shallow groundwater in the central sandy area, the average concentration of NO 3 - N clearly increased (about 2 g.m -3 in 10 years). This is related to the effects of manuring in the aerobic groundwater in the elevated ridges.

4.2. Aluminium

As for nitrate for aluminium the highest concentra- tions are found in some of the sandy areas. The con- centrations are significantly higher than the drinking water standard in the shallow groundwater under maize on sand. This was not expected.

The expectation of occurrences of high concentra- tions in groundwater under nature, was confirmed by the observations. At regional scale the northern and southern sandy areas show high, and very high percentages of surface area above the drinking water standard. Differences between sandy areas were not expected and are explained by the difference in geo- chemical situation of the sandy areas.

Acknowledgements

Ir W. van Duijvenbooden is kindly acknowledged for the design and development of the monitoring networks and encouraging co-operators to use the facilities for various purposes in the environmental field.

References

Boumans, L.J.M.. 1994. Nitrate in the upper groundwater under

natural areas on sandy soil in the Netherlands. RIVM-report 712300002, RIVM, Bilthoven (only available in Dutch).

Boumans, LJ.M., Meinardi, C.R., Krajenbrink, G.J.W., 1989. Nitrate concentrations and quality of the groundwater under grass in the sandy areas. RIVM-report 728472013, RIVM, Bilthoven (only available in Dutch).

Drecht, G. van, Boumans, L.J.M., Reijnders, H.F.R., 1994. National image of the groundwater quality, method and results for nitrate. RIVM-report 714801001, RIVM, Bilthoven (only available in Dutch).

Drecht, G. van, Reijnders, H.ER., Boumans, L.JM., Duijvenboc~len, W.V.. 1996. The quality of the groundwater at a depth between 5 and 30 meter in the year 1992 and the change in the quality in the 1984-1993 period. Rapport no. 714801005, RIVM (only available in Dutch).

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