Geochemical mapping based on overbank sediments in the heavily industrialised border area of Belgium, Germany and the Netherlands

Journal of Geochemical Exploration 56

as

a. *

9

’ Belgian Geolopiml ’ Geological

S!rrr,e_voj the Netlterhds,

’ Niedersiichsiscitcs

e

C

b 9

t B996) 9 1- 104

9

Sur~>e_~. Jertrterstr. 13. lOOO-Brtrssels. Belgiunt Richrd

Holklrde 10. P. 0. Box 157, 2000 AD Haarlent,

L.mtdesanz~,fiir Bodenforsclzung.

’ Geologisclses Lurtdesnntr Nordrltein-

Westf’len,

Net1terlclrtd.v

Postfach 510153, D-30631 Hanttor*er, Genttany Pos@tch 1080, D-47710 Krqfeid. Gertttart_v

’ F~.~ico-c.hentisrhe Geologic, K. C’. Lew*ert, Celestt’jttettlaurt 2OOC, 3001 h”etwlee-Lewen, Received

e 7

Belgium

15June 1995; accepted 19 March I996

The geochemical distribution patterns based on overbank and stream sediments were studied in the border region of Belgium, Germany and the Netherlands. In total 34 sites were sampled over an area of approximately 16,000 km’. Geologically the region comprises formations of the Palaeoloic Ardennes and Eifel, and the unfolded Mesozoic to Quaternary cover. Two overbank sediment samples were collected at each site; one from the lower part of the profile and another from the upper. In addition, samples of active stream sediment were taken from the same sites for comparisos and complementary geochemical information. Geochemical distribution patterns for the lower overbank sediment samples reflect the natural situation, with lithology as the main influencing factor. Natural anomalous patterns, due to Pb and Zn mineralization, are also detectable. The upper overbank sediment clearly shows the influence of Pb-Zn mining and metallurgy, as well as other anthropogenic chemical contamination. Pollution i*; even stronger in stream sediment, obscuring almost completely the natural pattern. The results of these investigations demonstrate the applicability of overbank sediment as a sample medium in regional geochemical mapping. However, for an efifective interpretation of the results a good knowledge of the sedimentological history and age of the sample sites is required in mining and industrial areas.

1. Introduction

A geochernical survey was carried out using overbank and active stream sediments in the border region of Belgium, Germany and the Netherlands * Corresponding author. 0375~6742/96/$15.00 PII

Copyright 0

SO375-6742(96)00009-X

(Fig. 1). The area was selected for an international research study at the initiative of the Working Group on Regional Geochemical Mapping (Biilviken et al., 19%), which was created by the Wec;tem European Geological Surveys (WEGS), presently the Forum of European Geological Surveys (FOREGS). In this study the recommendations made by the latter au-

1996 Elsevier Science B.V. All rights reserved.

92

of a simple and i

compasses the north-central part of the fold belt of the Ardennes and Eifel, including several mineralizations and a Caledonian inlier. The northern esozoic and Caenopart is composed of younger zoic sediments including a thick Quaternary cover (Fig. 2). Large parts of the area have been severely polluted by mining and industry, making it very difficult to define natural geochemical backgrounds by active stream sediment. The use of overbank sediment as a sample medium in geochemical mapping has already been discussed by Ottesen et al. (1989). The main advantages are representativity for large drainage areas, which allows a reduction of sampling density for general mapping purposes, and the possibility to sample both

are presumed to be pristine and another two based on riced by man-made pollution. The

survey in unpolluted areas. The other two maps will give a realistic picture of present-day geochemical lev4s. Furthermore, based on the geochemical signatures of overbank sediments a fourth map of the actual pollution could be generated. The main objective of this research study was to examine the usefulness of overbank sediment as a sample medium in regional geochemical mapping in a complex area typical for Western Europe. Further. stream sediment was also sampled in Belgium and

n

QUATERNARY

m

LATE

----i EARLY m

TERTIARY TERTIARY

CRETACEOUS TRIAS CARBONIFEROUS MIDDLE & LATE DEVONIAN EARLY DEVONIAN

m 3 m

CAMBRIAN, SILURIAN

0

20

ORDOVICIAN

40 I

Km

Fig. 2. Schematic geolo&al

map of the border area of Belgium. Germany and the Netherlands. compiled from De Bethune

and Vanhoome (1967). Walther and Zitzmann (1969). Arnold et al. (197

I ) and Van Staalduinen and Van Veen ( 1975).

( 1% 1). PaWe

the Netherlands from the same sites as the overbank sediment for comparative and complementary purposes. Due to river armouring it was, however, impossible to take stream sediment samples in Germany.

2. Geology and environmental framework Geological units outcropping in the study area range in age from Cambrian to Quaternary (Fig. 2). The lithology of the stratigraphic units in the study area is briefly described below. Palaeczoic rocks consist mainly of detrital sediments, mostly sandstone, quartzite, shale and phyllite, with some carbonate rocks of middle to late Devonian and early Carboniferous. Lead-zinc mineralization is commonly associated with limestone and dolomite, mainly as discordant veins (Fig. 3;

Dejonghe and Jans, 1983). In the Ordovician bordering the lower Palaeozoic Stavelot-Venn massif, manganese is enriched in sl,ate and shale. Small magmatic intrusions penetrating the massif show indications of polymetallic mineralization (Zn, Cu, MO, Pb, As, Co and Ti; Weis et al., 1980). In the centre of the area, Cretaceous chalk is present. In the eastern part, a Triassic wedge of conglomerate and sandstone occurs which hosts the Maubach-Mechemich PbS mineralization. Some limestone also occurs to the north of the Devonian of the Eifel. The Tertiary comprises mainly marine detrital sediments, without carbonates, whereas the Quaternaly deposits are chiefly fluviatile and peri-glacial. They are overlain by some aeolian deposits (not shown in Fig. 2), a large loess area in the central part and some scattered drift dunes in the northern section.

Fig. 3. Major geological and environmental features that may have an influence on the geochemical dispersion patterns.

main contamination the nineteenth century.

Table I Univariatc stalktics after loll,-lran~lbrtllationand correctionfor lohhon ignition of U- and L-samples(same sample locations:G-mean = wometric mean’. II = numberof’ samples) L’ Upper samples(11= 34) G-mea SiO, (‘4 1 TiO, A&O, Fe,O,

Min.

SO,

82.40 0.73 7.50 3.60 0.09 0.59 0.67 0.66 I .x0 0. I7 < 0.05

I.1 I 0.05 c 0.05

As (ppm 1 Bti

I6 10x

<5 360

Ml10

MpO cilo Nil,0 K,O P,Oi

7 I .HO 0.38 3.40 0.98 0.02 0. I 1

0. IL) 0.33

Lower samples(II = 33) MIX.

91.70 I .__ ‘? 14.30 6.80 0.99 I.10 2.42 I.15 3. IO 0.42 0.20

G-mean x3.00 0.75

7.30 2.80 0.06 0.5 I 0.63 0.66 I .80 0.07 < 0.05


28 720

IOH)

63

< IF

I86

5’ __

IO

<7

I I7 h

6-t d. .5

‘)‘I _-_ 77

III

CU

25

< IO

736 23 58 I.31

II 7 < 20 34

IO I8 90 1600 I46 I6

<5 IO 21 26 71 8

Mo Nb Ni Ph Rh SC


<5 <7 < IO 37 4

6

sr1

1

Sr

73

12

T;1 Th U V

< IO I2 <7 60

< IO c IO <7 IX

< IO 28 <7 12-I

< IO I2 <7 so

W Y Zn Zr

< IO 27 212 716

< IO
II 52 4670 2377

< IO 30 7x 792

<3

0.24

230

Cl Ca


0.16

x

IO

x c 20 31

I.10 2.73

360

60

Ga Hi La

94.60 2.3s 16.60 7.50 I .26 1.36 5.32

69

Co

I7

68.80 0.36 2.20 0.93 0.01 0.07 0.18 0.23 0.99 0.02 < 0.05

Max.

IOX

Cc

<7

Min.

9-l

I57

<3

73

5s
< 1047 <5 < 20 <5 <5 <5 <7 < IO 3s <5 <3

37 < IO < IO <7 I9 < IO 8 I6 300

30 428 21

21 253 Ill 34 37 66 506 147 I9 29 ISI IO 39 8 120 12 73 620 9265

W, De Vos et nl. /Jownni

96

of Geochentical Esplorntion 56 f 1996)YI- 104

three samples were collected: (1) an overbank sediment at depth (L-sample), (2) a near surface overbank sediment (upper 40 cm below the surface, U-sample) and (3) an active stream sediment (S-sample). Local field conditions dictated the final sampling method. The lack of relief and of active incisions in some alluvial plains in the Netherlands made it necessary to use an auger drill (see below). In the Belgian and German areas, samples were obtained from profiles presently being eroded by meandering rivers. In the basins of the Rur, Inde, Erft and Rothbach in Germany, Holocene terraces were sampled; their position and age are well documented by detailed mapping (Schalich, 1968). More detailed information on the vertical distribution patterns in these overbank sediment sequences, as well as the sampling strategy followed in the terraces, are described by Hindel et al. (1996). In the German (Fig.

1)

drainage basins the sample of the oldest terrace was classified as an L-sample in the statistical treatment and mapping, and the top of the youngest terrace as the U-sample (see below). Active stream sediment, in Belgium and the Netherlands, was sampled near the overbank sediment sampling site. This often proved difficult, because many rivers draining alluvial plains are canalised and armoured. Low water levels in the summer of 1991 allowed useful sampling to take place. Stream sediment samples were not collected in Germany because of canalisation. Analytica are, however, available from a regional stream sediment survey, based on small streams, carried out earlier in Germany (Fauth et al., 1985). The 20 kg samples were dried at 80°C, disaggregated and sieved to 125 microns. One gram of the < 125 km fraction was subsequently mixed with 5

5’

Iso Fig. 4. Spatial geocbemical pattern of Ai,O, &les were not taken in Germany).

30’

30’

in the lower overbank (L), upper overbank WI and active stream (S) sediment samples

To facilitate interpretation of the data, several methods were used to estimate the age of de of the overbank sediment sequences. T tern, of course, was that the lower o ment sample should be older than a few hundred years, before industrialisation started, i.e., “pristine”. b-r the Netherllands most L-samples were obtained through drilling, and determination of the sampling

logical dating tool was of the stratigraphy in this country is based on “pol(Zagwijn, 1975). acroscopic inspection of available profiles in lgium helped to select th most likely pre-instrial ones. Hn two cases ( 03 and 9306 respectively in the rivers Ambleve and Vesdre) even the lowest part, at about 2 m depth, contained slag and coal fragments. This points to rapid recent sedimen-

Fig. 5. SFatial geochemical pattern of Pb in the lower overbank (L). upper overbank (U) and active stream 6)

sediment samples.

98

W. De Vos et al. /Journal of Geochemical Explomtiorl 56 (NY61 YI- IO4

tation. Ten Belgian samples were selected for 14C dating, four of them on wood fragments, and the remaining ones on dispersed organic material. Eight samples yielded an absolute age between 1400 and 6000 BP while the Jeker sample (B07) was estimated to be about 270 years BP on a wood fragment. The Vesdre sample (BOS) gave a modem age on wood. In the German river plains of Inde, Rur, Elle, Rothbach and Erft a terrace system is present, which was first described in detail by Schalich ( 1968). Three postglacial terrace ages are distinguished, and the sampling in this area takes advantage of these relative ages. Samples at depth in the old terrace, dating from the beginning of the Holocene, were used as pre-industrial L-samples. Material of river loam from the youngest terrace, about one century in age, was used as U-samples.

4.2. Unicariate statistics of ocerbank sedimerzts The univariate statistics of the L- and U-samples are presented in Table I. Thresholds have been determined from histograms and cumulative frequency curves, and the raw results plotted on maps, only a few of which have been reproduced here (Figs. 4-7). The locations of sample sites and rivers are shown in Fig. 1. Major element concentrations for both samples are comparable except for P,O,, which is highest in the U-samples. Further, for most trace elements, such as Ba, Ce, Co, Cr, Cs, Ga, Hf, La, MO, Nb, Ni, Rb, SC, Sn, Sr, Ta, Th, U, V, W, Y and Zr, the mean concentration values are comparable. It is noted, however, that the maximum value for TiO, and some other elements (Ce, Cr, Hf, MO, Nb, Th, Y, Zr)

Fig. 6. Spatial geochemical pattern of Zn in the lower overbank (L), upper overbank (U)and active stream 6) sediment samples.

even more pronounced if the maxi

or U-samples (Table 2). A relative increase in mean and maximum values from older to younger material is apparent. An interpretation of these observations is given below. 4.3. Geochemistry qf the lower orerbartk sediments (L-samples) Silica contents are generally higher in the northem area, where sediments are more sandy, whereas

origin. Iron oxide is enriched in the sample sites along the rivers Our-the, Ambleve, Vesdre, euse, ur and Erft. This enrichment probably relates to Devonian and older rocks. anganese oxide showed a general enrichment in the Palaeozoic shale-bearing formations, with strong

‘5 -

jlo -

L5’ -

30’ -

I

0 < 25 n>,25

ppm n13m

I

_-

I6O

I 30

Fig. 7. Spati;qI geochemical pattern of Cu in the lower overbank $1. upper overbank (U) and active stream (S) sediment samples.

anomalies in the Amblkve and the Rur, which drain Salmian (Lower Ordovician) areas, where Mn-rich layers are known to occur. Calcium oxide has elevated values in the overbank sediments of the rivers Mehaigne, Hoyoux, Jeker, Geul, Meuse (NL06) and Niers, which are generally related to Cretaceous, Muschelkalk and Palaeozoic calcareous substrates. The distribution patterns of Na,O and Sr showed the same tendencies

as CaO, suggesting a similar enrichment of lithological origin. Among the trace elements Pb, Zn and Cu are the most interesting. Lead shows strong anomalies (Fig. S), which can be explained by known Pb-Zn mineralizations in the areas covered by Palaeozoic rocks (Fig. 3). The anomaly in Rothbach is downstream of the Mechemich deposit, and that in the Rur may be caused by the vein-type lead-zinc mineralization of

Table 2 Univariate statistics after log-transformation and correction for loss on ignition of active stream, lower and upper overbank sediments (only sample locations in Belgium and the Netherlands; G-mean = geometric mean: n = number of samples) Lower samples (II = 2 I)

Upper samples (n = 22) G-mean SiOz (c-/c) TiO, A120, Fe& MnO M@ CaO Na,O K?O PzOz) SO, As (ppm) Ba Q co Cr cs Cll Ga

Hf La MO Nb Ni Pb Rb SC Sn Sr Ta Th U V W Y Zn Zr

84.70 0.7 I 6.40 3.00 0.06 0.40 0.58 0.64 I .64 0.13 < 0.05 14 346 s4 8 109 5 I9 6 < 20 30 <5 9 17 52 62 7 3 64 < 10 13 <7 50 < IO 24 164 864

Max. 77.60 0.38 3.41 0.98 0.02 0.10 0.19 0.33 1.11 0.05 c 0.05 <5 260 17 <7 64 x5 < IO <5 < 20 6 <5 <5 <7 < 10 37 4 <3 42 < IO < 10 <7 18 < IO <5 29 427

Min. 91.70 I .22

Il.50 5.98 0.27 0.84 2.42 1.15 2.10 0.39 0.20 69 849 100 22 196 II 223 165 58 131 IO 16 52 437 87 13 94 90 < IO 28 <7 96 < 10 37 1237 2477

G-mean 85.30 0.76 6.17 2.31 0.07 0.41 0.60 0.65 I .64 0.06 < 0.05 8 322 58 9 112 <5 < 10 <5 < 20 31 <5 10 15 16 61 7 <3 66 < IO 13 <7 43 < 10 27 59 970

Max. 75.70 0.36 2.2 1 0.93 0.01 0.07 0.18 0.23 0.99 0.02 < 0.05 <5 240 < IS <7 55 <5 < 10 18 < 20 <5 <5 <5 <7 < 10 35 2 <3 37 < 10 < 10 <7 19 < IO 8 16 300

Active strearn sediment Min. 94.60 2.35 I I .90 6.13 0.34 1.01 5.32 1.10 2.43 0.15 0.24 28 510 IX6 29 428 I5 47 5 253 111 37 37 58 506 103 13 29 90 C 10 39 8 78 II 73 620 9265

G-mean 83. IO 0.67 5.48 3.17 0.06 0.44

1.20 0.58 I A4 0.38 0.13 I3 379 52 9 13s <5 34 x5 21 30 <5 8 23 94 53 6 7 74 < 10 10 <7 45 < IO 25 400 867

Max. 68.70 0.31 2.90

1.34 0.02 0.14 0.25 0.34 0.78 0.08 < 0.05 <5 242 29

7 62 <5 < 10 I8 < 20 18 <5 <5 <7 19 28 3 <3 45 < IO < 10 <7 25 < 10 12 93 386

Min. 91.20 1.21

12.20 6.04 0.49 1.91 9.26 0.93 2.00 2.67 I .37 I39 977 II8 35 I309 12

519 78 67 32 17 377 882 82 12 226 147 15 30 9 178 17 41 4412 3402

izations of Plombieres and La Calamine, and in the Inde, downstream of the Stolberg deposits. Samples NL06 on the Meuse (Maas) and NL03 on the lying in the natural enrichment trail. Zinc has a pattern similar to but the ratio, anomaly versus background, is les onounced (Fig. 61, a feature which may be explained by its greater mobility. The same influ es from the mineral deposits can be recognised. addition, the Neffel and the Ambleve show anomalous values. The latter were interpreted as due to anthropogenic influence. It is also noted that the relative increases in Zn content in the Rur could probably be linked to the mineralizations of Rescheid. This reflects the predominance of galena over sphalerite. Copper shows only weak cu t&lions with other elements like Zn, Ni and Pb. Etevatc>,dcopper concentrations are present in the rivers Rur, Swalm. Geul, Vesdre and Ambleve (Fig. 7). They can be traced to natural mineralizations in the rivers Vesdre, Geul and Rur (Fig. 3). It is noted that the contents of Pb, Zn and Cu in the L-samples never reach the high values of the anomalous U-samples, a feature also reflected in the univariate statistics. Nickel has different background values according

Table3 Multi-element anomalies in the upper (U-samples) not found in the lower overhank sediments (L-samples). and attributed to kuman activities River

Anomalous elements

Kleine Nete Grote Nete C’esdre Dommel Geul Inde Rur Wittsee Niers

Fe. P. As. Cu. Ni. Zn Fe, P. As As. Cr. Cu. Sn P. As. Zn P. Pb, S. Zn Fe, P, S, As. Cr. Cu. Ni. Pb. Sn, Zn P. As, Cu, Cr. Ni. Sn. Zn. Pb Cr. Cu. Sn Fe. P. As

part of the research area. This element association occurrence of heavy minerals, romite, rutile (and its

the heavy mineral content of Neogene formations in this area. without mentioning monazite, xenotimc, cassiterite and chromite. The very high heavy mineral content in overbank sediment sample B 10 could be due to a local palaeopracer, exposed by present n German L-overbank sediments a high Zr content has been attribuhed to the accumulation of zircon originating from the loess.

The upper overbank sediments show an Al,O, pattern similar to the lower samples, as illustrated in Fig. 4. The lithological contrast between Palaeozoic and younger rocks accounts for this difference. This distribution of lithological origin can also be observed for MgO, I&O, Rb, SC. Ga, V, Ce and Y. In the northern area a weak anomaly is present for Zr. Hf, TiO,, Cr, Nb, Th and La, related to the heavy mineral assemblage as explained above. Higher P,O,, As, Cu, Pb, Sn and Zn concentrations in the upper overbank sediments can be attributed to pollution by human activities. The Pb distribution map (Fig. 5) reveals strong anomalies downstream of known mining or metallurgical areas (Fig. 3; Vesdre, Geul, Meuse, Inde, Rur, Rothbach and Erft river plains). The Wittsee anomaly is not related to mining. It the same Gwsiwid i:, u& a~ for the L-samples, most of the heavy metal contents in the U-samples are anomalous (Fig. 5). The Zn distribution shows the strongest anomalies in the GeulMeuse and Inde-Rur valleys (Fig. 6). Mining works

102

W. De Vos et al. / Jownnl of Geochenukd Explordon

and metallurgical industry (Fig. 3), especially brass, are responsible for these anomalies. In the drainage basins of the rivers Dommel, Vesdre and Wittsee no mining occurred, but metallurgy and other industrial activities may account for the observed anomalies. The CU distribution (Fig. 7) shows strong anomalies in the Inde, Rur and Vesdre valleys, attributable to metallurgical industry. The rivers Kleine Nete, Swalm, W&see and Uffelsche Beek show weaker lution. This anomalies, probably due to local in the Geul pattern is similar to that for Zn, exe and Dommel valleys where Cu is not anomalous. Arsenic is enriched in the rivers Inde-Rur, Vesdre, Dommel and Kleine Nete, mostly in association with either Cu or Zn, and always in industrial areas. Several multi-element anomalies due to human activities, which are not obse ed in the L-samples, could be found, most notably, in the rivers Inde, Rur and Kleine Nete. They are listed in Table 3. In the case of Pb and Zn weaker anomalies do exist in the L-samples and have been described above. 4.5. Geochemistry of the stream sediments One apparent feature of the active stream sediment results (only data from Belgium and the Netherlands, Table 2) is that the average value of variables, such as Al 203, K,O and Rb, associated with the clay content, is comparatively low. This is probably related to the lower contribution from the Palaeozoic area, which mainly lies in Germany. The Al,O, map illustrates this feature fairly well (Fig. 4). High metal values could probably be explained by contamination due to human activities. Copper anomalies (Fig. 7) occur in the rivers Vesdre, Dommel, Rur, Demer, Herk and Meuse. High Zn contents (Fig. 61, due to the anthropogenic influence, are found in the rivers Geul, Meuse, Vesdre, Dommel and Rur, and possibly, to a lesser extent, in most of the other rivers. Lead has approximately the same pattern (Fig. 5). It is noted that some extremely high values correspond to very organic rich samples. Strong contamination occurs for Cr in the Demer valley (1204 ppm compared to 53 and 60 ppm respectively in L- and U-overbank sediment samples). Anomalous SO, contents occur in the river Vesdre. Phosphorus oxide has values above 1% in

56 (19961 91- 104

both the Vesdre and the Demer. The average P205 content in stream sediments is five times higher than in pre-industrial overbank sediments, which may be explained by fertilisers in agricultural practice, and detergents for domestic and industrial purposes. The enrichment of Ba in the Vesdre and Rur may be due to the paper industry. Regarding mineral exploration, key elements like Cu, Pb, Zn and Ni, cannot be used to make a valid interpretation of their natural occurrence. Their geochemical distribution apparent in stream sediments in this area is a combination of natural and human influences. 5. General conclusions The following conclusions can be drawn from the results of this geochemical mapping effort in a complex, heavily industrialised region of Western Europe. Geochemical distributions in the lower overbank sediment samples and available age dates, indicate that natural patterns are present, reflecting the pre-industrial situation. But whether these can be called pristine is not always clear. Geochemical distributions in the upper overbank sediment samples show for some elements (Zn, Pb, Cu, As) important differences when compared to the patterns of the lower samples, which can be explained by pollution through human activities. However, for several elements (e.g., Ba, Ce, Co, Cr, Cs, Ga, Hf, La, MO, Nb, Ni, Rb, SC, Sr, Ta, Th, U, V, W, Y, Zr) no systematic differences occur, suggesting that industrial activity in the catchment areas had no, or only a very limited influence, on the distribution of these elements. The contrast between the lower and upper overbank sediment samples proves that two distinct geochemical patterns could be mapped, one map representing the pre-industrial situation and the second showing the superimposed human influence. The active stream sediment is generally more polluted than the upper overbank sediment, and the patterns do not reflect the natural geochemical distribution. They essentially show the sum of natural and human influences, and could only give an indicative picture of element contents in surface materials at the present time.

The results prove t of anthropogenic con chemical pattern related to the original unm mineralization. This conclusion is similar to that reached by McConnell et al. (1993) in Newfoundland. The sampling of overbank sediment at depth is not a routine procedure. Careful profile selection, knowledge of the sample age and of basin and human history are highly recommended for an effective interpretation of the geochemical results, especialgy in areas with industrial and mining activities. ‘This increases the cost of the method but also the quality of the data. Macklin et al. (1995) came to similar conclusions. The latter authors as well as Ridgway et al. ( 1995) put forward valuable arguments which prejudice the viability of overbank sediment as a regional geochemical mapping medium. Geochemical mapping based on the systematic collection of low order stream sediment samples clearly is preferred by Ridgway et al. (1995). According to them this mapping has shown to be effective in discriminating between contaminated, mineralised and background regions (e.g. British Geological Survey, 1992). This is also the reason why this samping medium has been put forward by Darnley et al. ( 1995) for establishing a global geochemical database. However. as shown in this pilot study, the geochemistry of the stream sediments in this industrialised area reflects a combination of natural and human influences and for certain elements it is not possible to gather information on natural distributions. In this case it is obvious that overbank sediments form an alternative sampling medium. It therefore should not be disregarded in regional mapping programmer. Finally it should be noted that at some locations, due to the erosion regime. it is impossible to obtain pristine overbank sediments. A geomorphological landscape analysis. however. could help to identify suitable sites, where local sedimentation occurred.

ng. The comments m

Arnold, H., Braun. F.J., Heide, G.,

erbst. G.. Hoyer, P., Knapp, G., Knauff, W.. Quitzow, H.W.. Thiermann, A., Thome, K.N., Balke, K.D., Pirling, R. and Siebert, 6.. 1971. Geologie am

Niederrhein. Geologisches Landesamt Nordrhein-Westfalen, Krefeld, pp. 6-48. Biilviken, B., Demetriades, A., Hindel, R., Locutura. J.. O’Connor. P., Ottesen. R.T., Plant, J., Ridgeway, J.. Salminen, R., Salpeteur, I., Schermann, 0. and Volden, T. (Editors). 1990. Geochemical Mapping of Western Europe towards the Year 2OOO.Project Proposal. Geological Survey of Norway (NGU), Gpen File Report 90-106, 12 pp, 9 appendices. Bolviken. B., Bogen, J., Demetriades, A., De Vos. W., Ebbing, J.. Hindel. R.. Ottesen, R.T., Salminen. R.. Schermann. 0. and Swennen. R., 1993. Final Report of the Working Group on Regional Geochemical Mapping 1986-93. Forum of European Geological Surveys (FOREGS). Geological Survey of Norway (NGU) Open File Report 93.092, 18 pp., 6 appendices. British Geological Survey, 1992. Regional Geochemistry of the L,lke District and adjacent areas. British Geological Survey, Keyworth, Nottingham. Cleveringa, P., 1992. Pollenanalytisch onderzoek van de boringen Geleenbeek, Niers, Neerlem, Schalberg en Ten Graaf. Geological Survey of the Netherlands (RGD), Haarlem, Internal report 1163.4 pp. Darnley. A.G., Bjiirklund. A.. BSlviken. B., Gustavsson, N., Koval, P.V., Plant, J.A., Steenfelt, A., Tauchid, M. and Xie Xuejing, 1995. A global geochemical database for environmental and resource management. Final report of IGCP Project 259, Earth Sciences 19, UNESCO Publishing. De Bethune, P., 1961. Atlas van Belgi& Blad 8, Geologie. Militair Geografisch Instituut, Brussels. Dejonghe, L. and Jans, D., 1983. Les gisements plombo-zinciferes de I’Est de la Belgique. Chron. Rech. Min., 470: 3-24. Demetriaaes, A., Ottesen, R. and Locutura, J. (Editors), 1990. Geochemical mapping of Western Europe towards the Year 2OOO.Pilot Project Report. Geological Survey of Norway (NGU), Open File Report 90-105, 9 pp., 10 appendices.

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