4. Controls on the major groundwater composition
author: Frank Wendland (FZ Jülich, Germany)
Contents
4.1 Introduction
4.2 Groundwater composition in German hydrogeological units 4.2.1 Unconsolidated sand and gravel aquifers 4.2.2 Silicatic sedimentairy rock aquifers 4.2.3 Carbonate rock aquifers
4.2.4 Aquifers in igneous and metamorphic rocks
4.3 Electric conductivity: an important indicator for the classification of hydrogeologic units 4.4 References
4.1 Introduction
The general chemical quality of groundwater is determined by a variety of factors, where the petrografic properties of the rocks in the vadose and groundwater saturated zone, and the regional hydrological and hydrodynamic conditions are the major natural factors. Additional to these “natural” factors groundwater quality is also highly influenced by anthropogenic
influences, in particular land use. Figure 4.1 summarizes the factors influencing groundwater quality.
groundwater qualitiy
regional influences local influences
anthropogenic influences
geogenic, hydrologic, pedologic, biologic influences geogenic, hydrologic,
pedologic, biologic influences anthropogenic
influences
natural component influenced component
Figure 4.1. Factors influencing groundwater quality (Kunkel et al., 2004).
Petrografic properties of the vadose zone and the groundwater-bearing rocks
Dissolved solids in groundwater result from chemical interaction between the water and the rocks and/or the unconsolidated deposits through which the water moves in addition those dissolved in recharge water. Groundwater in hydrogeologic units (rocks or deposits) that
4-2
consist of good dissolvable minerals, will in general contain higher concentration levels than groundwater in hydrogeologic units consisting of less dissolvable minerals. It can be expected that aquifers or groups of aquifers with identical or similar petrografic properties
(hydrogeologic units) should causes a similar composition in comparable hydrodynamic and hydrologic environments. Thus the latter must be considered for the organization of the complexity of individual groundwater occurrences (aquifers) into simplified patterns, in order to provide a practical framework for an overview of major groundwater composition and related groundwater resources .
Physical hydrogeology of the geological setting
When dissolution of minerals is kinetically controlled, the length of time where the water is in contact with the aquifer minerals is a crucial factor. Thus, higher concentrations of dissolved solids are commonly found in groundwater displaying long residence times as long as no solubility equilibrium is attained with primary minerals. The residence time of groundwater is controlled by the permeability of the aquifer, the amount of groundwater recharge, the
hydraulic gradient and the length of ground-water flow path. Topography strongly influences the hydraulic gradient and groundwater recharge strongly depends on climate as well as topography. Here, climate controls the total amount of precipitation and potential evaporation and topography controls the surface and shallow subsurface runoff versus the groundwater runoff.
In some areas groundwater recharge is not more than 20 to 30 % of the total runoff. This situation is typical for regions close to the water table, e.g. in the marshy regions of coastal plains. Same situation applies for areas where groundwater runoff is bound at (to a large extent) impermeable basement rocks, like paleozoic and cristalline rocks and/or areas having high topography. Hence, in these regions direct runoff is the main runoff component. In areas where unconsolidated permeable rocks predominate groundwater runoff is to a large extent equal to the total runoff.
The permeability of an aquifer is directly related to the amount and type of porosity (open spaces between individual grains or rock particles) of the aquifer material in the case of the unconsolidated-deposit aquifers; there, the pore spaces are generally well connected. Short residence times of the groundwater can in general be expected in aquifers displaying high permeability. In the case of consolidated rock aquifers, permeability is the result of tectonic activity and bound at the amount and type of joints and fractures as well as porosity in certain cases (e.g. fragmented volcanic material, such as ash and tuff). In most cases the fractures and joint are not well connected. Assuming a similar permeability range in a certain hydrogeologic unit, the residence times depend predominantly on the groundwater recharge and the flow-paths distances. As a rule high hydraulic gradients and low flow-paths distances cause short residence times. Geologic structures, such as faults and folds, can accelerate or reduce the movement of groundwater at a local to regional scale.
We thus anticipate that for the classification of hydrogeologic units with respect to natural background levels, the hydrologic situation of the individual hydrogeologic unit has to be taken into account.
Anthropogenic influences
The anthropogenic influence of groundwater includes a modification of the composition of recharge water due to land use change (e.g. use of fertilizers, point pollutants, etc.) as well as changes in residence times of the percolation water (see Figure 4.1). Whereas the occurrence of some parameters (e.g. pesticides) is a direct indicator of human impact onto the groundwater,
most inorganic contents may originate from both natural and anthropogenic sources. Especially groundwater from shallow aquifers taking part in the active water cycle, which are used for water supply in most cases, are influenced since decades and centuries by anthropogenic activities. Consequently, the present composition of groundwater sampled from shallow aquifers does not strictly represent “natural” groundwater concentrations.
Groundwater from deeper aquifers may represent pristine groundwater in many areas, i.e., the groundwater composition is free from human impact. However, this groundwater may display higher solution contents than groundwater from shallow aquifers due to longer residence times in the aquifer and kinetically controlled dissolution, as explained above. Hence, taking
groundwater samples from deeper aquifers as reference values for the major groundwater composition in shallow aquifers may reflect on “untypical” groundwater conditions for upper aquifers.
Contrary to this, it is suggested to take predominantly groundwater samples from shallow aquifers for the definition of reference values for the major groundwater composition in the associated hydrogeologic units. The fate of individual contaminants is to a certain extent controlled by the major groundwater composition, irrespectice whether this composition may be considered as natural or not. Doing so, however, has the consequence that the omnipresent human impacts on groundwater quality of shallow aquifers have to be accepted to a certain degree. Thus, a more pragmatically understanding of the term “major groundwater
composition” and consequently also of the term “natural groundwater level”, which considers the anthropogenic influences on groundwater to a certain degree as inevitable (“natural”), needs to be developed. This is done by Schenk (2003), who considers natural groundwater
concentrations to be present, if “the concentrations of the most important cations and anions originate from not significant anthropogenic influenced soils and the rocks of a watershed, including groundwater from areas under agricultural use or from areas where land cover changes occurred over the last centuries”. Notice that it is implicitly assumed that agricultural activities have not negatively influenced the major groundwater composition, which does not hold in areas with intensive agricultural activities.
The subjects of 1. methodologies to establish natural background levels and 2. aquifer typology are extensively addressed in Work Package 2 of the BRIDGE project and reported by Pauwels et al. (2006). A German case study on major groundwater composition is presented below for illustration purposes.
4.2 Groundwater composition in German hydrogeological units
The major groundwater composition has a strong control on the behaviour of individual microcontaminantes due to varying acidity, redox status and salinity. The typical major groundwater composition varies between geographical regions due to differences in geology, climate and hydrodynamics as well as anthropogenic inputs. To illustrate the relationship between major groundwater composition and hydrogeology a case-study from Germany is presented below. The German study was executed in the framework of a study to derive natural background levels in Germany (Kunkel et al., 2004). The hydrogeologic units chosen for the description of the major groundwater environments take into account two major factors influencing groundwater composition as described in section 4.1:
Petrografic properties of the vadose zone and the groundwater bearing rocks Physical hydrogeology of the geological setting
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The principal water-yielding aquifers were grouped into four types:
unconsolidated sand and gravel aquifers, Silicatic sediment rock aquifers,
carbonate-rock aquifers,
igneous and metamorphic rock aquifers.
A further distinction was made on overall differences in water composition due to varying hydrodynamics and petrografic properties. The units distinguished and their typical major composition are described below.
Figure 4.2. Major hydrogeologic units in Germany (Kunkel et al., 2004).
4.2.1 Unconsolidated sand and gravel aquifers
Unconsolidated sand and gravel aquifers are grouped into two categories:
glacial-deposit aquifers alluvial aquifers
The two types have intergranular porosity, and all contain water primarily under unconfined or confined conditions. The hydraulic conductivity of the aquifers is variable, depending on the
sorting of aquifer materials and the amount of silt and clay present, but generally it is high.
Aquifer thickness ranges from a few meters or tens of meters up to several hundred meters in the ancient river valleys of the glacial deposit aquifers.
In addition to these reference units, the observation wells from Quaternary alluvial aquifers in consolidated rock regions were originally also planned in this group of units. These are thin, laterally not very extensive, Quaternary sediments in the low mountain range, which are nevertheless often significant for water management. The heterogeneous concentration distribution is determined by the degree of mineralization of the solid rock units in the
surrounding area. Hence, the groundwater composition needs to be regarded in accordance with the surrounding hard rock unit so that no “typical” groundwater composition of these aquifers can be found. These aquifers can thus not be considered as a separate unit.
Unconsolidated glacial sand and gravel deposits of the North European Plain
The glacially deposited unconsolidated sediments north of the low mountains are combined in the hydrogeological reference unit "Sands and Gravels of the North European Plain". Glacial deposits consist mostly of clay, silt, sand and gravel in various combinations, but also include cobbles and boulders. The general term "glacial drift" is used for all types of glacial deposits, regardless of the particle size or the degree of sorting of the deposits, or how the deposits were emplaced. The glacial drift was deposited during several advances and retreats of continental ice sheets.
Glacial ice and meltwater from the ice laid down several types of deposits. Till, which is an unstratified, unsorted mixture of material that ranges in particle size from clay to boulders, was deposited under the ice or directly in front of the ice sheet. Likewise tills are no productive aquifers. Outwash deposits, by contrast, generally consist of stratified sand and gravel that form productive aquifers. Most of the outwash deposits are in valleys; the intervening hills are mantled with till. Before or during the Pleistocene, some rivers in the glaciated area cut their channels as much as 300 feet deeper than their present riverbeds. In general, these channels appeared in front of the Pleistocene ice sheets which moved from northeast to southwest.
Consequently streams like the river Elbe or the river Weser typically flow from southeast to northwest and display deep so called ancient river valleys. Some of the deeply cut meltwater stream valleys were later filled to their present levels with glacial deposits and alluvium. Today they represent lowland regions close to the water table. Shallow Holocene deposits of marine, deltaic and fluvial origin are present in the transition zone to the seas (Baltic Sea, North Sea) and the river mouths. In these areas, seawater intrusion might typically occur.
Sand and gravel deposited as alluvium along the valleys of major streams also form productive aquifers. Some of the alluvium consists of reworked glacial deposits that were eroded and transported downstream during and following the last retreat of the ice. Most of the productive aquifers in the surficial aquifer system consist of valley-fill deposits of coarse-grained glacial or alluvial deposits, or both, and contain water under mostly unconfined conditions.
In the primary statistical analysis of the German groundwater data performed prior to the evaluation of natural background levels it was found, in actual fact, that no significant differences in substance concentrations occur if a classification according to
chronostratigraphic criteria was made. Hence, groundwaters from different glacification periods (Saalian, Weichselian) displayed the same composition. In case of a differentiation according to groundwater sampling depth, however, differences in the distribution patterns of the substance
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concentrations occurred. For this reason, the "Sands and Gravels of the North European Plain"
were evaluated according to the sampling depth. Here, 3 depth ranges were differentiated:
sampling depths of less than 10 m sampling depths between 10 and 25 m sampling depths between 25 and 50 m.
Table 4.1. Natural background concentrations (major groundwater composition) in Pleistocene gravels and sands in Germany based on ca. 8000 groundwater samples per parameter (Kunkel et al., 2004).
Parameter N Na mg/l 2019
K mg/l 2002
Mg mg/l 3307 Ca mg/l 3337 Fe mg/l 2097 Mn mg/l 2809 HCO3 mg/l 3369 SO4 mg/l 3387 Cl mg/l 3533 NH4 mg/l 3154 NO2 mg/l 1050 NO3 mg/l 3034 PO4 mg/l 1705 DOC mg/l 2720 LF µS/cm 2237
O2 mg/l 951
H µg/l 3401
pH - 3401
10. P 50. P 90. P 6,9 16,2 38,1
1,0 1,9 3,8
3,4 8,7 22,2
27,1 71 153
0,06 0,7 8,0 0,06 0,3 1,4 28,9 150 351 7,0 36,4 189 12,2 32,6 87 0,011 0,03 0,1 0,004 0,02 0,06
0,1 0,3 0,9
0,01 0,04 0,11
1,0 3,0 8,8
226 474 993
0,2 1,0 6,0
0,01 0,03 0,2
6,8 7,5 8,2
N 2197 2157 3962 4005 3012 3613 4149 4079 4207 3759 1215 3721 1814 3281 2875 1061 4070
10. P 50. P 90. P 6,5 14,4 31,9 0,8 2,1 5,1 3,0 8,9 26,5
20,9 69 149
0,02 0,7 4,8 0,05 0,2 0,7 32,2 167 332 4,3 29,2 197 11,3 29,3 76 0,001 0,02 0,4 0,006 0,02 0,04 0,1 0,3 0,5 0,01 0,03 0,08 0,9 2,3 6,2 191 440 1013 0,1 0,9 7,3 0,02 0,06 0,2 6,6 7,2 7,8
10. P 50. P 90. P
5,6 9,8 17,1
0,8 1,8 4,2
2,6 9,2 23,5
21,8 76 157
0,08 1,4 9,3
0,05 0,2 0,5
35,7 174 340
1,1 8,8 73
9,2 20,6 46
0,002 0,03 0,4 0,006 0,02 0,04
0,07 0,2 0,6
0,01 0,03 0,05
0,9 2,1 4,8
159 383 922
0,1 0,6 3,3
0,02 0,05 0,1
6,8 7,3 7,7
N 1977 1934 3733 3739 3130 3457 3897 3780 3951 3546 1089 3546 1564 3090 2843 749 3872 Parameter
N Na mg/l 2019
K mg/l 2002
Mg mg/l 3307 Ca mg/l 3337 Fe mg/l 2097 Mn mg/l 2809 HCO3 mg/l 3369 SO4 mg/l 3387 Cl mg/l 3533 NH4 mg/l 3154 NO2 mg/l 1050 NO3 mg/l 3034 PO4 mg/l 1705 DOC mg/l 2720 LF µS/cm 2237
O2 mg/l 951
H µg/l 3401
pH - 3401
10. P 50. P 90. P 6,9 16,2 38,1
1,0 1,9 3,8
3,4 8,7 22,2
27,1 71 153
0,06 0,7 8,0 0,06 0,3 1,4 28,9 150 351 7,0 36,4 189 12,2 32,6 87 0,011 0,03 0,1 0,004 0,02 0,06
0,1 0,3 0,9
0,01 0,04 0,11
1,0 3,0 8,8
226 474 993
0,2 1,0 6,0
0,01 0,03 0,2
6,8 7,5 8,2
Parameter N Na mg/l 2019
K mg/l 2002
Mg mg/l 3307 Ca mg/l 3337 Fe mg/l 2097 Mn mg/l 2809 HCO3 mg/l 3369 SO4 mg/l 3387 Cl mg/l 3533 NH4 mg/l 3154 NO2 mg/l 1050 NO3 mg/l 3034 PO4 mg/l 1705 DOC mg/l 2720 LF µS/cm 2237
O2 mg/l 951
H µg/l 3401
pH - 3401
10. P 50. P 90. P 6,9 16,2 38,1
1,0 1,9 3,8
3,4 8,7 22,2
27,1 71 153
0,06 0,7 8,0 0,06 0,3 1,4 28,9 150 351 7,0 36,4 189 12,2 32,6 87 0,011 0,03 0,1 0,004 0,02 0,06
0,1 0,3 0,9
0,01 0,04 0,11
1,0 3,0 8,8
226 474 993
0,2 1,0 6,0
0,01 0,03 0,2
6,8 7,5 8,2
N 2197 2157 3962 4005 3012 3613 4149 4079 4207 3759 1215 3721 1814 3281 2875 1061 4070
10. P 50. P 90. P 6,5 14,4 31,9 0,8 2,1 5,1 3,0 8,9 26,5
20,9 69 149
0,02 0,7 4,8 0,05 0,2 0,7 32,2 167 332 4,3 29,2 197 11,3 29,3 76 0,001 0,02 0,4 0,006 0,02 0,04 0,1 0,3 0,5 0,01 0,03 0,08 0,9 2,3 6,2 191 440 1013 0,1 0,9 7,3 0,02 0,06 0,2 6,6 7,2 7,8 N
2197 2157 3962 4005 3012 3613 4149 4079 4207 3759 1215 3721 1814 3281 2875 1061 4070
10. P 50. P 90. P 6,5 14,4 31,9 0,8 2,1 5,1 3,0 8,9 26,5
20,9 69 149
0,02 0,7 4,8 0,05 0,2 0,7 32,2 167 332 4,3 29,2 197 11,3 29,3 76 0,001 0,02 0,4 0,006 0,02 0,04 0,1 0,3 0,5 0,01 0,03 0,08 0,9 2,3 6,2 191 440 1013 0,1 0,9 7,3 0,02 0,06 0,2 6,6 7,2 7,8
10. P 50. P 90. P
5,6 9,8 17,1
0,8 1,8 4,2
2,6 9,2 23,5
21,8 76 157
0,08 1,4 9,3
0,05 0,2 0,5
35,7 174 340
1,1 8,8 73
9,2 20,6 46
0,002 0,03 0,4 0,006 0,02 0,04
0,07 0,2 0,6
0,01 0,03 0,05
0,9 2,1 4,8
159 383 922
0,1 0,6 3,3
0,02 0,05 0,1
6,8 7,3 7,7
N 1977 1934 3733 3739 3130 3457 3897 3780 3951 3546 1089 3546 1564 3090 2843 749 3872
10. P 50. P 90. P
5,6 9,8 17,1
0,8 1,8 4,2
2,6 9,2 23,5
21,8 76 157
0,08 1,4 9,3
0,05 0,2 0,5
35,7 174 340
1,1 8,8 73
9,2 20,6 46
0,002 0,03 0,4 0,006 0,02 0,04
0,07 0,2 0,6
0,01 0,03 0,05
0,9 2,1 4,8
159 383 922
0,1 0,6 3,3
0,02 0,05 0,1
6,8 7,3 7,7
N 1977 1934 3733 3739 3130 3457 3897 3780 3951 3546 1089 3546 1564 3090 2843 749 3872
# Left: shallow screens, middle: intermediate screens, right: deep screens. LF means electrical conductivity
Comparing the results of the 3 withdrawal depths it becomes clear that the concentration patterns are quite similar (Table 4.1). Compared to the other hydrogeologic units (see following chapters) the general solution content of around 1000 (µS/cm) reflects the petrografic as well as the hydromechanical conditions, i.e. intergranlar porosity allowing a good dissolving of
minerals and a long groundwater residence time due to shallow hydraulic gradients. For many parameters (e.g. Na, K, SO4,Cl) the solution content decreases with increasing depth. This is striking on first sight as in general it can be expected that the solution content will rise with increasing depth due to the longer residence times of groundwater. This behaviour can be explained however by the ubiquitous input of anthropogenic substances from the surface (e.g.
fertilizers) with the percolation water.
With regard to the redox status, the distribution of the parameters O2, Fe(II), Mn(II), NO3, DOC show, that in general the hydrogeologic unit Sands and Gravels of the North European Plain can be regarded as carrying reduced groundwater. Due to the long groundwater residence times and high redox reactivity of the subsurface, a complete use of the oxygen reaching the aquifers with the percolation water, is attained. An indicator for this is the decreasing DOC and the fact, that nitrate is present in very few samples only, while there is a general trend towards high Fe(II) and Mn(II) concentrations.
The absent nitrate is due to denitrification processes by which nitrate inputs to the aquifers is reduced to molecular nitrogen by microbially controlled redox reactions (associated with an increase of SO4 or alkalinity). If a groundwater is largely free of dissolved oxygen, certain micro-organisms are able to satisfy their oxygen demand by reducing nitrate. An important prerequisite for this reaction is the presence of organic carbon compounds and/or pyrite (FeS2) in the aquifer acting as reducing substances.
This hydrochemical groundwater condition is typical for the unconsolidated glacial sand and gravel deposits of the North European Plain. Due to the denitrification processes in reduced aquifers described above, it may be possible that a groundwater appears to be almost free from anthropogenic nitrate inputs, although the nitrate input with the percolation water is in fact very
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high. Hence, the low nitrate contents in the loose rock sediments of the North German Plane are part of the specific hydrogeochemical conditions in the aquifers of this hydrogeologic unit.
Fluviatile terrace deposits of major streams
The fluviatile terrace deposits of major streams consists of material that has been weathered and eroded from exposed consolidated rocks and was deposited in depressions formed by faulting (intracontinental rifts). In Germany the fluviatile terrace deposits of major streams of the river Rhine valley and the foothill deposits of the Alps have been summarized under this hydrogeologic unit.
The fluviatile terrace deposits of major streams are commonly rather thick (30 - 50 m) fluviatile deposits delivered from the Alps and the South and Central German low mountains.
Petrographically this hydrogeological unit consists primarily of siliceous material, which is not very soluble, but limestone, dolomite and other constituents are also present. In the aquifers of the foothills of the Alps the latter are even the primary minerals. The aquifer materials are commonly segregated by size into lenses and beds, which can affect the movement and availability of water. Beds and lenses of sand, gravel, or mixtures of the two yield large amounts of water.
Because of their thickness and their high water permeability the Fluviatile terrace deposits of major streams form productive aquifers. They receive most of their recharge from infiltration of precipitation that falls directly on the land surface. Groundwater recharge has been
calculated to represent more than 80% of the total runoff (Bogena et al., 2003) and is a good indication of the water-yielding capacity of the aquifer that provides the base flow.
As the aquifers are predominantly deep below from the land surface, the input of dissolved materials from the unsaturated zone can be rather high. Hence, although the most common mineral in the aquifers is quartz, the total solution content around 900 to 1000 (µS/cm) lies in the same range as it was already described for the Sands and Gravels of the North European Plain (see Chapter 4.2.1).
Table 4.2. Natural background concentrations (major groundwater composition) in Fluviatile Terrace Deposits of Major Streams in Germany based on ca. 2000 groundwater samples per parameter (Kunkel et al., 2004). Left: Rhine Graben, Middle: Rhine deposits in Nordrhein Westfalen, right: southern Germany.
Parameter N
Na mg/l 570
K mg/l 570
Mg mg/l 569
Ca mg/l 570
Fe mg/l 488
Mn mg/l 491
HCO3 mg/l 562 SO4 mg/l 562
Cl mg/l 572
NH4 mg/l 572 NO2 mg/l 369 NO3 mg/l 572 PO4 mg/l DOC mg/l 521 LF µS/cm 582
O2 mg/l 514
H µg/l 582
pH -
10. P 50. P 90. P 11,1 19,7 35,1
1,5 3,4 7,9
7,5 11,5 17,9 38,2 100 169 0,01 0,03 0,2 0,0001 0,0007 0,07 24,4 170 377
31,6 74 172
25,4 52 106
0,04 0,09 0,2
0,4 2,3 12,8
0,6 1,0 2,5
576 818 1161
0,1 3,9 9,1
0,06 0,2 0,4
6,4 6,8 7,2
N 1254 1255 1257 1243 1188 1255 1230 1193 1251 1240 767 1229
521 966 1235 1257 1260
Komponentensep.
10. P 50. P 90. P 6,2 10,8 18,9
1,1 2,3 4,7
7,0 15,2 33,1
74 121 197
0,005 0,04 3,3 0,01 0,09 0,6
277 348 438
16,7 65 249
9,8 31,2 99
0,002 0,01 0,04
0,05 0,3 1,2 0,01 0,03 0,2
0,7 1,6 3,7
450 763 1296
0,2 1,3 8,2
0,04 0,06 0,1
7,0 7,2 7,4
Komponentensep.
10. P 50. P 90. P 2,7 6,9 17,3
0,5 1,3 3,3
12,0 21,1 37,4
67 101 136
0,001 0,005 0,1 0,003 0,006 0,02
245 359 437
8,0 8,9 45
6,7 19,4 56
0,001 0,003 0,006
1,4 6,0 24,8 0,001 0,006 0,1
490 666 906
2,3 7,5 10,6 0,03 0,05 0,10
7,0 7,3 7,6
N 1310 1305 1368 1369 1356 1319 1362 1372 1374 1305 904 1374
997 344 1372 1308 1343 Parameter
N
Na mg/l 570
K mg/l 570
Mg mg/l 569
Ca mg/l 570
Fe mg/l 488
Mn mg/l 491
HCO3 mg/l 562 SO4 mg/l 562
Cl mg/l 572
NH4 mg/l 572 NO2 mg/l 369 NO3 mg/l 572 PO4 mg/l DOC mg/l 521 LF µS/cm 582
O2 mg/l 514
H µg/l 582
pH -
10. P 50. P 90. P 11,1 19,7 35,1
1,5 3,4 7,9
7,5 11,5 17,9 38,2 100 169 0,01 0,03 0,2 0,0001 0,0007 0,07 24,4 170 377
31,6 74 172
25,4 52 106
0,04 0,09 0,2
0,4 2,3 12,8
0,6 1,0 2,5
576 818 1161
0,1 3,9 9,1
0,06 0,2 0,4
6,4 6,8 7,2
N 1254 1255 1257 1243 1188 1255 1230 1193 1251 1240 767 1229
521 966 1235 1257 1260
Komponentensep.
10. P 50. P 90. P 6,2 10,8 18,9
1,1 2,3 4,7
7,0 15,2 33,1
74 121 197
0,005 0,04 3,3 0,01 0,09 0,6
277 348 438
16,7 65 249
9,8 31,2 99
0,002 0,01 0,04
0,05 0,3 1,2 0,01 0,03 0,2
0,7 1,6 3,7
450 763 1296
0,2 1,3 8,2
0,04 0,06 0,1
7,0 7,2 7,4
Parameter N
Na mg/l 570
K mg/l 570
Mg mg/l 569
Ca mg/l 570
Fe mg/l 488
Mn mg/l 491
HCO3 mg/l 562 SO4 mg/l 562
Cl mg/l 572
NH4 mg/l 572 NO2 mg/l 369 NO3 mg/l 572 PO4 mg/l DOC mg/l 521 LF µS/cm 582
O2 mg/l 514
H µg/l 582
pH -
10. P 50. P 90. P 11,1 19,7 35,1
1,5 3,4 7,9
7,5 11,5 17,9 38,2 100 169 0,01 0,03 0,2 0,0001 0,0007 0,07 24,4 170 377
31,6 74 172
25,4 52 106
0,04 0,09 0,2
0,4 2,3 12,8
0,6 1,0 2,5
576 818 1161
0,1 3,9 9,1
0,06 0,2 0,4
6,4 6,8 7,2
Parameter N
Na mg/l 570
K mg/l 570
Mg mg/l 569
Ca mg/l 570
Fe mg/l 488
Mn mg/l 491
HCO3 mg/l 562 SO4 mg/l 562
Cl mg/l 572
NH4 mg/l 572 NO2 mg/l 369 NO3 mg/l 572 PO4 mg/l DOC mg/l 521 LF µS/cm 582
O2 mg/l 514
H µg/l 582
pH -
10. P 50. P 90. P 11,1 19,7 35,1
1,5 3,4 7,9
7,5 11,5 17,9 38,2 100 169 0,01 0,03 0,2 0,0001 0,0007 0,07 24,4 170 377
31,6 74 172
25,4 52 106
0,04 0,09 0,2
0,4 2,3 12,8
0,6 1,0 2,5
576 818 1161
0,1 3,9 9,1
0,06 0,2 0,4
6,4 6,8 7,2
N 1254 1255 1257 1243 1188 1255 1230 1193 1251 1240 767 1229
521 966 1235 1257 1260
Komponentensep.
10. P 50. P 90. P 6,2 10,8 18,9
1,1 2,3 4,7
7,0 15,2 33,1
74 121 197
0,005 0,04 3,3 0,01 0,09 0,6
277 348 438
16,7 65 249
9,8 31,2 99
0,002 0,01 0,04
0,05 0,3 1,2 0,01 0,03 0,2
0,7 1,6 3,7
450 763 1296
0,2 1,3 8,2
0,04 0,06 0,1
7,0 7,2 7,4
N 1254 1255 1257 1243 1188 1255 1230 1193 1251 1240 767 1229
521 966 1235 1257 1260
Komponentensep.
10. P 50. P 90. P 6,2 10,8 18,9
1,1 2,3 4,7
7,0 15,2 33,1
74 121 197
0,005 0,04 3,3 0,01 0,09 0,6
277 348 438
16,7 65 249
9,8 31,2 99
0,002 0,01 0,04
0,05 0,3 1,2 0,01 0,03 0,2
0,7 1,6 3,7
450 763 1296
0,2 1,3 8,2
0,04 0,06 0,1
7,0 7,2 7,4
Komponentensep.
10. P 50. P 90. P 2,7 6,9 17,3
0,5 1,3 3,3
12,0 21,1 37,4
67 101 136
0,001 0,005 0,1 0,003 0,006 0,02
245 359 437
8,0 8,9 45
6,7 19,4 56
0,001 0,003 0,006
1,4 6,0 24,8 0,001 0,006 0,1
490 666 906
2,3 7,5 10,6 0,03 0,05 0,10
7,0 7,3 7,6
N 1310 1305 1368 1369 1356 1319 1362 1372 1374 1305 904 1374
997 344 1372 1308 1343
Komponentensep.
10. P 50. P 90. P 2,7 6,9 17,3
0,5 1,3 3,3
12,0 21,1 37,4
67 101 136
0,001 0,005 0,1 0,003 0,006 0,02
245 359 437
8,0 8,9 45
6,7 19,4 56
0,001 0,003 0,006
1,4 6,0 24,8 0,001 0,006 0,1
490 666 906
2,3 7,5 10,6 0,03 0,05 0,10
7,0 7,3 7,6
N 1310 1305 1368 1369 1356 1319 1362 1372 1374 1305 904 1374
997 344 1372 1308 1343
In spite of their lithogenic similarities, the statistical analysis of the groundwater data showed differences in the solution contents between the groundwater data of the Upper Rhine and those of the Lower Rhine (Table 4.2).
Due to the higher permeability of the gravels and sands and the steeper hydraulic gradients, the residence time of the groundwater in the aquifers is generally shorter compared to the
groundwater residence time in the hydrogeologic unit Sands and Gravels of the North
European Plain. A differentiation according to the withdrawal depths of the samples revealed
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no big concentration differences. Hence, the groundwater compositions were evaluated without a differentiation in depth.
The Fe-, Mn-, O2- and SO4 – data show, that the groundwater of the Lower Rhine valley can be classified as predominantly oxidized. Same is true for the groundwater of the foothills of the Alps. The groundwater of the Upper Rhine Valley, in contrast, show considerably the reduced groundwater type. Hence the groundwater of the Upper Rhine Valley displays normally lower NO3 and O2 concentrations as the groundwater of the Lower Rhine Valley.
The cobble and gravel deposits of the foothill of the Alps show similarities to the deposits of the Upper Rhine Valley with regard to the Na-, K-, Mg-, Ca-, and HCO3-concentrations. The reason for this might be the same provenance areas of sediments, which build up the aquifers, i.e., the Northern Alps, which consist to a considerable portion of carbonate rocks. As the portion of carbonate minerals in both hydrogeologic units is rather high, the concentrations of the related ions (Ca, Mg, HCO3) are too.
General conclusion about major groundwater environments and groundwater composition in unconsolidated sand and gravel aquifers
From the above mentioned similarities and differences between the groundwater composition in the different hydrogeologic units it can be concluded that the unconsolidated sand and gravel deposits should always be classified with regard to their genesis (glacial deposit or river valley sediment) and with regard to their redox status (oxidized groundwater or reduced groundwater) as the most important differentiation characteristics.
4.2.2 Silicatic sedimentairy rock aquifers
The silicatic sedimentairy rock aquifers are combined in this group. They include the Mesozoic sedimentary rocks (e.g. Lower Triassic) as well as the Palaeozoic sedimentary rock units.
(Triassic) sandstone aquifers
Aquifers in sandstones are more widespread in Germany than those in all other kinds of consolidated rocks. Although the porosity of well-sorted, unconsolidated sand may be as high as 50 percent, the porosity of most sandstones is considerably less. During the process of diagenesis of sand into sandstone (lithification), compaction by the weight of overlying material reduces not only the volume of pore space as the sand grains become rearranged and more tightly packed, but also the interconnection between pores (permeability). The deposition of cementing materials such as calcite or silica between the sand grains further decreases porosity and permeability. The average intergranular porosity of sandstone aquifers generally doesn’t exceed 10%. Thus, most of the porosity in these consolidated rocks consists of secondary openings such as joints, fractures, and bedding planes. Groundwater movement in sandstone aquifers primarily is along bedding planes, but the joints and fractures cut across bedding and provide avenues for the vertical movement of water between bedding planes.
Because the number of joints, fractures, and bedding-plane openings in all types of rock typically decreases with depth, permeability of the sandstone aquifer should similarly decrease until groundwater movement ceases at some depth. Decreased circulation of groundwater results in an increase of TDS in groundwater due to increased residence time and hence time to dissolve minerals. For the aims of the BRIDGE project however this is of minor importance, as
the major (typical) groundwater composition of sandstone aquifers is represented by samples of shallow withdrawal depths, i.e., of groundwater taking part in the active water cycle (cf. section 4.1).
In Germany, sandstone aquifers are parts of complexly embedded sequences of various types of clastic sedimentairy rocks, predominantly from the Triassic epoch. The sandstones display a red colour, which is due to the arid-continental deposition conditions and gave this epoch its name (Bunter Sandstone comes from the German for "coloured sandstone", where it is now known as
“Buntsandstein”). In general the sandstone aquifer system consist of layered rocks
differentiated vertically into fine-grained, low-permeability rocks such as shale or siltstone, and more permeable predominantly sandstones. This unit is found in South and Central Germany, above all from Rhineland-Palatinate, Baden-Würtemberg, Hesse and Thuringia (see Table 4.3).
Table 4.3. Natural background concentrations (major groundwater composition) in Sandstone aquifers in Germany based on ca. 1700 groundwater samples per parameter (Kunkel et al., 2004).
Parameter N Na mg/l 1617
K mg/l 1574
Mg mg/l 1629 Ca mg/l 1625 Fe mg/l 1552 Mn mg/l 1620 HCO3 mg/l 1604 SO4 mg/l 1622 Cl mg/l 1632 NH4 mg/l 1477 NO2 mg/l 1909 NO3 mg/l 1644 PO4 mg/l 439 DOC mg/l 453 LF µS/cm 1491 O2 mg/l 1599
H µg/l 1630
pH -
10. P 50. P 90. P 1,9 5,5 16 1,3 2,1 3,6 1,9 6,5 22 5,0 11,3 26 0,002 0,02 0,09 0,001 0,004 0,07
6,4 25 95
5,3 18 58
4,0 8,3 17,4 0,001 0,004 0,01 0,002 0,004 0,009
2,2 7,5 26
78 237 692
4,9 8,1 10,8 0,03 0,07 0,19 6,7 7,1 7,6
The sandstone sequences are in general characterized by a high significance for water management issues. Due to the silicatic rock attribute, however, the waters are have in many cases low TDS and threatened by acidification due to thin and buffer-poor soils. Thus more than 50% of all samples display an electric conductivity of less than 250 µS/cm and a pH less than 7.0 (see Table 4.3).
Due to the relative high permeability of the sandstone aquifers and the steep hydraulic gradients, the residence time of the groundwater is generally shorter compared to the groundwater in the unconsolidated rock units. Groundwater recharge has been calculated to represent about 50% of the total runoff (Bogena et al., 2003), which is a good indication of the water-yielding capacity of the aquifer that provides the base flow. The Fe, Mn, O2 and SO4 concentrations show that the groundwater of the sandstone aquifers can be classified as predominantly oxidized. Thus, low Fe and Mn concentrations coincide with high O2 and NO3
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concentrations. Although Ca is usually the major cation, Na and Mg are relevant as well. On milliequivalent basis, SO4 is the major anion instead of alkalinity.
Sandstones and silicatic alternating sequences
Observation wells that originate from alternating sequences of sandstones and claystones from different ages were allocated to this hydrogeological unit. Especially observation wells from the
“Rotliegend” as well as alternating sandstone and claystone sequences from the Mesozoic, which do not belong to the Lower Triassic, were allocated to this unit. The latter contain as a rule, although subordinately, carbonate rocks and carbonate-cemented sedimentairy rocks.
Table 4.4. Natural background concentrations (major groundwater composition) in sandstones and silicatic alternating sequences in Germany based on ca. 1700 groundwater samples per parameter (Kunkel et al., 2004).
Parameter N Na mg/l 1051 K mg/l 1040 Mg mg/l 1167 Ca mg/l 1170 Fe mg/l 1056 Mn mg/l 1006 HCO3 mg/l 1048 SO4 mg/l 1171 Cl mg/l 1192 NH4 mg/l 1117 NO2 mg/l 982 NO3 mg/l 1171 PO4 mg/l 720 DOC mg/l 344 LF µS/cm 1020 O2 mg/l 952
H µg/l 998
pH -
10. P 50. P 90. P 1,2 5,3 24 0,76 2,4 7,4 4,0 19,5 51
25 76 134
0,01 0,03 0,10 0,0001 0,004 0,07 60 280 403
14,2 37 95
5,2 16,8 55 0,0003 0,001 0,004
0,001 0,003 0,01 2,1 4,2 8,5 0,001 0,002 0,005
241 539 875 7,2 8,6 10,4 0,02 0,05 0,12 6,9 7,3 7,7
Comparing the major groundwater composition of the sandstones to the major groundwater composition of the silicatic alternating sequences, the solution contents display differences, which justify a separation into two hydrogeologic units: the 50-percentile value for EC is 539 µS/cm instead of 237 (cf. Tables 4.3 and 4.4). Thus the Mg-, Ca-, HCO3-, SO4- and Cl- concentrations in this hydrogeologic unit are considerably higher due to the presence of substantial amount of carbonates
The occurrence of redox-sensitive species indicates that groundwater is usually oxic. Calcium and HCO3 are the major ions, while Mg and SO4 are of secondary importance.
