Hydrogeol J DOI 10.1007/s10040-015-1321-7
REPORT
Dissolved noble gases and stable isotopes as tracers of preferential fluid flow along faults in the Lower Rhine Embayment, Germany L. P. Gumm 1 & V. F. Bense 2 & P. F. Dennis 1 & K. M. Hiscock 1 & N. Cremer 3 & S. Simon 3
Received: 6 March 2015 / Accepted: 30 September 2015 # The Author(s) 2015. This article is published with open access at Springerlink.com
Abstract Groundwater in shallow unconsolidated sedimentary aquifers close to the Bornheim fault in the Lower Rhine Embayment (LRE), Germany, has relatively low δ2H and δ18O values in comparison to regional modern groundwater recharge, and 4He concentrations up to 1.7×10−4 cm3 (STP) g–1 ±2.2 % which is approximately four orders of magnitude higher than expected due to solubility equilibrium with the atmosphere. Groundwater age dating based on estimated in situ production and terrigenic flux of helium provides a groundwater residence time of ∼107 years. Although fluid exchange between the deep basal aquifer system and the upper aquifer layers is generally impeded by confining clay layers and lignite, this study’s geochemical data suggest, for the first time, that deep circulating fluids penetrate shallow aquifers in the locality of fault zones, implying that sub-vertical fluid flow occurs along faults in the LRE. However, large hydraulic-head gradients observed across many faults suggest that they act as barriers to lateral groundwater flow. Therefore, the geochemical data reported here also substantiate a conduitbarrier model of fault-zone hydrogeology in unconsolidated sedimentary deposits, as well as corroborating the concept that faults in unconsolidated aquifer systems can act as loci for hydraulic connectivity between deep and shallow aquifers. The implications of fluid flow along faults in sedimentary
* V. F. Bense
[email protected] 1
School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK
2
Hydrology and Quantitative Water Management Group, Department of Environmental Sciences, Wageningen University, 6708 PB Wageningen, The Netherlands
3
Erftverband, Am Erftverband 6, 50126 Bergheim, Germany
basins worldwide are far reaching and of particular concern for carbon capture and storage (CCS) programmes, impacts of deep shale gas recovery for shallow groundwater aquifers, and nuclear waste storage sites where fault zones could act as potential leakage pathways for hazardous fluids. Keywords Fault zone hydrogeology . Noble gases . Unconsolidated sedimentary aquifers . Germany . Hydrochemistry
Introduction Large hydraulic head gradients observed across fault zones in unconsolidated sedimentary aquifers suggest that they often form effective barriers to lateral groundwater flow (Rawling et al. 2001; Bense and Van Balen 2004; Mayer et al. 2007; Bense et al. 2013). These barriers are created by the juxtaposition of aquifer-confining units such as clay beds with aquifer media at the location of the fault (Mailloux et al. 1999) and/or fault zone deformation processes such as cataclasis (Fulljames et al. 1997), diagenesis (Chan et al. 2000; Dewhurst and Jones 2003) and clay smearing (Lehner and Pilaar 1997; Bense et al. 2003; Egholm et al. 2008), which reduce the permeability of the fault zone itself. The role of faults as barriers to groundwater movement has been widely described in the literature but much less documented is the ability of faults to simultaneously act as conduits for sub-vertical flow along the fault (Roberts et al. 1996; Wiprut and Zoback 2000; Bense and Person 2006). Hydraulic head observations are typically not sufficient to infer the magnitude and direction of fluid flow along faults, usually due to the sparse distribution of observation boreholes. However, the application of noble gas tracers has been particularly useful in characterizing fluid flow associated with faults as conduit-barrier structures in both deep
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(Wiersberg and Erzinger 2011) and shallow (Kulongoski et al. 2003, 2005) settings. In general, the potential for fluid migration along faults is an important consideration for water resources management as well as proposed carbon capture and storage (CCS) programmes (Shipton et al. 2004; Bickle et al. 2007; Gilfillan et al. 2009), shale gas recovery from deep hydrological environments and its implications for shallow groundwater quality (Cassiat et al. 2013), and nuclear waste storage sites (Flint et al. 2001; Dublyansky and Spötl 2010) where faults could act as potential leakage pathways for hazardous fluids. Geochemical tracers are useful tools for investigating the origin and residence time of deep circulating fluids in aquifer systems worldwide (Kipfer et al. 2002; Lippmann et al. 2005; Ma et al. 2005). Dissolved noble gases and stable isotopes are particularly useful in studies of aquifer systems that contain pre-Holocene ‘palaeo’ groundwater (e.g. Vaikmäe et al. 2001) or fluids that have a deep crustal origin such as those observed in the Great Artesian Basin in Australia (Torgersen and Clarke 1985, 1987; Bethke et al. 1999), the Paris Basin in France (Marty et al. 1993, 2003; Castro et al. 1998; Lavastre et al. 2010) and the Witwatersrand Basin in South Africa (Lippmann et al. 2003; Lippmann-Pipke et al. 2011). Dissolved helium has been used extensively for estimating groundwater residence times and tracing the origin of crustal fluids (Torgersen et al. 1992; Ballentine et al. 2002; Gardner et al. 2012). Deep circulating groundwater typically has a radiogenic source of 4He from in situ α-decay of U-Th series elements. However, modern groundwater (>10 years) can also exhibit high concentrations of helium which originate from previously trapped reservoirs in the subsurface (Solomon et al. 1996). Helium enrichment in palaeo groundwater is reported in the literature at up to six orders of magnitude higher than expected due to solubility equilibrium with the atmosphere (Andrews and Lee 1979; Castro et al. 1998; Bethke et al. 1999). Terrigenic 3,4He is also significant in deep circulating fluids (Griesshaber et al. 1992; Stute et al. 1992; Ballentine and Burnard 2002; Castro 2004). The 3He/4He isotope ratio is often used to distinguish between helium of crustal or mantle origin (Oxburgh et al. 1986; Sano et al. 1986; Kulongoski et al. 2003, 2005). However, due to groundwater typically containing helium from several different sources (Solomon et al. 1996; Zhou and Ballentine 2006), helium age dating is usually only used as a technique for estimating groundwater age rather than a precise quantitative tool (Stute et al. 1992; Castro et al. 2000). More recently, however, Torgersen (2010) suggested that it is now possible to determine uncertainty limits associated with crustal fluxes of helium using the increased number of published data sets, thus improving the accuracy of 4He age estimates. In the unconsolidated sedimentary aquifers of the Lower Rhine Embayment (LRE), Germany, where large hydraulic gradients exist across many faults, suggesting that they act as barriers to groundwater flow, thermal anomalies in aquifer
units flanking the Rurrand fault have provided evidence for significant fault-parallel flow (Bense et al. 2008). Here, hydrogeochemical evidence is presented for the upward migration of deep crustal fluids along the Bornheim fault in the LRE.
Study area The Lower Rhine Embayment forms part of the Roer Valley Rift System (RVRS), which is the southward extension of the North Sea Basin and part of a Cenozoic mega-rift system that crosses western and central Europe (Ziegler 1994). The German region of the LRE (Fig. 1) covers an area of 3, 800 km2 and contains approximately 1,300 m of Oligocene to Pleistocene unconsolidated siliciclastic sediments that form a highly complex multi-layered aquifer sequence within six tectonic blocks. The central Erft block is adjacent to the Köln block to the east and the Rur block to the west, whilst the Krefeld and Venlo blocks are situated northeast and northwest, respectively. Each tectonic block consists of 5–15 different aquifers to a depth of 200 m and the Erft block has as many as 21 separate aquifer units to a depth of approximately 400 m. The LRE is intersected by numerous NW–SE-striking fault zones which have a significant impact on regional groundwater flow patterns (Wallbraun 1992). The formation of the LRE aquifer system began during the Oligocene and is described in great detail by Schäfer et al. (1996). The LRE primarily consists of Oligocene, Miocene and Pliocene marine sediments and the more recent Pleistocene sediments are mainly fluvial deposits derived from the Rhenish Massif to the south (Schäfer et al. 2005). The sediments derived purely from the Rhenish Massif are comprised of gravels that are characterised by having >90 % quartz material in the coarse gravel fraction and the remaining sediment is made up of quartzite (Boenigk 2002). Highly porous loess soils have formed due to the accumulation of silts, sands and clays, and these overlay the Pleistocene deposits (Kemna 2008). Boenigk (2002) and Kemna (2008) provide detailed mineralogical analyses of the shallow Pleistocene deposits, and Pliocene and lower Pleistocene deposits, respectively. The LRE contains two commercially important lignite seams. The main lignite seam Fig. 1 a Location of the study area in central Europe. b Hydraulic head distribution within the deeper aquifer units of the Lower Rhine Embayment (LRE), Germany, with spatial reference to the Hambach, Garzweiler and Inden open cast lignite mines. Contour increments vary due to the lower hydraulic head gradients that exist towards the River Rhine. A cone of depression is evident below the Hambach and Garzweiler mines, and the major graben faults frequently act as barriers to lateral groundwater flow. The shallow aquifer system is less impacted by lignite mine dewatering due to confining layers of clay that create relatively shallow perched aquifer conditions in many areas including the Brühl region. The observation boreholes not illustrated on the map are all situated to the north in an area less impacted by lignite mining
Hydrogeol J
76.6
150.0
169.5
27/957824
27/958062
27/958063
21.0
35.8
07/352721
54.4
27/957982
28/907271
9.0
14.5
28/908381
38.1
21/040331
80/030128
36.0
39.1
33.0
28/900601
5.15E-08
19.5
28/816011
28/816012
21/960511
5.72E-08
41.0
28/816061
3.67E-08
1.69E-04
1.28E-04
7.27E-05
8.63E-06
2.58E-06
7.36E-06
6.01E-08
4.18E-08
4.57E-08
4.76E-08
4.51E-08
4.40E-08
5.39E-08
16.4
54.0
28/900401
5.08E-08
5.59E-08
4.37E-06
1.03E-06
3.51E-08
3.91E-08
4.65E-08
5.40E-08
5.49E-08
5.58E-08
4.86E-08
5.01E-08
4.58E-08
5.59E-08
4.72E-08
5.09E-08
5.39E-08
1.14E-07
4.64E-08
Helium (cm3 STP g–1)
28/900171
12.8
67.0
01/040244
38.7
27/957823
28/806881
5.6
28.1
08/658086
33.4
21/861581
27/957822
25.9
34.4
27/937491
19.0
27/937881
21/863901
31.0
21.3
28/812531
07/355571
25.8
17.0
8.3
28/917191
59.5
27/831791
27/937921
28/806941
26.7
42.0
28/907892
28/806922
25.2
146.2
28/907641
28/806882
Filter depth (m)
27.5
29.3
74.6
31.8
35.8
20.7
11.6
10.8
7.9
8.3
7.9
8.2
22.6
31.8
18.4
7.6
21.6
24.4
22.0
-
8.3
7.8
18.8
14.6
25.1
17.8
12.3
14.8
12.9
26.7
14.5
24.8
5.4
35.5
5.8
Excess air (% ΔNe)
16.7
11.1
5.2
6.1
12.0
8.7
13.5
11.0
10.2
10.1
14.5
11.8
15.1
13.5
6.9
12.1
12.6
14.9
13.9
11.9
11.9
12.0
13.0
10.0
14.3
14.3
13.9
14.8
15.1
14.2
15.0
13.7
12.0
10.1
10.7
NGT (°C)
Hydrogeochemical data for the Lower Rhine Embayment, Germany
Borehole ID
Table 1
−59.1 −62.4 −62.7
−8.57 −9.18 −9.03
−52.3 −55.4
−7.45
−50.5
−7.00 −8.12
−50.8 −50.5
−7.38
−52.0
−7.50 −7.41
−49.0 −52.3
−7.13 −7.44
−47.1 −50.7
−7.09
−51.8
−7.58 −7.45
−52.8 −51.8
−7.56 −7.72
−49.2 −47.9
−7.17
−51.9
−7.42 −7.10
−48.2 −50.7
−7.38
−52.8
−7.66 −7.12
−52.4 −53.6
−7.45
−51.1
−7.28 −7.57
−50.7 −53.2
−7.32 −7.48
−51.3 −52.2
−7.43 −7.33
−52.9 −47.9
−7.40 −6.40
−49.5 −49.2
−7.65 −7.19
83.3
70.7
-
50.8
139.0
124.0
73.2
103.0
59.7
74.6
82.4
76.7
63.7
61.9
54.3
82.7
35.2
68.3
199.0
229.0
83.0
196.0
179.0
183.0
203.0
174.0
199.0
192.0
120.0
166.0
152.0
98.5
77.4
39.2
85.7
73.3
8.5
11.4
21.9
37.9
14.9
16.1
9.3
12.5
16.1
8.8
8.0
18.0
17.3
12.8
5.2
9.5
31.4
35.7
14.4
26.3
23.9
30.5
33.0
23.5
25.7
25.9
18.3
28.1
29.8
16.5
10.4
8.3
13.7
114.0
−49.4 −52.1
−7.25 −7.84
Ca2+ Mg2+ −1 (mg L )
δ2H δ18O VSMOW (‰)
1,860.0
1,700.0
426.0
99.4
66.4
128.0
81.7
25.8
18.9
28.6
19.9
19.1
15.2
39.3
22.3
25.4
14.4
21.9
31.7
26.0
24.9
18.0
21.9
18.0
17.8
15.9
9.3
38.2
16.1
13.1
54.7
7.5
8.6
13.0
21.2
Na+
38.7
33.5
9.7
15.0
10.9
14.7
6.0
1.6
2.9
11.9
4.1
5.2
5.0
7.2
9.4
4.9
2.2
3.3
1.8
1.5
16.9
3.0
1.3
1.7
1.6
1.1
2.0
3.6
2.5
1.5
1.8
1.5
2.3
5.8
5.5
K+
1,312
1,171
-
409
281
220
92
92
49
110
6
18
18
31
24
116
43
92
433
421
146
354
372
415
390
342
372
415
262
537
592
403
250
201
354
HCO3−
306.0
280.0
77.5
36.4
240.0
335.0
194.0
135.0
74.1
92.7
111.0
216.0
68.2
176.0
122.0
125.0
86.1
99.4
249.0
259.0
99.8
150.0
128.0
138.0
202.0
185.0
133.0
158.0
94.3
78.0
150.0
2.1
37.1
9.1
72.6
SO42−
2,460
2,200
360
34.5
91.5
205
124
52.7
42.2
75.6
57.1
68.1
29.6
69.3
46.4
54.4
26.1
33
75.6
76.9
64.3
107
75.3
84.4
85.6
41.7
106
62.3
49.8
27.7
84.7
7.81
15.2
5
37.5
Cl−
0.2
0.2
0.2
0.2
55.3
0.2
18.7
125.7
76.6
54
190.8
1.5
156.3
60.2
89.4
33.2
2.9
63.7
3.7
92.1
44.7
57.1
73.9
52.2
59.3
54.4
69.9
133.2
52.7
20.8
194.8
0.9
0.2
0.2
0.2
NO3−
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has a thickness of up to 100 m and the second ‘upper’ seam has a thickness of up to 40 m (Hager 1993). The Miocene lignite deposits of the LRE form one of the largest reserves in Europe with an estimated 55,000 Mt (Hager 1993) with an overburden thickness up to 300 m (RWE Power AG). The geological structures that underlie the Oligocene aquifers below a depth of 400–800 m are less well known. The oldest sequences identified from borehole records are Palaeozoic. It is known that Carboniferous strata with a total thickness of up to 2,500 m is underlain by approximately 2,000 m of clastic sedimentary rocks, limestone and evaporites, and a Devonian sequence of unknown thickness and composition (Geluk et al. 1994). Lower Jurassic sandstone and claystone sequences with a total thickness of up to 1,500 m are found beneath the Cenozoic sand deposits, and they overlie Permian-Triassic sandstone and conglomerate material (Geluk et al. 1994).
Methods Observation boreholes in the LRE were selected at locations in close proximity to faults, and where some historical monitoring records of well head parameters and hydrochemistry were available. Physical limitations of the pumping equipment restricted borehole selection and generally only shallow (250 m) aquifer units in the Brühl region are now influenced by the Hambach mine to the northwest. This deep hydraulic system is effectively isolated from the shallow aquifer system by a thick confining layer of clay shown in the borehole records for 958062/3 and 957981/2. The extent to which groundwater abstraction drives sub-vertical fluid flow along faults in the LRE is unknown.
Conclusions Palaeo groundwater with a deep crustal signature is potentially widely distributed at depth within the LRE aquifer system, but only observed at shallow depth in a few areas close to fault zones. It is possible that point source emanations similar to those described here occur frequently in close proximity to faults, but remain undetected due to the often sparse distribution of observation boreholes near faults. The influence of groundwater abstraction on sub-vertical conduit flow along faults remains uncertain. Similar crustal fluids have been reported at depth in numerous aquifer systems worldwide (e.g. Marty et al. 2003) but rarely at near-surface depths (
