5 Discussion
5.1 Porewater chemistry
5.1.1 Estimation of anion-accessible porosity
In clayrocks containing a negative structural charge, anions are affected by ion exclusion and thus only "see" part of the total porosity (e.g. Pearson et al. 2003). The anion-accessible porosity fraction can be derived from analysis of unreactive chloride by different methods, all of which are inflicted with some uncertainty. In this section, chloride data obtained from squeezing and aqueous leaching samples are evaluated. First, chloride porosity is estimated from the squeezing dataset (described in section 4.4), with aid of mass balance. Second, it is estimated from the comparison of squeezing data and re-calculated leaching data from the aqueous extracts (described in section 4.3), located in the vicinity of the respective squeezing samples. The chloride data is also compared with corresponding data from the advective displacement experiment and the groundwater sample in the Wedelsandstein Formation. Finally, bromide data obtained from different methods are compared.
Calculation of anion-accessible porosity using chloride squeezing data
The mass of Cl- released from samples by squeezing and subsequent aqueous leaching is listed in Tab. 5-1 and shown graphically in Fig. 5-1.
Tab. 5-1: Cl- inventory in samples subjected to squeezing and subsequent aqueous leaching.
Depth [m] Unit Core mass before squeezing [g] Cl- squeezed at 200 MPa [mg] Cl- squeezed at 300 MPa [mg] Cl- squeezed at 400 MPa [mg] Cl- squeezed at 500 MPa [mg] Total Cl- squeezed [mg] Cl- leached from squeezed core [mg] Total Cl- obtained by squeezing and leaching [mg] Total mass of water (squeezing + drying) [g] Cl- -accessible porosity fraction [-]
780.66 Parkins.-Württ. Beds 456.49 2.823 8.942 6.258 6.086 24.11 31.51 55.62 22.53 0.52 796.53 Humphriesioolith Fm. 380.93 9.416 9.987 7.112 4.846 31.36 32.62 63.98 23.06 0.56 807.51 Wedelsandstein Fm. 467.18 1.045 1.509 0.995 5.064 8.61 51.13 59.74 19.21 0.51 816.93 Wedelsandstein Fm. 356.50 6.254 8.899 5.873 3.501 24.53 29.75 54.28 20.80 0.55 825.65 Wedelsandstein Fm. 424.29 4.441 13.29 7.367 5.715 30.81 33.40 64.21 22.50 0.55 878.45N Opalinuston 411.54 1.270 9.559 4.912 3.777 19.52 34.88 54.40 19.63 0.54 878.45P Opalinuston 338.19 0.000 4.843 5.799 3.777 14.42 26.09 40.51 13.09 0.54 896.31 Opalinuston 383.83 0.736 6.549 7.260 4.826 19.37 33.93 53.30 17.27 0.49 915.87 Opalinuston 378.80 1.804 9.232 7.052 4.901 22.99 29.70 52.69 21.14 0.57 937.89 Opalinuston 359.54 11.62 7.478 5.469 3.666 28.24 24.74 52.98 20.27 0.51 958.21 Posidonienschiefer 385.72 0.000 6.664 9.351 5.543 21.56 19.73 41.29 15.52 0.47*
987.40 Psiloceras Beds 393.05 3.443 2.551 1.797 1.067 8.86 10.30 19.16 21.75 0.42
* As no water could be retrieved at 200 MPa, the calculation of the anion-accessible porosity fraction assumes that the Cl- content of the sample obtained at 300 MPa is representative for the in-situ value.
The total mass of water obtained from squeezing, followed by drying at 105 °C, is also shown (see also Tab. 5-1). Under the assumption that the Cl- concentrations in water squeezed at 200 MPa resemble the in-situ concentrations, and conceptually dividing the porewater to a
free-water reservoir that contains Cl- and a bound-water reservoir with zero Cl- content, the fraction of the total water-filled porosity (anion-accessible fraction) that is accessible to Cl -can be calculated from:
MPa Cl
Cl
C m
200
@
(5- 1)with
mCl- = total mass of Cl- obtained by squeezing and leaching (mg)
= total volume of water obtained by squeezing and drying (l) CCl-@ 200 MPa = Cl- concentration in water squeezed at 200 MPa (mg/l).Resulting Cl--accessible porosity fractions are also listed in Tab. 5-1 and are in the range 0.42 – 0.56 times water-accessible porosity (average: 0.52).
Fig. 5-1: Cumulative fractions of total Cl- in solution and porewater released by squeezing.
Calculation of anion-accessible porosity with chloride data from re-calculated leaching data and squeezing data at 200 MPa
Chloride concentrations at the lowest squeezing pressure (200 MPa) as a function of depth are illustrated in Fig. 5-2 (green triangles). A slight increase with depth is noted, but in view of the scarce data, an accurate profile cannot be recognised. However, the trend appears to follow that observed from the aqueous extraction (leachate) data at an S/L ratio = 1, re-calculated for water-loss porosity (red circles). Thus, squeezing data confirm a slight increase with depth down to 850 – 900 m, followed by a strong decrease until 1000 m depth. The squeezing data systemati-cally show higher concentrations pointing to an anion exclusion effect for all measured samples.
Under the assumption that chloride data at 200 MPa squeezing pressure reflect porewater con-centrations, the anion-accessible fraction factor can be deduced from the mass balance using re-calculated leaching data, whose samples are located in the vicinity of the corresponding squeezing samples:
MPa Cl
leach Cl w
L
S C
C w
r @200
, :
1 1 1
(5-2)
with
rS:L = S/L ratio of leachate
ww = wet water content (g/kgrock) determined for leachate samples CCl-@ 200 MPa = Cl- concentration in water squeezed at 200 MPa (mg/l).
CCl- = Cl- concentration in leachate (mg/l)
From this relationship, which is basically the same the one given in eq. (5-1), anion-accessible fractions of 0.44 – 0.65 are derived (average 0.55 ± 0.06). As depicted in Fig. 5-3, there is no clear correlation between anion-accessible fraction and clay-mineral content.
Fig. 5-2: Chloride profiles shown for 1:1 leachates (re-calculated to WL porosity), squeezed water at 200 MPa, advective displacement experiment and groundwater sample.
Fig. 5-3: Anion-accessible fraction for chloride derived by comparsion of squeezing data at 200 MPa and re-calculated leaching data of near-by samples as function of clay-mineral content.
Error of anion-accessible fraction taken to be 13 % based on linear error propagation of analytical errors of parameters in eq. 5-2.
The estimated range of the anion-accessible fractions is consistent with those estimated from the squeezing data alone (see above). Thus the average values 0.52 and 0.55 are similar for both methods. Such values are in line with general experience from clay-mineral-rich lithologies (e.g.
Pearson et al. 2003, Mazurek et al. 2012, Gaucher et al. 2009).
The chloride concentration from the groundwater sample in the Wedelsandstein Formation agrees well with the squeezing data at 200 MPa (Fig. 5-3). Thus, the Cl concentration in the groundwater is nearly identical to that obtained from three samples by squeezing at 200 MPa, which supports the assumption that squeezing data obtained at 200 MPa closely represent in-situ porewater compositions. A fourth squeezing sample in the same interval yields a somewhat higher value. The data from the advective displacement sample display somewhat lower con-centrations (Fig. 5-2) compared to those from squeezing. The reason for this difference is not clear.
In summary, with minor exceptions, the data set for Cl- is internally consistent among all data sources applied, which adds confidence in the applied methods and their evaluation. This also supports the assumption that chloride data obtained from 200 MPa squeezing pressure repre-sents in-situ conditions reasonably well.
It could be a priori expected that the anion exclusion effect depends on the clay-mineral content since it is related to the permanent negative surface charge of clay phases, such as smectite and illite. As illustrated in Fig. 5-3, such a correlation is not clearly supported by the squeezing data, which suggests a near-to-constant anion exclusion volume independent of clay-mineral content.
It should be noted however that the clay-mineral content in squeezing samples is rather high (> 50 %, except for two samples), hence hampering the interpretation of anion exclusion pheno-mena at lower clay-mineral contents. Overall, the data suggest an anion-accessible fraction close to 0.5 at clay-mineral contents > 35 wt.-% for the rocks studied. The samples from the Effingen Member and some from the 'Brauner Dogger' exhibit clay-mineral contents below 35 wt.-% (but these were not included in the squeezing tests). Further data would be required to establish a relationship between anion exclusion effect and clay-mineral content for these types of rocks.
The uncertainty in chloride porewater data for the upper part of the cored sequence (above 800 m) originates from the fact that many of the samples are relatively poor in clay minerals, so the anion-accessible porosity for these rocks is not well known. Two limiting cases, illustrated in Fig. 5-4, can be defined: In the first case, a constant anion-accessible fraction of 0.5 is applied for the entire sequence. This leads to a relatively small concentration decrease towards the top (shown as open squares). In the second case, the anion-accessible fraction is varied between 0.5 and 1, depending on the clay-mineral content above 800 m depth (shown as open circles). A linear function is assumed with an anion-accessible fraction of 1 for the lowest clay-mineral content (17 %) and an anion-accessible fraction of 0.5 for all those with a clay-mineral content
≥ 35 %. Note that the the second approach reflects a bounding case to highlight the span in chloride concentrations in the upper sections, but does not have any geochemical basis.
Fig. 5-4: Chloride profile for two different assumptions with regard to anion-accessible fraction.
Case 1 (open squares): anion-accessible fraction = 0.5 for all leachate data.
Case 2 (open circles): = 0.5 for samples > 35 % clay-mineral content; for clay-mineral content < 35 %, is a linear function of clay-mineral content (see text).
Chemical composition of squeezed waters compared to evidence from the Benken borehole
Cl- concentrations and profile shapes for Cl- are very similar for Benken and Schlattingen (compare Fig. 5-4 and Fig. 5-5). Both profiles are characterised by a major decrease with depth in the Lias. In Benken, a groundwater sample from the Keuper had 520 mg/l Cl-, which fits well
with the trend of the porewater data (Fig. 5-5). In Schlattingen, there is no evidence for an increased hydraulic conductivity in the Keuper, and no groundwater could be obtained. Never-theless, the trend of Cl- in porewater suggests a low-salinity water in the Keuper, analogous to Benken.
Waber et al. (2003) documented the results of squeezing experiments on two samples of Opalinus Clay from Benken. These data were obtained at 512 MPa, as no water fractions were taken at lower pressures. The water types are Na-(Ca)-Cl-SO4 to Na-Ca-(Mg)-Cl-SO4, i.e. simi-lar to the water types of Schlattingen samples squeezed at 500 MPa. The biggest difference is the higher sulphate concentration at Benken (2288 – 3810 mg/l) compared to 1360 – 1413 mg/l for Opalinus Clay from Schlattingen squeezed at 500 MPa.
Fig. 5-5: Profile of Cl- concentration in pore and groundwaters from the Benken borehole.
Cl- concentrations refer to the mass of chloride per volume of Cl--accessible porewater, assuming that 50 % of the physical porosity is accessible to anions. Data from Gimmi &
Waber (2004), Figure from Mazurek et al. (2009).
Comparison between chloride and bromide
The depth profile of bromide is shown in Fig. 5-6. It reveals similar features as chloride (Fig. 5-2), both in terms of behaviour with increasing squeezing pressure and profile shape. The Br/Cl mass ratio is mostly in the range 0.96E-03 – 1.25E-03 and does not vary systematically with depth or with squeezing pressure. All Br/Cl mass ratios of squeezed water are considerably below that of seawater (3.6E-03). Br- in the groundwater sample in the 'Brauner Dogger' as well as the value from the advective-displacement experiment are slightly below those of squeezing at 200 MPa (but close to those at 500 MPa). Note that because Br- concentrations in leachate samples are close to detection limits, no comparison between these data and those from the other methods is possible.
Fig. 5-6: Depth profiles for bromide from all available methods.
Br- contents in sample 987.40 at squeezing pressures of 400 and 500 MPa are below the detection limit of 1.6 mg/l.