4 Results
4.5 Data from advective displacement
Sample dimensions and measured physical properties
Sample SLA 779.87 remained in cold storage in a perfectly sealed state until April 5, 2011, when it was opened and prepared for the experiment. The infiltration with artificial porewater started on April 21, 2011. The experiment was terminated on February 18, 2013. Only results for the first year are presented here. Post-mortem analysis of the core material is presently on-going, as of April, 2013.
Measured dimensions, sample mass, wet water content and derived physical properties are summarised in Tab. 4-28 for the advective displacement experiments with sample SLA 779.87 (lab no SLA-779.87-AD1) from the Parkinsoni-Württembergica Beds. The methods and equa-tions for water-loss porosity and densities are given in section 3.3.2. The calculated bulk dry density and grain density are not needed for data interpretation but serve as additional parameters for a consistency check. The calculated wet water content in the core is the basis for converting experimental time to pore volume fraction. A density of 1 g/cm3 is assumed for 1 g of porewater which is a reasonable approximation at these relatively low salinities.
The wet water content of 4.843 wt.-% is an average of four measurements, two from sub-samples above and two from below the core sample. The data suggest a slight heterogeneity, with a range of 4.937 – 4.945 wt.-% in the upper part, and 4.741-4.747 in the lower part. For comparison, grain density in the nearby sample SLA 778.7 (SLA-7) is 2.769 g/cm3 and com-parable, but its wet water content of 3.69 wt.-% is distinctly smaller.
Tab. 4-28: Sample dimensions and measured and calculated physical properties for sample SLA 779.87.
Experiment / Sample ID SLA 779.87
Parameter Units
Core length cm 10.13
Core diameter cm 9.64
Cross-sectional area (calc.) cm2 80.56
Core volume (calc.) cm3 776.2
Core mass (water saturated) g 1958.8
Bulk wet density (calc.) g/cm3 2.524
Wet water content (105 ºC) mass fraction 0.04843
Water mass in core (calc.) g 94.86
Water-loss porosity vol.-% 12.22
Bulk dry density (calc.) g/cm3 2.402
Grain density (calc.) g/cm3 2.736
Evolution of pressures, temperature, hydraulic and electric conductivity
Temperature variations (22 ± 4 C, Fig. 4-52) reflect the seasonal fluctuations in a thick-walled basement laboratory, but are virtually free of diurnal fluctuations.
The confining pressure (P(conf) in Fig. 4-52) was set to 75 bar and remained steady, rising to 78 bar during the period with highest temperatures in the laboratory. The pressure was applied 13 days before injection of the artificial porewater (APW). The reason for this was that the APW had to be replaced with a new batch, and this required some time. The sample core may have consolidated during this time to some degree. The confining pressure is set by a com-pressed argon head space in a liquid/gas pressure compensation cylinder connected to the water-filled confining volume of the pressure vessel. The purpose of the compensation cylinder is to dampen pressure changes due to changes in room temperature.
The infiltration pressure (P(inf) in Fig. 4-52) of artificial porewater (APW) was initially set to 44 bar and was increased to 60 bar after 34 days to force a larger sample flux, starting with sample #3. Infiltration pressure gradually decreased from 60 bar to 55 bar over 350 days. This gradual decrease is mostly due to the fluid volume continuously pushed out from the tank through the sample core, and possibly also due to any small helium leakage from the head space of the tank.
Fig. 4-52: Experimental conditions (pressures, temperature) during the porewater displace-ment experidisplace-ment, sample SLA 779.87.
The dead volume contained in the titanium/Teflon filter is approximately 1.8 ml on either end of the sample core. The amount of gas initially contained on the injection side was removed by applying vacuum prior to injection. The gas contained in the dead space of the extraction side was flushed with argon by a few alternating cycles of moderate vacuum and overpressure. This does not reduce the dead space gas, but reduces atmospheric contamination. During the first two weeks of the experiment, only gas from the dead space was expelled and released from the sampling syringe. The first drop of displaced porewater reached the syringe 15 – 16 days after start of injection. The amount of gas expelled during this initial phase could not be quantified, but was larger than 1 ml.
The hydraulic conductivity K [m/s] (Fig. 4-53) of the sample can be calculated for each sampling interval from the average volumetric flow rate, the sample length, the cross section, and the average difference in hydraulic head applied during displacement, according to eq. 3-20.
The first 3 data points are apparent values. The first sample is dominated by expelled gas from the dead volume of the filter and this leads to a low apparent porewater flux. The second and third sample contained only minor amounts of gas. It is hypothesized that the low K values may be due to the pre-consolidation of the sample before injection started (see note regarding con-fining pressure above). A relatively steady value for K is reached at approximately 4·10-14 m/s.
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Fig. 4-53: Hydraulic conductivity (K) derived from individual sampling intervals and tem-perature, sample SLA 779.87.
Open squares represent minimum estimates due to interfering small gas fluxes from the outflow side.
The electric conductivity was monitored continuously as a summation parameter (Fig. 4-54) similar to ionic strength. A very small conductivity cell from Metrohm was used, inner diameter approximately 0.75 mm, with a cell constant of 22.7 cm-1, connected to a Metrohm 712 conduc-tometer. No temperature compensation was done because of technical difficulties in connecting the very small built-in thermistor. Instead, the readings were recorded manually at regular laboratory checks, along with temperature. A continuous record was also collected in parallel with a data acquisition system. The cell and instrument were calibrated with a conductivity standard of 12.9 mS/cm (at 25 °C) that gave a reading of 12.63 mS/cm at 21.1 °C. The artificial porewater gave a reading of 17.7 mS/cm at 22 °C. The conductivity readings are subject to drift due to electrode corrosion and cannot be strictly interpreted over long time periods without frequent re-calibration. Another advantage of a continuous conductivity log is that the passage of gas bubbles contained in the effluent is clearly visible as a period of very low conductivity.
The early samples elute at a conductivity of 13.5 mS/cm (referenced to 20 ºC), and after 120 days a maximum of 15 mS/cm is reached which is lower than that of the APW (17 mS/cm at 20 ºC). A gradual decrease is observed after 295 days, and this may indicate the onset of electrode corrosion. The difference between the maximum conductivity and the APW is either due to passivation of the electrode (low apparent readings) or mineral precipitation along the flow path that would reduce the dissolved salt concentrations.
16
Time at midpoint of sampling interval [d]
Hydraulic conductivity Temp
Fig. 4-54: Electric conductivity (EC, EC-20: temperature-corrected) measured in-line and variation of room temperature, experiment SLA 779.87.
Chemical evolution of eluted porewater
The total mass of displaced porewater up to sample #14 amounts to 53.9 g. This is equivalent to a pore volume fraction of 0.568 relative to a total wet water content of 94.9 g determined for the core sample. The wet water content is used to convert experimental time (duration) to pore volume fractions in graphs for chemical evolution. This time scale is more meaningful in terms of advective-diffusive transport. The sampling schedule up to 350 days – or 0.57 pore volume equivalents – is shown in Fig. 4-55. The excursion to a zero syringe number at the end of sampling syringe #4 reflects the time when pH was measured in-line.
Fig. 4-55: Plot of time intervals for each sample, experiment SLA 779.87.
Sample numbers 9-11 were skipped, and the square symbol represents the time when pH was measured in-line.
Sample aliquots of displaced water were analysed for major cations and anions by ion chroma-tography in-house. Alkalinity was measured on selected samples by titration. The analyses of samples and of the injected artificial porewater (APW) are given in Tab. 4-29. Low-molecular weight organic acids were also reported in some samples, as well as dissolved silica. Only incomplete analyses (no alkalinity) could be obtained from samples #1 and #2 due to their small sample size. Nitrate was below a detection limit of 0.016 mg/l for all analysed samples. The time reported reflects the midpoint for each sampling interval, and so it marks the position where the averaged concentration in a sample syringe should match the averaged time.
Tab. 4-29: Chemical composition of sample aliquots from the advective-displacement experi-ment with sample SLA 779.87, including artificial porewater (APW).
The APW is a simplified porewater traced with deuterium (stable isotope analyses not yet available) and does not contain bromide (Tab. 4-29). The water isotope tracer 2H, 18O and Br -are non-reactive tracers whose breakthrough is indicative of the proportions of APW contained in the extracted sample aliquots, and also provides information on the advective-dispersive properties including electrostatic effects such as ion exclusion. While δ2H and δ18O can be observed as a breakthrough curve in the outflow (gradually increasing δ2H), Br- is contained initially in the porewater and is observed as a break-out tracer (decreasing concentrations).
Likewise, Cl- may also serve as a breakthrough or break-out indicator in case the difference in concentration between APW and in-situ porewater is significantly larger than the analytical error.
Data from Tab. 4-29 is depicted in Fig. 4-56 for major components, and in Fig. 4-57 for minor components. The graphs represent break-through curves with the in-situ porewater displaced at earliest time, followed by an increasing proportion of admixed APW. The composition of the injected APW is shown along the right margin of the graphs. Non-conservative components are affected by ion-exchange processes or mineral dissolution/precipitation.
A striking feature of the elution curves is the break in slope between sample #3 and #4, after an initial smooth trend from samples #1-3. An analytical artefact can be excluded because samples
#1 and #2 were processed in the same series, as well as samples #3-14. The position of the 1st sample is not well defined due to the displacement of gas from the filter dead volume. But starting with sample #2, this should no longer be an issue. This elution behaviour is not typical when compared to previous experiments (Opalinus Clay, Effingen Member) and it is presently not resolved. The results from stable isotope analysis may add to clarification.
Fig. 4-56: Chemical evolution of eluted samples for major components, plotted against time expressed in pore volume fractions, experiment SLA 779.87.
APW is plotted along the right side
Fig. 4-57: Chemical evolution of eluted samples for minor components, plotted against time expressed in pore volume fractions, experiment SLA 779.87.
APW is plotted along the right side.
The key features of the chemical evolution are a relatively fast approach of Na+ and Ca2+ to the APW concentration, while Mg2+ and K+ are being retained in the core. Sr2+ is not present in the APW, but is eluted at approximately constant concentration. Cl- reaches its full breakthrough concentration at 0.28 pore volume fractions, and even appears to elute at slightly higher con-centration than in the APW, but this is just within the analytical uncertainty of ca. ± 300 mg/l at this concentration range. SO42- is clearly retained in the core. The initial increase for Br- is unexpected because it is to be just gradually flushed out of the core. Furthermore, the Br
-0
Concentration [mg/L] Na, Cl, SO4, acetate
Time in pore volume fractions
Concentration [mg/L] Si, Br, Alk [meq/L]
Concentration [mg/L] K, Sr
concentrations of 4 – 6 mg/l are unusually low. A striking feature are the large alkalinities (21 meq/l in sample #4) eluted in the early samples that correlate with very high measured acetate concentrations (1580 mg/l in #3, 1350 mg/l in #4). There are no measurements of alkali-nity or organic acids for the first 2 samples: the trends suggest that these might be even sub-stantially higher than the ones measured in subsequent samples.
Break-through of anionic species
The breakthrough concentrations of the anionic species Cl-, SO42-, Br- and acetate are shown in Fig. 4-58. The trend in chloride breakthrough is smooth apart from the break in slope (com-mented above) between sample #3 and #4. A full breakthrough may be expected after ca. 0.5 pore volumes if the accessible porosity is ca. 50 %. The data presented here therefore suggest an accessible porosity of less than 50 %. The exact value is best evaluated by modelling, whereby the presently missing data for δ2H will additionally constrain the breakthrough behaviour for water. A full breakthrough may be expected after ca. 0.5 pore volumes if the accessible porosity is ca. 50 %, and if transport is in a homogeneous porous medium.
The breakthrough trend for bromide is enigmatic and presently not understood. It was observed to be a smoothely decreasing trend in previous experiments with Opalinus Clay or Effingen Member. It should be noted that the conservative nature of bromide, as well as iodide, in the presence of poorly characterized organics may be questionable (e.g. Gilfedder et al. 2011).
The trend in the evolution of the acetate concentrations is smooth, and roughly in agreement with a gradual flusing out of the organic acid. The only other low-molecular weight organic acid detected is formate (Tab. 4-29) present at low concentrations of 4 – 6 mg/l. This strong domi-nance of acetate over other carboxylic acids is unusual compared to previous (qualitative) obser-vations, and may suggest that other acids are being degraded to acetate by bacterial activity either in the filter or during sample storage. Nevertheless, the observed initial high concentra-tion of a low-molecular weight organic acids is real and suggests a substantial dissolved organic carbon content in the porewater, and also likely a distinct solid organic carbon content.
Sulphate at ca. 1300 mg/l in early samples, first decreases slightly and then gradually increases to 1600 mg/l in sample #14. This value is distinctly below the 2180 mg/l present in the APW.
This linear trend strongly suggests some solubility control by a solid phase, likely by precipita-tion of a sulphate phase. This issue can be further addressed by examining the calculated satura-tion indices in the eluted samples.
Fig. 4-58: Chemical evolution of eluted samples for anionic species, plotted against time expressed in pore volume fraction, experiment SLA 779.87.
APW is plotted along the right side.
Break-through of cationic species
The early rapid increase in all species is mainly due to an early partial breakthrough of ionic strength driven by the increase in chloride concentrations that is largely balanced by sodium (Fig. 4-57, Fig. 4-58). The overall smooth approach of cation concentrations to those in the APW suggest that the proportion of cations in the APW relatively closely match those on the exchanger in the sample core. This is not strictly true for Mg2+ that is being retained in the core, and the same can be said for potassium. It is not a priori possible to relate these processes solely to ion exchange, because the retarded sulphate breakthrough suggests the involvement of a precipitating sulphate phase. This process couples the cation present in the sulphate (possibly Ca2+ or Sr2+) to the ion exchange processes. By the same token, Sr2+ eluted from the core may not only derive from displacement from the exchanger but may also be leached from a Sr-phase if present. Thus, an interpretation will require detailed coupled reactive transport modelling.
Mineral saturation states in the eluted porewater samples
The eluted porewater compositions have been modelled using PHREEQC in order to assess the saturation states with respect to diagnostic minerals, and also to compute carbonate alkalinity and a partial pressure of carbon dioxide in the absence of direct measurements of DIC. The following assumptions were made to prepare the input concentrations for the calculations and modelling constraints:
A pH of 7.65 was used for all samples that corresponds to the pH measured in-line during sampling syringe #4.
A temperature of 22 ºC was used for all calculations corresponding to the average labora-tory temperature.
Acetate concentrations were neglected in the input, but were "substituted" by chloride via a charge-balance constraint. This results in Cl-+SO42-+Br- balancing the total measured cation charge. The singely-charged acetate anion is thus substituted by additional chloride, and this is a good approximation for the ionic strength and its effect on the activity coefficients.
Calcite saturation was assumed for all samples to constrain the carbonate alkalinity and partial pressure of CO2 at fixed total Ca concentration and pH.
A concentration of 21 mg/l for Sr corresponding to sample #3 was used for samples #1 and
#2 where Sr could not be measured.
The results of the calculations are summarised in Tab. 4-30. The column with "Cl-added" lists the amount of chloride that was added by PHREEQC to achieve charge balance with the sum of cationic charge. This amount is substantial, but in accord with the large measured acetate concentrations for which this added chloride is substituting. There is a net excess of cationic charge on average compared to the measured acetate concentrations and sum of anions, but this difference is entirely within analytical uncertainty. Carbonate alkalinity is negligible in this context.
The calculated carbonate alkalinity decreases from 1.3 meq/l in the first sample to 0.9 meq/l in the latest samples, as a result of gradually increasing Ca concentrations in the eluted samples (Tab. 4-30). The calculated partial pressures of CO2 are within a narrow range of -2.9 to -3.1 log bar units.
The eluted solutions are distinctly undersaturated with respect to gypsum and strontianite, and only slightly undersaturated with respect to dolomite. A striking feature is that all but one composition is at celestite saturation within ± 0.09 units, more or less evenly distributed around zero SI.
While there is uncertainty in the assumptions (e.g., constant pH, acetate 'substitution'), the calculations are thought to be relatively robust with respect to celestite because this mineral equilibrium does not sensitively depend on assumptions regarding pH and the carbonate system.
A conclusion from these calculations is that celestite may be a candidate sulphate phase that is limiting the elution of sulphate via precipitaion of celestite during permeation through the core sample. The Sr required for precipitaion would be derived from the exchanger complex via dis-placement by the incoming APW that is more Na-rich compared to the initial in-situ porewater.
This does not necessarily imply that celestite is present initially in the sample for this process to operate during the experiment. The fact, however, that also the very first displaced sample aliquots are saturated with respect to celestite would suggest that celestite may be a solubility-controlling phase present initially in the core sample.
Tab. 4-30: Results of PHREEQC simulations of sample aliquots from the advective-displace-ment experiadvective-displace-ment with sample SLA 779.87.
'Cl-added' is the chloride required by charge balance and substitution of organic acids.
pCO2 is computed at calcite saturation, pH = 7.65 and measured Ca concentration.
Summary and conclusions from the advective displacement experiment
A sample core from 779.87 m depth from the Parkinsoni-Württembergica Beds was examined.
The experiment performed technically flawless for a duration of almost a year. A break in slope in all trends of chemical evolution after the 3rd sample is not understood, but may hint at a more complex flow regime than simple advective-dispersive flow. The hydraulic conductivity of this sample is approximately 4 × 10-14 m/s. Also, initially, hydraulic conductivities were distinctly lower than the value of 4 × 10-14 m/s approached at later times.
A distinct feature are the very large acetate concentrations of > 1000 mg/l observed in the early samples. The absence of other low-molecular weight organic acids apart from a trace of formate
A distinct feature are the very large acetate concentrations of > 1000 mg/l observed in the early samples. The absence of other low-molecular weight organic acids apart from a trace of formate