Centrifuge Extraction and Chemical Analysis of Interstitial Waters Wyndham M. Edmunds" and Adrian H. Bath Hydrogeological Department, Institute of Geological Sciences, Exhibition Road, London SW7 2DE, U.K.
Centrifugation has been investigated as a method for recovering interstitial fluids from consolidated geological materials, for subsequent chemical analysis. Practical aspects of the technique, illustrated using results mainly from the English Chalk (Cretaceous), are described and the relationship between centrifuge drainage and pore size distribution is discussed, as are fractionation effects related to partial fluid extraction and problems of sampling and contamination. Microanalytical techniques for 18 parameters in fresh waters are outlined. Fractionation effects and small sample volumes may introduce errors of around f10% in the pore water analysis. The technique has applications not only in the investigation of natural water geochemistry, but also in the field of aquifer pollution.
Sampling of small rock volumes is necessary in order to describe accurately geochemical processes and element distributions involving the water-rock system. I t is frequently impossible to assign an exact location of origin to borehole depth samples because of vertical and lateral flow within the water column; interstitial water derived from borehole core material can provide a precise log of in situ hydrochemical variations and enable the water chemistry to be related directly to the host rock composition. Methods of fluid extraction described in this paper have been developed principally in conjunction with research on natural water geochemistry ( I ) , but the techniques are being applied increasingly to the study of groundwater pollution where dispersion of pollutants from landfills can be monitored by investigation of pore waters in the surrounding rock matrix. Interstitial water extraction, particularly from marine cores, has usually been carried out using hydraulic or gas operated squeezers (2-5) and these methods are especially suited to unconsolidated sediments. Elutriation or leaching of sediments with distilled water and other solvents has also been used (6, 7) but this method, although useful in providing estimates of salinity, is clearly unsuitable for accurate work on water composition. A multiple washing technique has been described by Devine et al. (8) which attempts to differentiate between micellar solutions around clay minerals and the main pore solution. A tensiometric method for in situ extraction of fluids from lysimeters or shallow excavations has been described by Wengel and Griffin (9) which in theory a t least, offers the possibility of continuous monitoring of unsaturated zone groundwaters. Centrifugation has been used for fluid removal from various saturated or partly saturated geological materials for several decades (10-14) where the application has been the study of fluid saturation in relation to pore water pressure in porous hydrocarbon and groundwater reservoir rocks. Emphasis in such work is placed on a measurement of the rock physical properties rather than fluid removal for chemical analysis. Centrifuge extraction for geochemical work does not appear to have been widely favored, although Jones et al. (15)and Sholkovitz (16) used the method in conjunction with other techniques. The present paper sets out specifically to describe the techniques of fluid extraction by centrifuge and to discuss the sampling, handling, and microanalytical procedures de-
veloped during our investigation of the Cretaceous Chalk and Triassic sandstone aquifers of the U.K. Emphasis is placed on the practical rather than the theoretical aspects of the method. The work is described in two sections-first, the physical aspects of fluid removal are described including experimental studies of fluid yields with time and with different rotation speeds; the second half of the paper deals with sampling problems, contamination risks and with an analytical scheme devised for the extraction of pore waters from terrestrial borehole core material.
Physical Basis The physics of fluid removal from porous earth materials by centrifugation is fairly well understood, although the precise force distribution developed during the process is difficult to determine. Consider a column of water-saturated rock under centrifugation. Then the applied tension T , developed a t a point within the column, r2 cm from the center of rotation, will be given by the expression ( I 7 ) :
where T , is in centimeters of water, w is the radial velocity in radians per second, g is the acceleration due to gravity in cm sec-2, r1 is the distance (cm) from the base of the column to the center of rotation. Clearly, applied tension is a function of distance from the rotor and of centrifugal speed. I t will be the same magnitude whatever the density and nature of the material, and the pattern of interstitial water removal will depend on the pore size distribution of the material. According to the formula given by Washburn (18)for the capillary pressure in a pore 4u cos0
T, = pd where T , is the tension (in Nm-2) due to capillary action, cr is the surface tension in Nm-l, 8 is the contact angle between the porous solid and the liquid, p is the specific gravity, and d is the effective pore channel diameter in m. The applied tension will vary across the sample in the centrifuge bucket and Figure 1 illustrates the relationship between w and radius of emptied capillaries, and its dependence on the distance from the rotor. I t can be seen for example, that, in theory, rotation speeds in excess of 7000 rpm, for the equipment described are required to completely drain pores of 0.1 pm radius or less, but that significant drainage of pores greater than 1 pm radius will occur a t low speed centrifugation (EROC K
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Figure 3. Sample processing scheme for interstitial fluid extraction and analysis using the centrifuge method
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Figure 5. Pore size distribution curves for Chalk samples, determined by mercury injection porosimetry Upper Chalk samples from 30-40-m depths: Lower Chalk sample from 490-m depth
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time (mins) Figure 4. Pore water loss from samples of Chalk (lower curves) and Bunter sandstone (upper set of curves) as a function of time of centrifugation Results are for 14-mm diameter fully saturated plugs with physical properties as shown, = effectiveporosity, % ; k = intergranular permeability,mi,lidarcys. Centrifuge speed (ca. 1300 rpm) regulated so as to give same applied tension at midpoints of all plugs. The upper curves are from Lovelock ( 79)
Sampling, Contamination, and Modification The small volumes involved in interstitial water investigations, together with the number of operational steps involved, demand detailed attention to sampling methods. Previous studies (20, 21) have highlighted many of the extractiodhandling problems particularly as applied to marine pore waters. The principal opportunities for contamination or modification of samples occur during the following three stages: Sample contamination as a result of the coring process Contamination/compositional change/evaporative loss during fluid extraction from cores Chemical changes/evaporative loss during storage. The problems associated with each of these aspects are discussed below in the light of experiences with the extraction of pore waters from continental drilled cores. Core Recovery. Cores of 140-mm diameter recovered by water-flush rotary drilling from depths of up to 500 m below ground level, have formed the standard starting material for investigations. In view of a potentially wide range of aquifer lithologies that might be investigated, it was de-
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Figure 6. Weight percentage of total pore water drained by centrifugation Of Chalk samples plotted as a function Of the induced tension (referred to midpoint of Chalk column in centrifuge bucket)
cided to pay special attention in early projects to effects of contamination during drilling. In two hydrogeological studies in which cores were recovered, a LiCl tracer was added t o the formation water-based drilling mud specifically to investigate the extent of fluid invasion into the cores. Tracer investigations were carried out during core recovery from the Cretaceous Chalk of Southern England. A tracer strength of 2 mg/l. Li was maintained and it was demonstrated that core invasion was limited to the outer 1-1.5 cm. Of some 150 samples extracted from a single coring operation only three showed contamination-the remainder being used for geochemical studies of the natural water including lithium. Water or mud flush drilling is therefore satisfactory for coring the saturated zones of Chalk or formations with analogous physical properties for interstitial water studies, although checks using a suitable tracer are clearly desirable. In contrast, pore fluids extracted during hydrogeological investigations of the highly permeable Triassic Penrith sandstone (22) showed significant contamination. A tracer strength of 30 mg/l. Li was used and it was found that Volume 1 0 , Number 5, May 1 9 7 6
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about half of the samples after removal of the outer 2 cm showed more than 2% fluid replacement, and that less than 20% of the samples could be considered uncontaminated. By deduction it is clear that a tracer in the drilling water is essential if similar permeable material is to be investigated; drilling by air flush methods might be preferable for water quality studies on such formations. Fluid Extraction. Chemical changes in pore fluid composition during storage of marine argillaceous sediments prior to pore water extraction have been demonstrated by Bischoff et al. (23) who found that warming of samples from in situ temperatures changes the temperature-dependent ion-exchange selectivities. While isothermal sampling is virtually impossible, during the present studies an attempt was made to minimize these water-rock reactions by extracting fluids on site, within l/2-2 h of recovery, where feasible. In the studies described here, extraction temperatures were within a few degrees of the formation temperatures. Immediate extraction of fluids in this way also enables pH and bicarbonate to be measured with minimum risk of alteration from in situ values. Chemical variations resulting from the fluid extraction process may also occur. Various examples of decreasing salinities of squeezed pore fluids with increasing compaction pressures have been noted by Rieke and Chilingarian ( 2 4 ) . To check possible fractionation during centrifugation, sequential extracts from the series of Upper Chalk samples, described for the yield experiments above, were analyzed for the cations Na+, K+, Ca2+,Mg2+,Sr2+.These analytical results have been used to plot the curves in Figure 7, illustrating the variation in overall extracted fluid composition as progressively more fluid is removed. Two trends are apparent from the curves in Figure 7 . The alkali cations, Na+ and K+, generally show a progressive depletion as a greater proportion of fluid is extracted; an upturn or leveling-off of the trend occurs over the last 10-20% of fluid extracted. On the other hand, Ca2+exhibits depletion which continues unabated to the final effluent fluid (Le., 85-90% of total pore fluid); Sr2+ and Mg2+ show no distinct fractionation patterns during the extraction. These trends must be considered in the context of the analytical precision of about f2-3% (Table I). However, it is clear that the patterns noted above represent variations outside these limits, as well as being reasonably consistent between different chalk samples. A detailed discussion of the possible causes of this fractionation is beyond the scope of this paper and will be discussed elsewhere. The general trends of cation depletion as extraction progresses are similar to those observed in mud compaction experiments ( 2 4 ) .At present, it can be suggested that the upturn in fractionation of Na+ and K + (Figure 7), a feature which has not been observed in previous studies, is merely the result of concentration of the cation in residual fluids due to earlier depletion in the extracted fluids. The apparent cation retention itself may be a hydraulic drag effect on hydrated cations, as postulated by Kharaka and Berry (25) although direct evidence of this process operating in a lithology not dominated by clay minerals (Chalk has about 1-2% clay phases) is not available. I t is tentatively suggested that the contrasting behavior of Ca2+ in the late extracts from chalk may be due to the buffering of this cation in residual fluids by calcite, this effect being superimposed on the cation retention effect. Next, we may consider how representative of bulk pore water chemistry are the partial samples obtained by lowefficiency extraction techniques. The low-speed centrifuge (MSE Ltd., Mistral 2L) method described previously in this paper gave yields of 20-30% of total pore water from chalk samples. If it is assumed that the final ca. 10% of re470
Environmental Science & Technology
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Figure 7. Graphs showing changes in cumulative composition of extracted fluids from four Upper Chalk samples against percentage of total pore fluid removed. Data points have been calculated from analyses of stepwise extracts and the relative yields of these extracts
tained fluids do not deviate excessively from the compositional trends in Figure 7, then an overall pore water composition for each sample may be estimated by extrapolation of these trends. For only 20-30% pore fluid extraction from chalk, the greatest discrepancies between in situ levels and those in the extract are expected for Ca2+and Na+ compositions; the trends in Figure 7 indicate enrichment of the initial extracts. Of the other cations analyzed, K+ also shows some enrichment in initial extracts, though to a lesser extent, while Mg2+ and Sr2+ compositions as measured on early extracts would appear to be satisfactorily representative of the overall pore-water composition. Degradation of Water Sample During Storage. The acidified water samples are stored in polystyrene bottles with polypropylene screw caps. Analysis is carried out as rapidly as possible to minimize sample deterioration. Par-
Table 1. Summary of Analytical Methods Used for Microanalysis of Interstitial Waters Mlnlmum solute weight required for volume and method stated Na Ca K
Mg Sr Li tic03 Son
