Environ. Sci. Technol. 1983, 17, 362-368
Extraction and Chemical Analysis of Interstitial Water from Soils and Rocks David G. Kinniburgh” and Douglas L. Miles Hydrogeology Unit, Institute of Geological Sclences, Wallingford, Oxon OX 10 8BB, U.K.
Interstitial water was extracted from field-moist soils and chalk by immiscible displacement with a dense, inert fluorocarbon liquid. The only special equipment required is a high-speed centrifuge. Yields of interstitial water from soils at field capacity are typically 20-50% of the total water present; yields from chalk range up to 90%. The interstitial water was analyzed for about 20 solutes principally by inductively coupled plasma emission spectrometry. The water extracted from a chalk sample appeared to be representative of all the water present, but with a clay soil, there was evidence of some fractionation of NO3-N. Organic carbon ((2-7) X M) was frequently the most abundant solute in the solutions extracted from ten topsoils. Other typical solute concentrations were in the range 10-3-104 M for NO3-N, Na, Mg, Si, S, C1, K, and Ca and less than lo* M for the trace elements Cu, Zn, and Ba. H
Introduction Interstitial waters play an important role in many aspects of environmental chemistry, particularly those concerned with the movement of solutes in soils, sediments, and rocks. The chemistry of interstitial water provides an important link between field and laboratory studies and may be used as a sensitive means of calibrating and validating theoretical models of solute transport. It can also provide information about the kinetics of solid/solution interactions in situ (1). The extraction and analysis of interstitial water has already been used extensively in the study of ocean sediment pore waters (2), and more recently, a similar approach has been used to monitor the movement of fertilizer nitrate through the unsaturated zone (3). Interstitial water chemistry profiles have also provided useful information about the origin and evolution of interstitial water in aquifers (4). There is relatively little direct information on soil and clay interstitial water chemistry, partly no doubt because of the difficulty of extracting sufficient water to carry out a detailed chemical analysis. We have therefore extended the range of application of the centrifuge extraction technique of Edmunds and Bath (5) by replacing the centrifuge drainage method with a more versatile method based on the displacement of interstitial water with a dense, immiscible liquid. In this paper, we outline this method and apply it to some soil and chalk samples. The Chalk is a pure and highly porous limestone that forms England’s most important aquifer, but the methods described are applicable to other porous materials. Centrifuge Methods of Interstitial Water Extraction Background. No single method of extracting interstitial water from porous media is suitable for all samples, but centrifuge methods offer the greatest potential for the routine extraction of interstitial water for chemical analysis. These methods are inevitably destructive in that the sample must be brought to the laboratory and subjected to a high, and potentially deforming, driving pressure to extract the water. However, the residual solid is available for subsequent chemical analysis if desired. 362 Environ. Sci. Technol., Vol. 17,No. 6,1983
The centrifuge may be used for extracting water in two ways (Figure 1): either the water is allowed to drain freely through a porous plate supporting the sample and collected in a cup at the bottom of a specially made centrifuge tube (5) or an excess of a dense, immiscible liquid is added to the sample and the displaced water collected after it has floated to the top (6-9).Whelan and Barrow (8) used the term “displacent” for the immiscible liquid, and we adopt that term here. We have employed the immiscible liquid displacement method because it can be used successfully for most soils, including clay soils. Although sample compaction is a problem with the drainage method, it may aid water extraction with the immiscible liquid displacement method. A Beckman J21C high-speed refrigerated centrifuge was used in this study. It has a maximum speed of 21 000 rpm, but the maximum usable speed depends on the particular rotor used and the tube weights. If the loaded tube weights were greater than those specified by the manufacturer, the maximum permissible speed was derated accordingly. The rotors tested, their loadings, and the driving pressures exerted are given in Table I. No attempt was made to exclude air during handling and centrifugation although, if necessary, it would be feasible to carry out all operations in an inert atmosphere. Choice of Displacent. A number of displacents, mostly halogenated hydrocarbons, can be used. Desirable properties are high density, low water solubility, low volatility, low toxicity, high chemical inertness, and low price. Carbon tetrachloride was used by Mubarak and Olsen (6) but has the disadvantages that its vapor is quite toxic and it attacks most of the plastics (e.g., polypropylene) used for making centrifuge tubes. Batley and Giles (7) used a high molecular weight fluorocarbon (FC78; 3M UK Ltd) that does not have these disadvantages, but its manufacture has now been discontinued, and although related fluorocarbons are available, these are expensive for routine use. Whelan and Barrow (8)discussed the merits of a number of displacents and suggested that perchloroethylene was the best. However, perchloroethylene requires the use of nylon centrifuge tubes, and these are currently difficult to obtain. We have used trifluoroethane (F113), which is compatible with polypropylene, has a low toxicity and a density of 1.57 g/cm3, and is relatively inexpensive. It has a low boiling point (47 “C) and is therefore relatively volatile at room temperature, but this should not present a serious problem; however, a well-ventilated room is a sensible precaution. F113 is available under the trade names Arklone P (IC1 Ltd) and Freon TF (DuPont). I t can be redistilled for reuse, and unlike some other displacents, it contains no stabilizer. Water has a solubility of about 90 ppm (w/w) in F113; this limited solubility does not significantly affect the volume of water displaced. We have not experienced high blank values due to impure Arklone, but should this occur, the Arklone may be purified by distillation and repeated extraction with high purity water. Principle of the Immiscible Liquid Displacement Method. Although the immiscible liquid displacement method successfully displaces interstitial water, the precise
0013-936X/83/0917-0362$01.50/0
0 1983 American Chemical Society
Table I. Characteristics of Several Rotors Used for Extracting Interstitial
water^ from Soils and Rocks rotorsa
type of rotor method used
a
JA14
JA17
fixed angle immiscible displacement 6 X 250 mL 14 000 13 000
fixed angle immiscible displacement 14 x 50 mL 17 000 17 000
fixed angle immiscible displacement 8x50mL 20 000 20 000
41 150 380.0 11.7
16 40 65.0 11.3
16 40 65.0 9.8
9.7, i
10.3, i
8.8, i
2.3 4-25
2.0 1-6
2.3 1-6
4 x 250 mL no. of tubeslbottles 7500 max permissible speed, rpm 7500 recommended speed, rpm approx wt, g 280 empty tube (bottle) 380.0 + sample not used + sample t ArkloneP 8.0 dist from center of rotation to midpoint of sample, cm 11.0,o dist from inner, i, or outer, 0,surface of sample, cm 1.8 driving pressure at midpoint,b MPa 4-25 vol of water extracted/tube ( bottle),c mL Calculated using eq 4. For the Beckman J21C centrifuge. lo) Drainage method
~
Q J
detachble collecting cup
centre of rotation
W
sample
'
JA20
557.5 swinging bucket drainage
disc
May be less if particularly dry or clayish.
point in the sample. This model may be formulated as follows. At equilibrium, the driving pressure will everywhere be balanced by a capillary pressure, pc. Consequently, the capillary pressure gradient at a point P distance r from the center of rotation (Figure 1)is related to the angular velocity, w , and the density difference between the fluid phases by dp,/dr = Apw2r
[b) Immiscible liquid displacement method
(1)
where = P d - Pw
(2)
w = 2m/60
(3)
and w
%
displaced water displocent
sample
e, P,
pc2
Figure 1. Two centrifuge methods of interstitial water extraction: (a) the drainage method using a swing-out rotor: (b) the immiscible liquid displacement method using a fixed-angle rotor.
mechanism of this displacement remains unclear, particularly with regard to the fate of trapped air and the continuity of the liquid phases. Displacent probably penetrates the sample by progressively displacing water upward from the larger cracks and pores. Any air that was initially present in the larger pores will be either expelled or dissolved in the displacent. The simple model considered here assumes that, at equilibrium, there is a network of displacent-filled cracks and pores that are connected, albeit in a tortuous way, to the surface layer of displacent. We also assume that the water phase forms a similar network of interconnected pores. Water is displaced only from those pores in which the driving pressure, due to the density difference between the displacent and the interstitial water, exceeds the capillary pressure tending to retain the water in the pores. The driving pressure at any point in the sample depends on the distance to the center of rotation and will vary continuously through the sample. For water to escape from the sample, the upper (inner) layer of the sample must be saturated with water, and so this layer will have a capillary pressure close to zero. This contrasts with the centrifuge drainage method (Figure 1)where the sample is saturated at its base (7). The average residual water saturation is a weighted average of the residual water saturation at each
where Pd and pw are the densities of displacent and water, respectively, and n is the centrifuge speed in rpm. Integrating eq l between the inner surface of the sample, distance rl from the center of rotation, to point P and assuming that the capillary pressure at the inner surface, pcl, is zero gives the driving pressure at P Apw2
pc(r,w)= f Arl p w 2 r dr = -(r2 2
- r12)
(4)
In particular
where pc2is the capillary pressure at the base of the sample distance r2 from the center of rotation. Equation 4 may also be used for calculating driving pressures with the drainage method, but here the displacent is air ( p d = 0) and rl is the distance to the outer surface of the sample (Figure 1). The average saturation of the sample, S ( w ) can be related to the saturation vs. capillary pressure curve by invs. distance curve for all tegrating the saturation, s@,), points in the sample. For a sample of uniform cross section
or using eq 4 and 5 and changing variables give
Environ. Sci. Technol., Vol. 17, No. 6, 1983
363
Table 11. Variation in t h e Recovery of Water from a Chalk Sample as a Function of t h e Amount of Displacent Useda wt of water wt of wt of displacent, recovered, g g recovery, % chalk, g 29.60 30.01 30.07 30.02 29.07 29.65 29.95 30.20
169.70 169.70 219.99 220.19 270.29 270.11 310.03 310.66
4.828 5.175 5.234 5.078 4.770 4.898 5.096 5.054
80.7 85.4 86.2 83.7 81.2 81.8 84.2 82.8 mean 8 3 . 3
a Centrifuge parameters: rotor = JA14; speed = 1 3 000 rpm; time = 1 h , temperature = 5 'C; displacent = Arklone P,initial moisture content = 202 g/kg wet.
For a nonuniform cross section, as shown in Figure Ib, eq 6 should include a term for the variation of cross section with radius. The percentage yield of interstitial water is given by w t of water extracted/g of sample % yield = X w t of water initially present/g of sample 100 =
so - S ( 0 ) SO
X
100 (8)
where Sois the initial gravimetric moisture content of the sample here expressed on a dry weight basis. Equations 5, 7, and 8 relate the percentage yield to the centrifuge speed, the geometry of the centrifuge/sample arrangement, and the average saturation vs. capillary pressure curve of the sample. In principle, the saturation vs. pressure curve may be derived from the variation of the percentage yield with centrifuge speed by solving the integral equation (eq 7) (IO),but we have not attempted to do this. Any compression during centrifuging will tend to aid the extraction of interstitial water. If compression is significant, then it is the Sb,) - p c curve of centrifuging that is relevant. An important prediction of the proposed model is that the driving pressure, and hence the percentage yield, should be independent of the amount of displacent used, providing of course that there is sufficient displacent to prevent the expelled water from reentering the sample when the centrifuge stops. In practice, this was found to be the case (Table 11), which corroborates the observation of Whelan and Barrow (8). Therefore in order to maximize the amount of water extracted, the maximum practicable ratio of sample/displacent should be used, as recommended in Table I. This contrasts with the recommendation of Batley and Giles (a,who suggested that a small sample/displacent ratio should be used for maximum yield. Procedure Adopted. The optimal choice of centrifuge speed, centrifugation time, sample weight, and displacent weight depends to some extent on the particular sample and the volume of interstitial water required, but Table I gives the combinations that we have found useful. The weight of displacent recommended is the minimum required to ensure that, when the centrifuge rotor stops spinning, the displaced water does not contact the solid. Standard 250-mL polypropylene centrifuge bottles or 50mL Oak Ridge centrifuge tubes (Nalgene) were used. The centrifuge bottles and tubes were thoroughly cleaned with detergent, acid, and distilled water before use. The appropriate weight of sample was loaded into a preweighed centrifuge bottle and tamped down, and the 364
Envlron. Scl. Technol., Vol. 17, No. 6, 1983
neck and sides of the bottle were cleaned with tissue. The centrifuge bottle was then reweighed, the weight of sample found by difference, and the requisite amount of displacent added. Centrifuging for 1h was sufficient for most samples, but yields from clays significantly increased, albeit at a diminishing rate, with centrifuge times of up to 4 h or more. Duplicate samples were weighed into beakers for moisture-content determination. After centrifuging, the displaced water was withdrawn with an automatic pipet, removing as little displacent as possible. The translucent nature of polypropylene pipet tips and the sharp displacent-water interface means that it is relatively straightforward to make a further separation, within the tip, of the displaced water from any displacent inadvertently drawn up into the pipet. The displaced water was transferred to a weighed 7 or 30-mL polycarbonate vial. The weight of extracted water was recorded. If the extracted water contained a considerable amount of suspended solid, it was centrifuged briefly at low speed in a bench centrifuge. Finally, the solution was filtered into a clean vial through a Millex (Millipore) 0.45-pm membrane filter by using an all-glass syringe. Usually the solution was divided into two parts: one was left unacidified while the other was acidified with concentrated AnalaR grade HC1 to give a final concentration of 1% HC1. The extracted water samples were stored at 5 or -18 "C until required for analysis. The percentage of the total water content extracted was calculated from the weight of water extracted, the sample weight, and the initial moisture content.
Chemical Analysis Interstitial Water. An ARL 34000C inductively coupled plasma (ICP) emission spectrometer linked to a PDP-11/03 minicomputer was used for most of the analyses and in the present study was calibrated for the following 22 elements (figures in parentheses are the approximate 3a detection limits in pg/L): Li (2), B (4), Na (1%Mg (15), A1 (20h Si ( 7 ) ,S (251, K (110),Ca (6), V (11, Mn (0.5), Fe (2), Co (3), Ni (12), Cu (l), Zn (l), Sr (0.2), Y (0.3), Mo (20), Cd (l), Ba (0.3), and Pb (40). Measured concentrations were based on the average of three 10-s integrations, which with preflush required about 3 mL of solution persample at an uptake rate of 2.4 mL/min. Acidified samples and standards (in 170HC1) were always used. Calibration curves were calculated by using the manufacturer's software, and blanks and standards were analyzed at intervals to allow corrections to be made for any slight within run instrumental drift. Arklone has a solubility of about 170 ppm (w/w) in water, but this had no effect on the ICP standard curves. The remaining analyses were carried out on a filtered, but unacidified, sample. A Technicon AutoAnalyzer I1 system was used for the C1 and NO,-N analyses: C1 by the mercuric thiocyanate method and NO3-N by the sulphanilimide/N-(1-naphthy1)ethylenediaminemethod with reduction by a Cd-Cu column. Maximum use was made of the sensitivity of the methods to minimize the volume of sample used for analysis. For example, with high NO3-N concentrations (20-40 mg/L), a 20-pL aliquot of sample was mixed with 380 pL of distilled water directly in the AutoAnalyzer sample cup. With care, good precision (&3%)was obtained. pH was measured with a Radiometer PHM84 meter with Radiometer pH (G2222C) and saturated calomel reference (K4040) electrodes, with pH 7.00 and 4.01 buffers for standardization. A Pye Unicam SP1800 UV-visible spectrophotometer with 10-mm silica cells was used for recording the UV-
Table 111. Chemistry of Interstitial Water Extracted from a Chalk Sample as a Function of Centrifuge Speeda driving pressure centrifuge at midspeed, point, rpm MPa
5000 6000 7 000 10000 14000 18000 20000
0.1 0.2 0.3 0.6 1.1 1.9 2.3
electrical conductivity yield, at 25.0‘C, % pScm-‘
34.0 50.0 64.5 77.7 84.0 86.8 89.5
784 780 787 776 774 777 779 4b
concentrations, mg/L Ca
Mg
Sr
133 135 139 134 134 134 138 2b
2.79 2.81 2.86 2.69 2.63 2.71 2.79 0.05b
0.362 0.353 0.351 0.342 0.346 0.355 0.357 0.004b
Na
16.1 16.2 16.0 15.9 16.2 15.7 16.3 0.2b
Si
NO,-N
S
1.26 1.29 1.22 1.27 1.19 1.29 1.18 0.06b
53.6 53.6 55.3 53.0 53.3 52.3 55.5 0.6b
25.0 24.7 25.0 24.5 24.4 24.4 24.7 0.4b
a Centrifuge parameters: rotor = JA20; time = 1 h; temperature = 5 “C; displacent = Arklone P. All results are the means Standard error of reported mean values. of duplicate determinations. Initial moisture content = 231 g/kg wet.
visible spectrum and for the absorbance measurements. Electrical conductivity was measured with a Philips PW 9513/00 microcell thermostated at 25.0 f 0.1 OC and connected to a Wayne Kerr Autobalance B642 universal bridge. Dissolved organic carbon (DOC) was measured by using a Phase Sep TOCsin I1 aqueous carbon analyzer. Normally all of these analyses could be carried out on 5-7 mL of solution. Soil Analyses. Moisture contents were determined from the weight loss at 110 “C and are expressed on a wet weight basis. Loss-on-ignition (LOI) was determined from the weight loss of approximately 5 g of soil heated between 110 and 540 “C for 2 h and is expressed as a percentage loss of the 110 OC weight. “Soil pH” was the pH of the supernatant solution obtained by adding 25 mL of 0.01 M CaC12to the weight of field moist soil required to give 10 g of dry weight of soil.
Representativeness of the Extracted Water Since the immiscible liquid displacement method often extracts only a small fraction of the total water present, it is important to establish whether the extracted water has the same chemical composition as the remaining water. The variation of interstitial water composition with yield should indicate whether systematic differences are significant. We tested this with a well-mixed sample of Upper Chalk. The yield of water extracted from separate subsamples was varied from 34.0% to 89.5% by varying the centrifuge speed. We estimated the measurement error, urn,from the median value of the absolute difference between duplicates (11). It combines the errors resulting from subsampling and analysis and represents the standard deviation of a single measurement; the standard error of the mean of duplicate determinations is arn/v’2. Small differences in the chemical composition of the extracted solutions were found, but there was no systematic variation (Table 111). Therefore, it is reasonable to assume that, for this sample at least, if only a small fraction of the total water present had been extracted, it would have been a representative sample of all the water present. This will not necessarily be the case with soils. Many soils have a wide range of pore sizes with some of the pores sufficiently small to be affected by surface electrical interactions. For example, it is well-known that the presence of electrically charged surfaces can give rise to a nonuniform, but equilibrium, distribution of ions around clay particles. This is reflected in the cation exchange, anion exclusion, and “salt sieving” properties of clays and soils (12, 13). Indeed, in dry clays and soils, where overlapping electrical double layers may occur, the concept of an interstitial solution of uniform composition is unrealistic.
Table Iv. Interstitial Water Nitrate Concentrations of a Soil (Harwell Soil Series) Estimated by Extracting Nitrate with 0.1 M CaC1, Using Various Solid/Solution Ratios (4-h Equilibration)
wt of wet ~ 0 i i , 4g
wt of 0.1 M CaC1, added,b g
solid/ soh ratio, g/g
measd [NO,-N] estimated in super- [NO,-N] in natant, interstitial mg/L water,c mg/L
27.4 25.2 26.3 26.9 26.6 27.3 27.8 26.4 26.7 27.5 29.4 25.0 27.3 27.3 27.0 26.6 mean 26.9 a Moisture content = 250 g/kg of wet soil. p (0.1M CaC1,) = 1.009 g/cm3. Compare with interstitial water NO,-N concentration (mg/L) of eight replicate samples extracted by immiscible liquid displacement: 33.8,32.4, 32.9,32.2,33.7,33.2,34.6,34.0 [mean= 33.41;average yield = 43.2% using JA20 rotor at 20 000 rpm for 1 h. 0.997 0.997 1.001 0.993 1.027 1.027 1.013 1.996 2.016 2.002 1.993 2.017 5.002 4.995 4.998 5.002
1.009 2.007 3.001 4.007 4.997 9.994 20.017 1.012 1.982 4.011 9.997 20.023 2.022 1.037 9.998 20.001
0.988 0.497 0.334 0.248 0.206 0.103 0.051 1.97 1.02 0.499 0.199 0.101 2.47 4.82 0.500 0.250
5.46 2.81 2.04 1.58 1.31 0.69 0.35 8.77 5.46 3.07 1.41 0.62 10.5 15.0 3.02 1.58
With the Beckman J21C centrifuge used in this study, it is usually not possible to extract more than 50% of the total water from clay soils, so a study of the variation of interstitial water chemistry with yield is likely to be of limited value. Instead, the N03-N concentration of the interstitial water in a moderately clayish soil was estimated in two ways: first, by immiscible liquid displacement, and second, by displacing all of the N03-N with an excess of 0.1 M CaC1, and then back-calculating the apparent interstitial water concentration from the initial moisture content and the amount of CaC12added (Table IV). In order to ensure adequate precision, interstitial water was extracted from eight replicate samples, and sixteen solid/solution ratios were used for the CaC1, displacement. The two extractions were carried out concurrently so as to minimize any spurious effects arising from the mineralization or immobilization of soil nitrogen. Each analysis was also duplicated to minimize analytical errors. The N03-N concentration found by immiscible liquid displacement was significantly ( P < 0.001) greater than that estimated by CaC1, displacement (Table IV). This Envlron. Scl. Technol., Vol. 17, No. 6, 1983
385
-
Table V. Selected Properties of t h e Ten Soils Used for Interstitial Water Extraction
soil series Harwell Icknield Grove Rowsham Fyfield Thames Marcham Denchworth Sout hampton Berkhamsted
soil type brown earth (lessive') rendzina gleyed calcareous surface water gley brown earth (lessive') groundwater gley brown calcareous surface water gley podzol gleyed brown earth (Iessive')
texture silt loam silt loam clay clay loamy sand clay sandy loam clay loam sandy loam clay loam
national grid ref 4516 1886 4492 1836 4459 1927 4436 1933 4451 1988 4335 2002 4344 1972 4369 1899 4558 1692 4597 1753
moisture content, "soil LOI, g/kg yield: pH" % wet %
land use grassland (pasture) grassland (gallops) grassland (rough pasture) arable (ploughed) grassland (rough pasture) grassland (perm pasture) arable (ploughed) grassland (perm pasture) grassland (heath) grassland (perm pasture)
5.1 8.2 7.8 8.0 8.2 8.5 8.3 8.2 3.7 8.3
5.2 14.0 4.5 5.6 2.6 14.6 4.1 15.1 20.0 6.7
250 345 227 254 103 396 196 402 370 228
a Centrifuge parameters: rotor = JA14, speed = 1 3 000 rpm, time = 1 h ; temperature 5 "C; displacent = Arklone P. ple floated during immiscible displacement, therefore mixed (1:1)with clean silica sand.
is consistent with the NO3- ion being partially excluded from a near-surface volume of solution. The conclusion from this experiment is that the total NO3-N content of a soil is not necessarily equal to the NO3-N concentration in the extracted water multiplied by the moisture content of the soil (13). However, the difference is not large in relation to the overall variation found between different soils, and so the immiscible liquid displacement method can provide information on soil solution chemistry that should be adequate for many comparative purposes.
Interstitial Water Chemistry of T e n Soils Five-kilogram samples (0-15 cm) of soils from ten soil series of the Oxford-Reading area were collected on Jan 6, 1982. The moisture contents of the soils were close to field capacity. Some properties of these soils are given in Table V; additional details are given in the soil survey memoirs of the area (14, 15). All of the soils, except the Harwell and Southampton soils, contained free CaC03. The large loss-on-ignition (LOI) values are associated with the organic-rich soils (Southampton, Denchworth, Thames, and Icknield). The moisture contents of the soils reflected their texture, with the clay soils tending to have relatively high moisture contents (Table V). Yields of interstitial water varied from 19% to 49% for eight of the ten soils. Yields from two of the soils (Thames and Denchworth) were not calculated since the soils floated to the surface of the Arklone during centrifugation. This was because the bulk density of these two soils, both high in organic matter and clay content, remained less than that of the Arklone. The centrifuging was therefore repeated with a layer of clean silica sand placed on top of both samples. This proved successful with the Denchworth soil but only partially so with the Thames soil. A piece of nylon gauze placed on the surface of the soil with a layer of glass beads placed on top of the gauze, combined with the use of a swing-out rotor, has subsequently proved effective for some awkward soils (D. Campbell, personal communication). With highly organic soils such as peats, it would be better to use the centrifuge drainage method. The concentrations of 17 solutes are given in Table VI. The concentrations of those elements that were sought but that are not listed in Table VI were below or close to their detection limits. The median soil solution concentrations, expressed in milligrams per liter and as -log (molar concentration), are shown at the bottom of Table VI. Median concentrations of the major solutes ranged from to 366 Environ. Sci. Technol., Vol. 17, No. 6, 1983
'
15'
01 200
1
1,
b
49.1 b
30.9 45.1 Sam-
I
Rowsham
'
38.3 25.3 19.0 21.8 42.5
10mm cell pH 8 0
250
300
350
LO0
L50
500
550
600
Wavelength ( n m )
Flgure 2. UV-visible absorption spectrum of Rowsham soil solution
about lo4 M while the minor elements were mostly below M. The only major solute not shown in Table VI is HCO,. The measurement error, a,, expressed as a relative standard deviation (RSD = 100 a,/mean concentration), was less than 10% in most cases but was higher for some of the trace elements, notably Cu, Zn, and B. These high RSDs do not appear to be due to analytical errors since the largest differences between duplicates were found for those soils with relatively high soil solution concentrations. The variation in soil solution concentrations between the different soils was surprisingly large, often varying by an order of magnitude or more. The acidic Southampton soil was notable for its high concentrations of Zn, Fe, and A1 and low concentrations of N03-N and Ca. The high Ksupplying power and high alkali-soluble Si content of soils from the Harwell soil series have been discussed previously (16).
A characteristic feature of the soil solutions was that they were all brown and contained high concentrations of dissolved organic carbon (DOC). In fact, organic carbon was the most abundant solute on a molar basis in many of the soil solutions (Table VI). The DOC concentrations were in the range (2-7) X M and were more than an order of magnitude greater than those found in the groundwaters of the area. This highlights the importance of the cycling of organic C in soil and confirms a previous finding that the chemistry of stream waters can be quite different from the chemistry of soil solutions from their catchments (17). Figure 2 shows the UV-visible spectrum of the Rowsham soil solution. The continuous increase in absorbance with decreasing wavelength and the absence of any marked peaks or inflections are characteristic of the spectra of
InwN0.10-Jwrlt--t-0.1
In
Table VII. Absorbance of Ten Soil Solution Extracts at 600,400, 300, and 250 nm Compared with Absorbance at 350 nm absorbance ratio
+-I
0.10r1r100-JddaNd 0
w o ?????o??????? ooooooooooo~om $
e4wOInw0-JdOdm0.1
4r 0
0000000000
A,,,
0d0a
Harwell Icknield Grove Rowsham Fyfield Thames Marcham Denchworth Southampton Berkhamsted mean a
+~
, m r l 0 0 0 0 d m ( D m
m o o
0
0000000000
0u50
b
mOrlr1r-mwr-wOO
d
v v
v
In
A 400 i A 350
A,,,/
A350
A350
A 310
0,009 0.024 0.023 0.020 0.018 0.022 0.041 0.018 0.030 0.019 0.022
0.364 0.379 0.393 0.414 0.398 0.288 0.436 0.369 0.375 0.410 0.39 3
2.29 2.28 2.20 2.20 2.26 2.30 2.03 2.33 2.24 2.13 2.23
4.43 4.76 4.20 3.96 4.77 4.93 3.59 4.83 4.09 4.20 4.38
A 6WI
r l o N d o N r 1 N o o r l 0 - J o w
1 f
a
0.403 0.238 0.172 0.254 0.139 0.126 0.146 0.401 0.407 0.422 0.271
A2Wl
10-mm cell, ambient pH.
natural humic materials (18). The color of the soil solutions was conveniently characterized by the absorbance at 350 nm (A350).A 350-nm wavelength was chosen as the reference because it usually gave an absorbance reading in the range 0.1-0.5 in a 10-mm cell. However, additional measurements at other wavelengths showed that the shapes of the UV-visible spectra of the various soil solutions were quite similar (Table VII). There was a poor correlation between A350 and DOC as has also been found recently for New Hampshire river and lake waters (19). One possibility that must be considered is that Arklone might extract significant quantities of organic matter and associated metals from soils and soil waters. Batley and Giles (7) found that FC78 extracted 3% or less of the DOC from natural waters and that the use of FC78 resulted in no significant change in the UV spectra of a number of natural waters. The metal content of the FC78 after use as displacent was also small. Although Arklone would not be expected to behave in exactly the same way as FC78, it is unlikely to behave very differently. We have found that the A350of Arklone remains insignificant after being used as a displacent and that shaking a naturally organic-rich lake water with Arklone does not result in a decrease in A350 of the water. Therefore, Arklone is unlikely to extract significant quantities of organic matter from soils or soil solutions.
Conclusions The immiscible liquid displacement method is a simple method of extracting samples of interstitial water directly from soils and porous rocks. The method requires no special equipment other than a high-speed centrifuge, and since the sample is in contact with only plastic containers and an organic liquid, trace element contamination is potentially low. The multielement capability and high sensitivity of ICP emission spectrometry make it an ideal method for the analysis of interstitial waters, but a preconcentration step will probably be necessary if data for a wide range of trace elements are required. Studying field-moist soils requires considerably more care in storage and handling than studying dried soils, but these disadvantagesmay be more than offset by the benefit gained. I t has often been said that the soil is a dynamic system: it is the medium of considerable biological activity and provides the mechanical and chemical environment for plant growth. The chemical composition of the soil solution both reflects this activity and to some extent controls it, and it therefore reflects the balance of a large number of physical, chemical, and biological processes. Environ. Sci. Technol., Vol. 17, No. 6, 1983
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These many processes and their complex interactions are likely to remain difficult to model reliably, but the immiscible liquid displacement method provides a method for determining the soil solution composition directly. A systematic study of soil solution chemistry should provide clues to the dominant processes controlling soil solution concentrations, which in turn may enable simplifications of more complex models to be made. Alternatively, but of no lesser significance, the temporal and spatial variation in soil solution concentrations may prove to be so complex that attempts to model soil solution chemistry using simple models may be shown to have little chance of success. We have shown here that there are large differences in the chemical composition of soil solutions from different soils, and it remains to be seen if the dominant processes controlling these concentrations can be identified. It could well be the variation of solute concentrations with time, depth, or site that proves most revealing and the immiscible liquid displacement method should be useful for monitoring such variations.
Acknowledgments We thank R. Andrews for assistance with the centrifuge extractions and chemical analyses, C. J. Smith for the DOC analyses, and J. A. Barker for discussions on the physical basis of the immiscible liquid displacement method and for deriving eq 7. Registry No. Ca, 7440-70-2;Mg, 7439-95-4;Sr, 7440-24-6; Na, 7440-23-5; Si, 7440-21-3;S, 7704-34-9; C1,7782-50-5;K, 7440-09-7; Mn, 7439-96-5; Fe, 7439-89-6; Cu, 7440-50-8; Zn, 7440-66-6; Ba, 7440-39-3;B, 7440-42-8;NO,, 14797-55-8;DOC, 7440-44-0;water, 7732-18-5; Freon TF, 76-13-1.
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(2) Manheim, F. T. In “Chemical Oceanography”, 2nd ed.; Riley, J. P., Chester, R., Eds.; Academic Press: London, 1976; Chapter 32, pp 115-186. (3) Wellings, S. R.; Bell, J. P. J. Hydrol. 1980, 48, 119-136. (4) Bath, A. H.; Edmunds, W. M. Geochim. Cosmochim. Acta 1981,9, 1449-1461. (5) Edmunds, W. M.; Bath, A. H. Enuiron. Sci. Technol. 1976, 10,467-472. (6) Mubarak, A.; Olsen, R. A. Soil Sci. SOC.Am. J. 1976, 40, 329-331. (7) Batley, G. E.; Giles, M. S. Water Res. 1979, 13, 879-886. (8) Whelan, B. R.; Barrow, N. J. J. Environ. Qual. 1980, 9, 315-3 19. (9) Kittrick, J. A. Soil Sci. SOC.Am. J. 1980, 44, 139-142. (10) Hassler, G. L.; Brunner, E. Trans. Am. Znst. Min. Metall. Eng. 1944,155, 114-123. (11) Thompson, M.; Howarth, R. J. Analyst 1976,101,690-698. (12) Nielsen, D. R., et al. In “SoilWater”; Nielsen, D. R., Jackson, R. D., Cary, J. W., Evans, D. D., Eds.; American Society of Agronomy: Madison, WI, 1972; Chapter 6, pp 121-154. (13) Bolt, G. H.; de Haan, F. A. M. In “Soil Chemistry. B. Physico-Chemical Models”, Bolt, G. H., Ed.; Elsevier: Amsterdam, 1979; Chapter 7, pp 233-257. (14) Jarvis, R. A. “Soils of the Reading District [Sheet 2681”; Memoirs of the Soil Survey of Great Britain: England and Wales, Agric. Res. Council: Harpenden, 1968. (15) Jarvis, M. G. “Soils of the Wantage and Abingdon District [Sheet 253]”, Memoirs of the Soil Survey of Great Britain: England and Wales, Agric. Res. Council: Harpenden, 1973. (16) Talibudeen, 0.;Weir, A. H. J. Soil Sci. 1972,23,456-474. (17) Hinkley, T. Nature (London) 1979, 277, 444-446. (18) Swift, R. S.; Thornton, B. K.; Posner, A. M. Soil Sci. 1970, 110, 93-99. (19) Truitt, R. E.; Weber, J. H. Environ. Sci. Technol. 1981,15, 1204-1208.
Received for review July 30,1982. Accepted January 10, 1983. This research was supported by the Natural Environment Research Council (NERC)Geochemical Cycling Programme and is published with the permission of the Director of the Institute of Geological Sciences (NERC).
