Anomalous transmission of water through certain peats

Journal of Hydrology, 22 (1974) 213--218 O North-Holland Publishing Company, Amsterdam -- Printed in The Netherlands

ANOMALOUS TRANSMISSION OF WATER THROUGH CERTAIN PEATS

H.A.P. INGRAM, D.W. RYCROFT and D.J.A. WILLIAMS

Department of Biological Sciences, The University, Dundee (Great Britain) Field Drainage Engineering Unit, A.D.A.S., Anstey Hall, Trumpington, Cambridge (Great Brita in) Department of Chemical Engineering, University College, Singleton Park, Swansea (Great Britain) (Accepted for publication December 20, 1973)

ABSTRACT Ingram, H.A.P., Rycroft, D.W. and Williams, D.J.A., 1974. Anomalous transmission of water through certain peats. J. Hydrol., 22: 213--218. Field experiments ave described in which seepage tubes were used under variable-head and constant-head conditions. The estimates of "hydraulic conductivity" obtained from humified peat showed time dependence, but they also increased with head, indicating departure from Darcy's law. It is suggested that this law is only obeyed in unhumified peat. INTRODUCTION The use of seepage tubes ( " p i e z o m e t e r s " ) has b e c o m e well established as an e x p e r i m e n t a l t e c h n i q u e for estimating the h y d r a u l i c c o n d u c t i v i t y o f undisturbed soils (Boersma, 1965). This is a brief, p r e l i m i n a r y r e p o r t o f an extensive series o f e x p e r i m e n t s in w h i c h the t e c h n i q u e was used in peat. The w o r k was carried o u t during 1 9 6 7 - - 7 0 at D u n Moss, a raised bog in the G r a m p i a n foothills o f eastern Perthshire, S c o t l a n d (grid. ref. NO. 167 558). METHODS AND RESULTS

Variable head e x p e r i m e n t s Our e x p e r i m e n t s were p e r f o r m e d in the field using various versions o f the m e t h o d of L u t h i n a n d K i r k h a m (1949). Our seepage t u b e s were cylindrical, unplasticised p.v.c, pipes o f k n o w n internal d i a m e t e r {about 11 cm), driven into the in-situ peat to d e p t h s o f 5 0 - - 1 5 0 c m to f o r m tightly fitting linings to o p e n wells. At the f o o t of each t u b e an unlined, cylindrical cavity was f o r m e d using specially c o n s t r u c t e d augers. In o u r earlier e x p e r i m e n t s an artificial h y d r a u l i c p o t e n t i a l gradient was i m p o s e d on the s y s t e m either by rem o v i n g w a t e r f r o m the t u b e ( d e p l e t i o n m o d e ) or by adding w a t e r (recharge

214 mode). The time course of return to equilibrium was followed by observing the rise or fall of water level in the tube. Darcy's law implies that the flux of water through a saturated soil is proportional to the hydraulic potential gradient. On this basis, Kirkham (1945) derived an analytical solution to the Laplace equation which gave the relationship between the m o v e m e n t of water level in the tube and the hydraulic conductivity, K, of the soil surrounding the cavity: In hi

y2

hj

K .......... A t.i ti

(1)

Here r is the radius of the cavity (equal to the internal radius of the tube) and A is a function with dimensions of length whose value depends (Youngs, 1968) on the shape and position of the cavity, while hi, j are water levels in the tube, measured as displacements from equilibrium level at times tij respectively. C o n f o r m i t y with eq. 1 is indicated in Fig.l, which shows a linear relationship between log h and t, the slope giving a value of K close to 3.1 • 10- ~ cm sec -~ . The data of Fig.1 relate to the infilling of an old drain with fresh Sphagnum magellanicum and S. cuspidatum peat of low humification H1--H2 (Von. Post and Granlund, 1926). In general, values between 10-3 and 3 • 10 -* cm sec-' were f o u nd in overgrown ditches and marginal water tracks (Ingram, 1967) with Carex rostrata or Sphagnum recurvum (Rycroft, 1971).

~15~ ~lOfi u 5¢

g g 0

;

1;

1;

Time (minutes)

2'o

Fig.1. L o g a r i t h m s o f water level d i s p l a c e m e n t p l o t t e d against time for a seepage t u b e with its cavity in the fresh to slightly h u m i f i e d infill o f an old field drain at the mire margin.

Most of our experiments were carried out in m o r e highly humified peat (H3--H6) from the mire expanse (Ingram, 1967). This peat was mainly derived from Sphagnum (S. irnbricatum and sect. Acutifolia) with which were associated varying a m ount s of Eriophorum vaginatum and dwarf shrubs, especially Calluna uulgaris. The results of some 80 such experiments are typified by Fig.2, which shows a curvilinear relationship between log h and t in both recharge and depletion modes. On attempting to use eq.1 to c o m p u t e

215

100

50 u x~ > 10

gl

. . . .

2 4 5 Time {minutes) xlO -2

8

Fig.2. T i m e c o u r s e o f t h e l o g a r i t h m of w a t e r level d i s p l a c e m e n t for a seepage t u b e with its cavity in t h e m o d e r a t e l y h u m i f i e d peat of t h e mire e x p a n s e , used in b o t h d e p l e t i o n a n d recharge modes.

an estimate o f K between each successive pair of readings, the results showed a rapid initial fall, followed by a more gradual decline (Fig.3). Treated in this way, the majority of our results f r om more humified peats gave values of hydraulic conductivity which declined in a similar manner, usually by an order of magnitude and typically from about 2 • 10 -4 to 2 • 10 -5 cm sec -~ Fig.3 illustrates a further finding, general in peat of this kind, namely the t e n d e n c y for results from a corresponding pair of recharge and depletion experiments to converge.

12-

recherge

x

4-" u~ E

B-

0 0

2

4 6 Time (minutes) xlO -2

Fig.3. Time course of a p p a r e n t values of h y d r a u l i c c o n d u c t i v i t y (K I), c o r r e c t e d for viscosity effects t o a t e m p e r a t u r e of 20.2°C a n d based o n t h e data of Fig.2.

Constant head experiments The imposed hydraulic potential gradient diminished during each of the above experiments. A c o n c o m i t a n t decline in hydraulic conductivity could therefore either have been a function of potential gradient or else the result

216

of o t h er time-dependent processes. To separate these two possibilities we devised a further series of experiments in which the potential gradient was held virtually constant for the duration of each test. This was achieved by maintaining a nearly constant water level in the seepage tube by means of a graduated Mariotte bottle device, used to deliver water in the recharge m o d e or, with the aid of a hand-operated vacuum pump, to aspirate water o u t of the seepage tube in the depletion mode. If q is the rate of inflow to or out fl ow from the Mariotte bottle which is necessary to maintain at the seepage tube a constant level displacement h, then according to Kirkham (1945): K = q/A.

(2)

1/h

where A is the shape f unct i on of eq. 1. Typical results from peat in the mire expanse are plotted against time in Fig.4, which relates to two experiments carried out on the same day. The first ex p er imen t at h --- 56.3 cm showed a decline in K which was n o t repeated in the immediately following test at h = 50.3 cm. This and similar results suggest that, when an artificial gradient of hydraulic potential is first imposed within the peat, some time elapses before the conductivity becomes constant. 10-

w

~5

o

h = 50.3cm 0

o

5'o

I~o

40

Time (minutes)

Fig.4. Plots of a p p a r e n t h y d r a u l i c c o n d u c t i v i t y (Kt) against t i m e for t w o tests w i t h t h e same seepage t u b e a n d cavity in t h e m i r e e x p a n s e . B o t h were re".harge e x p e r i m e n t s at cons t a n t w a t e r level d i s p l a c e m e n t (h), carried o u t o n t h e same day w i t h the larger displacem e n t i m p o s e d first.

We suppose this time to be occupied in a gradual adjustment of the system to the newly-induced flow. Similar tests were carried out unde r a series of different but constant potential gradients. After preliminary periods of adjustment, which lasted between 10 min and 3 h depending on the previous history of the seepage tube installation, we obtained a series of values of conductivity which increased with potential gradient in the manner shown in Fig°& These results were obtained during a 7-day period at the same position and depth and with the same

217

20-

~o 15x

T'-u~

x-10-

5

~0

20

6b

Level displacement (cm)

Fig.5. Variation of the apparent hydraulic conductivity value (K t), obtained after allowing for time-dependent effects, with imposed constant level displacement. Experimental details as in Fig.4.

cavity. They show that the apparent hydraulic conductivity varies with the potential gradient under which it is measured. Such a variation is, of course, a contradiction in terms for, according to Darcy's law, the flow rate should be proportional to the gradient of hydraulic potential, the hydraulic conductivity being defined as the constant of proportionality. CONCLUSIONS

It therefore appears that humified Sphagnum peat does not transmit water in accordance with Darcy's law and that, as with certain clays (Swartzendruber, 1966), the concept of hydraulic conductivity does not apply. Eggelsmann (1964) found Darcy's law to be obeyed in such circumstances. On the other hand, both Yamamoto (1970) in Japan and Dai and Sparling (1973) in Canada have used seepage tubes and obtained results similar to ours. Galvin and Hanrahan (1967) performed laboratory permeameter tests which showed a non-linear increase in conductivity with pore water pressure, a result which they ascribed to air entrapment. This they also offered as an explanation of conductivity anomalies in field tests with pumped wells. In conclusion, we suggest that it may become necessary to divide peats into two categories with respect to the transmission of water and that, on present evidence, Darcy's law can only be assumed to apply in peat of low humification. There is clearly a need for further experimental work on this aspect of peat hydrology, beginning with laboratory experiments using rectilinear flow geometry.

218 ACKNOWLEDGEMENTS T h i s w o r k was f i n a n c e d w i t h t h e aid o f a g r a n t f r o m t h e N a t u r a l E n v i r o n ment Research Council and carried out with technical assistance from the D e p a r t m e n t s o f B i o l o g i c a l S c i e n c e s a n d Civil E n g i n e e r i n g o f t h e U n i v e r s i t y o f D u n d e e , f o r w h i c h we are g r a t e f u l . REFERENCES Boersma, L., 1965. Field measurement of hydraulic conductivity below a water table. In: C.A. Black (Editor-in-qhief), Methods of Soil Analysis. Agronomy, 9: 222--233. Dai, T.S. and Sparling, J.H., 1973. Measurement of hydraulic conductivity of peats. Can. J. Soil Sci., 53: 21--26. Eggelsmann, R., 1964. Verlauf der GrundwasserstrOmung in entwasserten Mooren. Mitt. Deut. Bodenkundl. Ges., 2: 129--139. Galvin, L.F. and Hanrahan, E.T., 1967. Steady state drainage flow in peat. Nat. Res. Counc. -- Highway Res. B o a r d - - Res. Rec., 203: 77--90. Ingrain, H.A.P., 1967. Problems of hydrology and plant distribution in mires. J. Ecol., 55: 711--724. Kirkham, D., 1945. Proposed method for field measurement of permeability of soil below the water table. Proc. Soil Sci. Soe. Am., 10: 58--68. Luthin, J.N. and Kirkham, D., 1949. A piezometer method for measuring permeability of soil in situ below a water table. Soil Sei., 68: 349--358. Ryeroft, D.W., 1971. On the Hydrology of Peat. Thesis, Dundee University, Dundee, 268 pp. Swartzendruber, D., 1966. Soil-water behaviour as described by transport coefficients and functions. Advan. Agron., 18: 327--370. Von Post, L. and Granlund, E., 1926. Sodra Sveriges torvtillgangar I. Sver. Geol. Unders., C335. Yamamoto, S., 1970. Soil permeability with particular reference to peat soils. Mem. Fac. Agric. Hokkaido Univ., 7: 307--411. Youngs, E.G., 1968. Shape factors for Kirkham's piezometer method for determining hydraulic conductivity of soil in situ for soils overlying an impermeable floor or infinite ly permeable stratum. Soil Sci., 106: 235--237.