Lysimeter installations in sandstone at Styrrup, Nottinghamshire

in The Netherlands

UYSIMETER I N S T A L L A T I O N S IN S A N D S T O N E

AT S T Y R R U P ~

NOTTINC-iHAMSHIRE

eal Sciences, London (Great Britain)

ABSTRACT Kitching, R. and Bridge, L.R., 1974. Lysimeter installations in sandstone at Styrrup, Nottinghamshire. J. Hydrol., 23: 219--232. A description is given of the construction and operation of two relatively large lysimeters instaBed to measure the infiltration from rainfall on undisturbed blocks (100 m 2 ) of Bunter Sandstone in north Nottinghams~ ire, England.

I N T R O D U C T I O N A N D AiMS O F T H E W O R K

Groundwater in most water-bearing formations has been derived from precipitation which has passed through the base of the soil zone, through the vadose or unsaturated zone and entered the saturated zone below. Some assessments of regional groundwater resources have been based upon estimates of infiltration derived from measurements or estimaces of precipitation less evaporation, but the values allocated to both precipitation and evaporation~ and consequently their difference, may be subject to ,error. The programme of infiltration r e ' a r c h , aimed at measuring the races of movement of water i n t h e vadose zone of various British aquifers, of which the present study forms a part,, is concerned with the application of different techniques at a series of re~earch sites. An earlier paper (Kitching, 1974) describes work with radioactive tracers at a site on the I o w e r Greensand outcrop in Hampshire where considerable dispersion of the tracers was expe~:ienced. The present study aimed at enclo~ing an undisturbed block of Bunter Sandstone within impermeable boundaries and maintaining the conditions within the block as close as possible to the natural conditions pertaining outside. Lysimecers used to measure infiltration at other sites in the United Kingdom have tended to be of small dimensions and have gener~ly been filled with artificially repacked local soils and strata {Anonymous, 1969). Moist ,~.~e and. wacer table conditions inside and ou.~" le the lysimeters have

not been comparable~nd thus have given riseto uncer%ainty in the relationship

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between '~he infiltration in the lysi~eter and ~he n ~ l conditio~, The work described in this repor~ dens with lysimeter~ 100 m 2 The lysimeters consist of undisturbed blocks of Bunter Sandstone surrounded by impermeable membranes installed in ~enches to mL~aimize

221

are grouted Bunter Sandstone. The groundwater level inside ~he tysimeC~s is maintained at the same

lp into a measuring sys~m.'Mois~ure contents insi=de and ouiside the lysimeter are als~o measured using neutron probe techniques. SITE DESCRIPTION

The site (Fig.l) is located near Styrrup, Nott'nghamshi~e (NGR SK 607898) on the East B',etford (101) Sheet of the Geologic~d Survey New Series One,inch map. I~ is close to the western boundary of the Lower Mottled Sandstone (Permo-Triassic age). The base of the Lower Mottled Sandstone is not clearly defined ir~tthis area and is characterised by a gradual transition from the underlying Upper Permian Marl. A lithological description of the northeast corner of Lysimeter 2 is given in Table I: and illustrated in Fig.2, whilst the intergranulax permeabilities of specimens taken from that section are given in Table IL The values were determined lay gas permeameter for vertically orientated plugs. The site of I acre is located at about 16 m O.D., slopes gently to the south and is enclosed by a 2 m high chain-link and barbed-wire fence. So far as hydror~eteorological factors are concerned, the site e x p e s ~ e is considered reasonably typical of that for much of the outcrop of the Lower Mottled Sandstone. A hedge, which is approximately 2 m high before trimming, runs parallel to and some 4 m from tl~e western fence, whilst the southern side is bordered by a ple~tation containing Crees approximately 12 m high. The water table is about 2--3 m below ground surface and the area drains into the river R yCon. Lc,cal meteorological conditions are recorded at quart~r hour intervals by ~m automatic weather station on the site. The mean annual rainfall for ~he area is 600 ram. CONSTRUCTION OF THE LYSIYIETERS

Common feat~rer Two square lysimeters have b ~ n cons~ucted a t t h e site about 40 m apm~, the main difference b e t w e e n them being in the construction of the membrane. A ~ e n c h 10 m in ]e::~gth was excavated b y a mechanical digger along each side of the lysimeCers. They were approximately 0.80 m wide by 5.40 m deep

222 TABLE I Description of lithology at the northeast corner of Lysimeter 2, northern side of trench Bed No.

Description

Thickness (m)

Depth (m)

18 17 16

Topsoil with rounded pebbles 0.04 m diameter Very weathered orange fine sand Orange brown cross stratified fine sand; b o t t o m 0.13 m brownish with green spots and highly micaceous; upper 0.61 m very heavily weathered and friable Red mudstone (irregular layer 0--0.15 m); mudflakes in fine sand Brownish pink fine sand with green spots and occasional mudflakes; black stained bedding planes and micaceous Pink fine to very fine micaceeus mudstone cross stratified in units of 0.03 m thick; bedding planes dip gently W Some sandstone as underlying but above seepage at 2.84 m Brown fine to very fine sandstone; cross stratified bedding planes every 0.04 m in these slope very gently W or SW; top of this sandstone is marked by seepage at 2.84 m which runs down the face of it Mudflake conglomerate consisting of galls of red mudstone up to 0.08 m diameter in greenish brown fine sand -- no seepage at this level at this position Green sandstone showing graded bedding; lowest 0.15 m is a medium yellow/green sand; green mudflakes in bottom 0.03 m in places; becomes browner between 3.58 and 3.63 m and 3.40 to 3.46 m; between 3.46 and 3.58 m mainly pale green fine sandstone; massively bedded on a cut face but when broken by hammer divides into 0.03 m units apparently saturated -- on the whole moderately hard Green mudstone -- plastic clay; base rather irregular undulating but sharp colour junction with underlying red mudstone (base of this zone rises to W); continuous sharp junction at top (3.90 m) Red well bedded mudstone strongly micaceous units up to 0.08 m usually 0.04 m; some thin vertical joints; 2 main sets of joints WNW/ENE; other minor sets E/W and N/S; all appear closed Thin bedded mainly green mottled mudstone Mainly green mottled mudstone; well bedded in 0.03 m units Red mudstone slightly thicker bedding Red thin bedded mudstone Mainly green thin bedded mottled sandstone Red and green mottled mudstone thin bedded

0.46 0.30

0.46 0.76

1.07

1.83

0.05

1.88

0.20

2.08

0.61

2.69

0.15

2.84

0.48

3.32

0.06

3.3{}

0.52

3.90

0.06

3.96

0.34 0.20

4.30 4.50

0.08 0.18 0.18 0.13 0.30

4.58 4.76 4.94 5.07 5.37

15 14

13

12 11

10

4 3 2 1

223

Fig.2. Photograph o t section described in Table I.

and their inner walls defined the central block of the lysimeter which had a t o p surface area of a b o u t 100 m 2 . The width of the trench permitted examination of the lithology. All observation bores were drilled with a small power auger. In the early

stages 110 m m diameter bores were drilled and lined to the fulldepth with perforated P V C lining tubes but experience gained from the excavations showed that it was only necessary to line the top 1.50 m of each bore. A second seriesof boreholes, 190 m m in diameter, were drilled and lined only to that depth with unpefforated P V C tubes. With the exception of the central

224 TABLE II Permeability values for specimens taken from section described in Table I* Laboratory No.

789

--31V --30V --26V --25V --24V2 --24VI --23V2 --23Vl --22V --21V --20V --19V --18V2 --18Vl --17V2 --17V1 --16V2 --16VI --15V2 --15Vl --14V2 --14V1 --12V

--llV --10V2 --10Vl -- 9V2 -- 9 V l -- 8V 7V -- 6V -- 5V2 -

-

Bed No.

Depth (m)

Intergranular permeability millidarcies

14 14 13 13 12 12 11 11 11 11 11 9 9 9 9 9 9 9 9 9 7 7 7 7 5 5 4 4 3 1 1 1

1.98 2.06 2.49 2.59

310.1 268.9 40.2 13.25 42.5 19.43 184.3 116.1 90.2 42.8 47.8 103.1 110.3 74.46 205.8 137.4 23t~.1 186.6 131.3 130.5 0.004 0.066

-- 5V1

1

-- 4V

I

} 2.75 } 2.90 3.05 3.20 3.28 3.51 } 3.58 } 3.66 } 3.91 } 3.96 } 4.06 4:09 4.24 } 4.57 } 4.67 4.78 5.13 5.34

0.201

0.0067 0.006 0.182 0.135 0.175 0.007 0.164 0.007 0.013

} 5.44

0.195

5.49

0.023

*Permeability determined from vertically orientated specimens.

pumped bore in each lysimeter and some observation bores in the vicinity of Lysimeter 1. all bores are of this latter type. The water ~evels observed in the bores, together with the neutron probe measurements, are all referred to a common datum, taken for convenience as the top of neutron access tube NI 8 (Fig.l).

225

Lysimeter I The membrane of Lysimeter I consists of three contin'aous sheets of 1,000 gauge polythene weighted at their bottom margins and suspended from t h e surface along the inner wall of the trench. They were positioned after the trench had been pumped dry and their lower edges were grouted into the Lower Permian Marl with bentonite. The whole trench was then backfilled to the surface using the spoil which had been removed frcm it originally. Subsequently it was noted that whereas water levels inside the lysimeter were of the same order of elevation and followed each other closely, those immediately outside differed from each other by up to 0.60 m, there being a ~ a d i e n t from north to south. This was undesirable because of the possibility of uncontrolled leakage into and out of the lysimeter as a result of differential heads across the membrane. It was also difficult to select the appropriate water level outside the membrane as a control for that inside. To overcome this difference in levels, a second trench some 0.33 m wide was excavated around the eastern, northern and western sides to a depth of 2.40 m and backfilled to within 0.90 m of the surface with gravel of approximately 10 mm grain size. The remainder was filled with soil. Perforated lining tubes, to act as observation points, were placed vertically at the centre of each section of the trench prior to backfilling (.859, B60 and B61 in Fig.3A) and water-level recorders were installed in them. The gravel pack did not greatly improve the situation, particularly when the water levels were 1.60 m or more below d a t u m - - the level in B60 remained 0.10 m or more, higher than the level in B59 or B61. The three observation tubes in the trench were connected by siphons which were capable of equalising the levels, if they were re-primed daily. Deepening of the boreholes to a thick sandstone band (Bed 9, Table I) at approximately 3.70 m was also tried but did not improve communication appreciably. Because the siphons appeared capable of reducing the difference in levels, boreholes of 190 mm and 3.70 rn depth were located about 1.0 m outside the existing trenches (Fig.3) at each corner and at the centre of the western and southern sides. These bores, together with B59 and B60, were then interconnected by siphons (10 mm diameter tubing). A small submersible pump was fitted to one end of each siphon and the siphons were automatically re-primed simultaneously every half hour by pumping for about 30 sec. This arrangement reduced the difference in level between north and south to 0.15 m and avoided interference to the traces on the recorders. I t also ensured t h a t a failure by one pump in the series would leave the affected bore connected hydraulice.lly to the rest of the system by a siphon which was still being primed. The difference in water levels has since been reduced to about 0.06 m by comlecting B60 directly to B77 by a separate pump and siphon. Pumping between these two bores is controlled using the water-level recorders in them so that the pump operates when the level in B60 is higher than that in B77. The water levels remain connected by the siphon after the pump has been switched off by the balancing of the recorders.

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Lysimeter 2 The excavations for the second lysimeter were as before, although all the marl derived from below the thick sandstone band at approximately . 4 ~ 4 . 0 m (part of Bed 9) was removed from the site. The trench was dered and k e p t : d ~ for a period of:one week using a sludge pump. The entraI block was thus also de-watered down to the thick sandstone b a n d as shown by the observed fall in the water level in a bore B66 drilled at the centre of the block and used subsequently for the control pump. The vertical faces of the block were then sprayed with three coats of a resin known by the trade name of "Geophil". This substance was claimed to be capable of forming an impermeable skin under the conditions which existed in the trench after de-watering. Pumping was continued for three days after the final coat had been applied to ensure that the Geophil had cured. At the end of this time, examination showed that although the Geophil had adhered to the faces at and above Bed 9 it did not appear to have done so to the finer and wetter material below it. Pumping was discontinued and the water level was seen to rise in the central bore. After allowing water to stand in the trench for three weeks, it was pumped out in stages over a period of five days. This allowed the block to drain gradually, reducing the possibility of the excess head of water within the block forcing the Geophil away from the faces. Examination showed that rupturing of the Geophil had occurred below Bed 10. There aiso appeared to be some evidence of weathering near to ground level. The trench was therefore filled with a bentonite cement m ~ (5% bentonite/25% ordinary Po~land cement byweight) to the top of the thick sandstone in Bed 10, approximately 3.40 m below the surface, and allowed to refill with water. The water level again rose in the central bore indicating that some leakage was still occurring. The trench was pumped down to the bentonite mix and the Geophil examined. Damage was visible at some points particularly along the southern face and it was decided to fill the trench with more bentonite/cement to within 1.50 ~: ot the surface. During the final stages of emplacing this slurry, three continuous sheets of polythene were suspended round the inner wall so that the~ b o t t o m edges were grouted in by the bentonite. After setting, the top surface o[ the bentonite was between 1.25 and 1.64 m below surface and therefore normally above the highest point likely to be reached by any rise in the water table. The bentonite was covered with polythene sheeting and the trench filled in with the material which had been retained when the trench was excavated. In view of the d~ficulties experienced with d~fering water levels in the bores outside Lysimeter 1, bores 190 mm in diameter, were drilled to a depth pf 3o70 m close to the cornets of the trench and about 1.0 m from it. They were interconnected by a system of PVC pipes 30 mm in diameter at about 3.0 rn below surface prior to the second stage of bentonite emplacement. This arrangement was later supplemented o~th a pump/siphon system° Bores

228

(190 mm) were drilled to 3.7 m at the centre of each side and about 1.0 m outside the line of the trench (Fig.4). Pumps were installed in each of these as well as in B 73, and

ystem which ad vat trench was ~lowed:to for subsidence before clover mixture. The gl to keep its height to C edges of the polythene membranes emerge at the surface, they have been protected with strips of aluminium edging m a r t i a l , These rise 0.02--0,03 :m above ground and serve to prevent surface wa~er from running o n ~ or off the lysimeters. The siphon hoses, as well as the hoses from the con~ol pumps in the central bores of the lysimeters which run to ~he measuring tank in ~he control hut, have all been laid through conduits buried at least 0.40 m below ground to avoid blockages caused by freezing. The electric cables for the control equipment are also laid in these conduits.

Neutron probe access tubes A total of fourteen neutron probe access tubes have been inserted to depths ranging from 2.20 to 4.10 m The tubes are stainless s~eel and were inserted using a tripod rig which ensured that the rubes were vertical and that there was minimum disturbance around them. This was achieved by augering through and about 0.10 m in advance of the tube and then driving the tube down. The k~wer open ends of the tubes were sealed using neoprene bungs together with a quantity of epoxy resin. OPERATION OF THE LYSIMETEP~S

In order to simulate natural conditions as closely as possible, the wa~er table within the lysimeter was controlled a t the same level as in the surrounds of the lysimeter. Controlling the level in this way further ensured that any leakage from the lysimeter was minimized, Control of the level was achieved by using modified float~operated Munro water-level recorders in boreholes inside and outside the lysimeter, These recorders were modified by the inclusion of 10 turn electrical potentio.. meters coupled to the pulley wheels. The potentiometers were w~ed in parallel across a tow voltage d.c. source so that the electrical output from the tapping e~ each potentioraeter was directly related to the water leveh The elect,rical outputs from inside and outside the lysimeters were amptified b y d.c, differential amplifiers located in the control hut nearby, The oUtput from these ampligiers wa~ arranged to operate a submersible pump in a borehole at the centre of the lysimeter when the water level in t h e lysimeter rose above that outside. An e~.ectrical sensing and control system was included to prevent

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the pumped water level from falling to that o f the pump and a time delay prevented hunting. The water pumped from each lys~meter was supplied to measuring tanks in the hut fitted with water4evel recorders and the measuring tanks were allowed to fillto a given level (detec~d: ~y electrical sensors) and then:empied b y a solencid valve: To avoid 10ssof Water, the sUbmersible i n each lysimeter was prevent;ed from operating b y a r e l a y when the m e a s ~ n g tank was being emptied. Counters recorded the number of times o f operation of the submersible pumps and solenoid valves. When t h e water level m the lysimeter rose slightly above the level outside, t h e submersible pump operated until the water level in the lysimeter was reduced to the same level as outside. The sye,tem was capable of controlling the water level to better than 10 mm (Fig.5). A typical example of the controlled level is shown in Fig.6. The quantity of water pumped from the lysime~er less the reduction in water stored in it, gave the infiltration in any given time: The change in water stored in the lysimeter was assessed from the change in water level and the specific yield obtained from laboratory measurements and neutron probe logging. The presence of the impermeable membrane and the control of the water levels minimized the leakage from the lys~me~ers. During early 1972 pumping tests were cerried out on each lysirneter to determine the rate of leakage under known head conditions. Each lysimeter was pumped continuously until the leakage into the lysimeter due to the head difference across the memurane was balanced by the water pumped out. This condition was recognised by a stable water level in the lysimeter (Fig.7). The leakage ra~e~ found for the ~ys~meters were as follows: Lysirneter 1 0.00417 cm water/day/cm head Lysimeter 2 0.00554 cm water/day/cm head The two lysimeters have !::eenoperating continuously since spring I~72 and the results of this operation and future experiments will be published in a subsequent paper. ACKNOWLEDGEMENTS

The field work and installationof the control systems was carried out by Mr. T.R. Shearer, Mr. S.L. Shedlock and Mr. P°D. Hilly~d, The description of the lithology {Table ~) at the northeast corner of Lysimeter 2 was made by Dr. P.E.Ro Lovetock° Permeability values (Table II) for specimens taken from this section were determined by Mr. M.J. Bird. REFERENCES

Anony~nous, 1989. Manual of BrRis~ Water S~ppl~y Practice, VoL2o Heffer~ Ca~nbridge0 655 pp. ~itching~ R.~ 1974. InfiRration studies on the Lower Greensand outcrop a~ L~s~, Hampshire. Rept. Set. h~t. Geol. Sci., Lond., ]n press