Soil-water sampling using pan and deep pressure-vacuum lysimeters

Journal of Hydrology 11 (1970) 1-21; © North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

SOIL-WATER

SAMPLING

USING

DEEP P R E S S U R E - V A C U U M

PAN AND

LYSIMETERS

R I C H A R D R. P A R I Z E K

Associate Professor of Hydrogeology and Staff Geologist, Mineral Conservation Section, The Pennsylvania State University, University Park, Penna. U.S.A. and B U R K E E. LANE

Staff Hydrologist, Water and Air Resources Commission, Dover, Delaware, U.S.A. Abstract: A description of two soil-water sampling devices and success achieved in obtaining soil water on a routine basis at depths of I to 36 feet below land surface are described. In one facility, galvanized, 16 gage metal pans, 12 × 15 inches, with a copper spout, are driven into the wall of an open trench to intercept gravitational water at 1-foot intervals to a depth of 17 feet. Water samples are diverted to sample bottles attached to the inside walls of a protective housing by way of plastic tubing. Flooring is required to prevent quicking of stratified residual silt loam to sandy loam soils at the base of the trench. From 4 to 6 inches of water are irrigated adjacent to the trench on a weekly basis and a floor drain or sump pump is required to prevent flooding of the sampling trench. The deep trench lysimeter provides gravitational water samples on a routine basis when waters are artificially applied to the site but failed to yield samples during the growing season in the absence of irrigation under Pennsylvania's climate. A modified version of a commercially available lysimeter constitutes the second device which can extract soil water under tension from depths in excess of 50 feet. The commercial lysimeter contains a porous ceramic cup attached to a 2-foot section of plastic pipe with a one-hole rubber stopper. In the modified version a two-hole rubber stopper is used and two copper tubes are inserted through the stopper, one for evacuating the tube, the other to force out water. This improved soil-water sampler has been named a pressure-vacuum lysimeter or "suction lysimeter." One or more lysimeters may be installed in a 6-inch diameter drill hole. The ceramic tip is embedded in a pulverized silica deposit to insure hydraulic continuity with the soil-water reservoir. Native soil, bentonite plugs, or grout plugs are used to backfill holes to prevent channeling. A hand pump may be used to evacuate the tube and to blow out samples. When sufficient soil-moisture is available 500 to 970 mls of water may be obtained on a weekly basis. During prolonged dry periods, no samples can be collected when soil water has been depleted around the ceramic point. This may be four to eight weeks after pan lysimeters fail to yield water samples.

Introduction T h e s o i l - w a t e r r e s e r v o i r is a v i t a l s e g m e n t o f t h e h y d r o l o g i c cycle. G r o u n d -

2

RICHARD R. PARIZEK AND BURKE E. LANE

water recharge and discharge, changes in water quality, renovation of potential pollutants and contaminants, processes of evaporation and transpiration, all may involve the soil-water reservoir. In many hydrologic and land-use investigations it is desirable to determine what water-quality changes are taking place with respect to the residence time and distance of travel of chemical constituents within the soil-water reservoir. This is particularly true when determining the fate of pesticides, herbicides, fungicides, nitrates, detergents, radioactive isotopes and related chemical constituents contained within the soil-water reservoir. Soil water is largely under a state of tension and is held within soil by capillary forces. Water in excess of field capacity, or gravitational water, is free to drain under the influence of gravity through the soil. Gravitational water may be intercepted within the soil-water reservoir by drains, well points, by impermeable interceptors, or it may collect within perched ground-water pools above semi-confining layers of silt and clay. Under these circumstances water samples may be collected using rather simple sampling devices. More commonly, perched ground-water pools are ephemeral, do not necessarily appear at convenient or desired depths within the soil-water reservoir and at best are transient. Similarly, free flowing gravitational water is available only after field capacity water requirements are met. Hence, gravitational water samples normally are limited in volume and available only from time to time, which may not be frequent enough to allow detailed changes in soil-water quality to be defined. In more arid regions, gravitational water rarely may be available. All remaining moisture is held within the soil until removed by plants, lost by evaporation, or consumed in chemical processes. This water will not enter tile or bore-hole openings unless special conditions are imposed at the sampling point. This means that soil-water samples have to be collected using special sampling devices if they are to be obtained on a nearly routine basis. Not all sampling procedures will be reviewed here, rather only two methods are discussed that are being used on research projects in progress at The Pennsylvania State University. On one project it was necessary to determine the degree of renovation being achieved within the soil-water reservoir where sewage effluent was being irrigated on a variety of crops, at various rates and on various soils on a year-round basis1). The object was to determine the optimum rates and volumes of sewage effluent that might be applied to achieve a high degree of renovation of waste water at least cost and at the same time to achieve artificial recharge with water of favorable quality. Soil water was contained in stratified residual soils varying from less than 5 to more than 100 feet in thickness that were derived from dolomite and sandy

SOIL-WATER SAMPLING

3

dolomite bedrock. The regional water table was entirely within the underlying carbonate rocks at depths of 150 to more than 250 feet below ground surface. In a second project soil-water samples were required in a similar soil and bedrock setting from below sanitary landfills to determine the time-rate of change of leachate quality as a function of depth of leachate penetration through the soil, residence time within the soil, and age of landfill z). In both projects it was considered desirable to obtain soil-water samples at either 1 or 5-foot intervals, down to 20 to 35 feet below ground level for the effluent project and 36 feet below the floor of a landfill cell at the sanitary landfill project. Various sampling methods were considered for these projects. Two were adopted because of their supposed simplicity, efficiency and limited cost. The nature of each installation will be discussed along with advantages and disadvantages of each for the benefit of other investigators who face similar soil-water sampling problems. A number of requests for this type of information have been directed to the authors indicating that there is interest in this problem. Previous methods

An approach to study the movement of moisture under natural conditions was attempted as early as the end of the eighteenth century by the introduction of the drain-gauges, later known as lysimeters (ref.Z), p. 280). During the last century they have been used to study the losses of fertilizer ingredients and soil amendments under cropped and fallow conditions 4-7) and to study the leaching losses of N from a 40 cm deep forest profile which was treated with urea enriched with N is (ref.8), p. 280). A multitude of lysimeter studies reported in the literature, were cited and evaluated by Kohnke, Dreibelbis and Davidson a). Most lysimeters were constructed to serve as impermeable barriers to gravitational water which collects within the soil above the interceptor and is allowed to flow to a drain point. Only gravitational water can be collected by this means, hence the sampling frequency and amounts of water obtained are highly variable and depend upon day-to-day changes in climatic conditions. A trench lysimeter in which galvanized metal pans were installed at 1-foot intervals to a depth of 17 feet is described later in this report. Wallihan 9) offered a suggestion for overcoming the major objections to lysimeters. He proposed to modify the lysimeter by using a tension meter cup to suck water from the soil under a tension equivalent to that of a continuous column of soil and tested his suggestion in the laboratory. Colman 10) developed suction beneath a 6-foot column of soil in the labora-

4

R I C H A R D R. P A R I Z E K A N D B U R K E E. L A N E

tory using a ceramic plate as a further elaboration on the procedure and Colen), and Cole et a1.12), made such tests in the field using an alundum tension plate which he called a tension lysimeter. Wagner (ref.la), pp. 379386) reported on a porous ceramic cup assembly which was specially tooled by the Soil Moisture Equipment Company, Santa Barbara, California. Field results were summarized for installation at the 6, 12, 24, and 36-inch depth. At the 6-inch depth, a sample of water was obtained by this procedure whenever the percent water by volume in the soil was greater than 29. The soil-water content at which the first water sample could be collected was found to be variable. The device described by Wagner is a prototype which was modified for the studies at Penn State to allow soil-water samples to be collected at any depth. Anions and cations contained in soil water can be obtained from soil samples by leaching them from dried or moist soil samples collected from beneath research plots. The number and type of constituents that can be analyzed using this procedure is somewhat limited and requires new drillholes and soil samples from time to time. The volume of samples is limited and chemical changes can occur for certain constituents particularly where soil samples have been dried and stored. This procedure is being used on Penn State's waste water renovation and conservation research project by L. T. Kardos and is suitable when infrequent soil samples are required. D. W. Cole (ref.14), p. 176) described a direct method for analyzing for selected constituents contained in soil water. The equipment is rather elaborate, costly but capable of providing highly reliable results. It is an integrated system for collecting, recording, and analyzing water samples obtained from the soil-water reservoir. It includes tension lysimeters of fused aluminum oxide disks which are maintained under a partial vacuum. Solutions collected during periods of flow are passed through flow cells measuring conductivity, acidity, and rate of water flow. Electrical output from the flow cells is coupled to a data-logging facility and printed as perforations in paper tapes. Direct readings on other chemical constituents probably could be obtained with minor modifications in the system. Cole's system is more sophisticated than the ones reported here but also is more costly.

Pan lysimeters D E S C R I P T I O N OF A T R E N C H LYSIMETER

Pan lysimeters were designed and installed at The Pennsylvania State University Waste Water Renovation and Conservation site by W. E. Sopper, Professor of Forestry (ref.1), p. 33). These were installed at 6 and 12-inch

SOIL-WATER SAMPLING

3

depths below irrigated and control plots and along the walls of a 6-foot trench where 4 inches of effluent per week were applied. A somewhat similar trench lysimeter was constructed at our winter irrigation site and contained sampling pans spaced at 1-foot intervals from a depth of 1 foot to 17 feet below ground level. Galvanized, 16 gage, metal pans, 12x 15 inches long were fabricated (Fig. 1), copper tubing was soldered to a raised end of the pan to allow perched soil water to drain into a sample container located inside the sampling pit. A 4-foot wide, 12-foot long trench was excavated to a depth of 10 feet using a back hoe. The hole was braced with timbers and siding to allow safe access to the trench. The trench was then hand-dug to a 17-foot depth and braced. The entire seepage face was inclined 1 to 5 degrees from the vertical and sloped toward a hill down which soil water and interflow was expected. The residual soil contained resistant chert and quartzite cobbles and boulders and was reasonably well "cemented" with iron-oxide and clay. As a result, the pans could not be inserted into the soil profile without first providing an opening. A sheet metal blade 4 inches wide and 2 feet long was hammered into the overhanging bank with a sledge hammer to provide access for the pans. Pan lysimeters were tapped into these openings and allowed to slope gently toward the trench. Voids above and below the pans were back-filled

Fig. 1. Pan lysimeter. The galvanized, 16 gage metal pans are 12 × 15 inches and contain copper tubing fitted with tygon tubing and a sample bottle.

6

RICHARD R. PARIZEK AND BURKE E° LANE

2x12" Siding and 4 x 4 " Timbers. All Wood Treated wilh Preservative.

Lystmeler ,~ -

~

~__

~ L I f ~.'"~'_'_" Tubing I~r~.~Sornple

, ° ...

~. . : "'.".

Bottle

k

==~

HI --,-:-:_.-~Residuol Soil II-.--.. . . . . Stratified Silt, I1-- -Y~--IZ-'---- Cloy' and sand

r~

~/.~ •

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.~_7 o°

Pipe

Fig. 2.

Schematic diagram of trench lysimeter.

with soil and tamped into place. As siding was added to the trench walls, holes were cut to allow the copper tubings to project into the sampling pit. Spaces between the original trench faces and siding were filled with native soil and washed pea-gravel to allow water to flow freely toward the pit floor (Fig. 2). After the walls and braces were emplaced, tygon tubing was connected to the copper tubing and inserted into plastic sampling bottles (Fig. 3). The sampling pit was covered with a sloping roof and a half-round drain pipe was used to divert roof water away from the installation (Fig. 4). A ladder was placed at one end of the house to allow access. PERFORMANCE AND LIMITATIONS

During the first winter (1964-65), 6 inches of effluent per week were applied to the slope above the sampling pit. No soil water samples were collected the first week indicating that the soil was well below field capacity.

SOIL-WATER SAMPLING

7

After a total of 12 inches of irrigation water had been added by the second week, underflow became excessive and the pit was flooded nearly to the roof by perched ground water. To protect the sampling station and avoid contaminating the soil adjacent to the pit, a floor drain was drilled 90 feet through residual soil into unsaturated carbonate bedrock. The drain contained a screen to prevent it from plugging.

Fig. 3.

Inside view of trench lysimeter showing sampling bottles attached to the wall.

Seepage pressures became excessive at the bottom of the trench and the unprotected floor soon became quick. A considerable effort was required to clean the pit floor, line it with flooring and replace lost ground outside the housing walls. The sampling facility has functioned suitably for 6 years since these

8

RICHARD R. PARIZEK AND BURKE E. LANE

corrections were made. A sump pump would have to be used if a floor drain could not be provided. If less than 4 to 6 inches of water per week were irrigated, less flooding would have resulted at the base of the slope being irrigated, and a sump may not have been required. Stratified soil also facilitated lateral flow of perched soil water which further aggravated the flooding problem at this site.

Fig. 4.

Oblique view of trench lysimeter. Note effluent irrigation lines in the background.

Soil-water samples were available from the facility after each irrigation and after major storms the first year. During alternate growing seasons the renovation site above the pit was not irrigated to allow effluent constituents stored in the soil in winter to be removed or renovated during the following growing season. Several weeks after irrigation was terminated, usually in April or May, side hill and subsurface seepage became minimal and no further samples could be collected until irrigation was resumed again in the fall. Evapotranspiration losses of soil moisture during the growing seasons of each of 5 years depleted soil water to the extent that little or no gravitational water could be collected from the facility. In summary, flooding problems could make deep trench lysimeters somewhat troublesome when sampling soil water where more than 2 inches of water per week are artificially applied to land in a humid region. An automated sump pump would be required to prevent flooding under some field conditions, hence electricity should be available. At State College the

SOIL-WATER SAMPLING

9

pit was drained successfully on site to underlying unsaturated carbonate rocks. Gravitational water samples can be obtained only rarely during the growing season in the absence of irrigation waters and when precipitation is equal to or less than field capacity requirements. These factors, together with the expense involved in constructing such a sampling pit (mainly manpower requirements) lead us to conclude that other sampling procedures are more desirable particularly where a number of sampling points are required at various depths and at various widely scattered sites. There is risk of cave-ins when working in open trenches more than 5 feet deep in soil. Timbers and siding must be used to protect workmen during construction. This was mandatory when working at the 17-foot level at the waste water research site. Further, all timbers and sidings were treated with liquid wood preservatives to prevent decay and to insure integrity of supporting members. The supports in question appear sound indicating that the life of such a facility should be at least 5 to 10 years. Plywood siding is not recommended because laminations easily pull apart under semi-saturated field conditions.

Pressure-vacuum lysimeters Soil water samplers of the type shown on Fig. 5 have been used to obtain soil water samples on two separate research projects currently underway at The Pennsylvania State University. This improved soil water sampler has been named a pressure-vacuum lysimeter or "suction lysimeter" by workers at The Pennsylvania State University. This improved lysimeter was first used in conjunction with the University's Waste Water Renovation and Conservation Project1). Prototypes, described by Wagnerla), and used by W. E. Sopper and L. T. Kardos on the Penn State Waste Water project, were manufactured and distributed by Soil Moisture Equipment Company, Santa Barbara, California. The prototype was modified by Parizek to allow soil water samples to be collected from depths greater than 6 feet. A description of this modified device follows along with some of its advantages and disadvantages. Pressurevacuum lysimeters were one element in a much larger soil-water monitoring system that was designed to evaluate the effectiveness of the soil profile as a "Living Filter" in purifying sewage effluent that was irrigated on crop and forest lands. Several dozen of these devices were installed at depths of three to twenty-six feet beneath ground surface. Many of them have been in operation for five years or longer and are still in good operating condition. Seventeen of these lysimeters were installed in the soil beneath a sanitary landfill trench. This soil-water monitoring system is part of another research

10

RICHARD R. PARIZEK AND BURKE E. LANE

project currently underway at The Pennsylvania State University2). In this project the qualitative changes that take place in the leachates derived from the refuse in a sanitary landfill trench were required as they migrate through and interact with soil. The depths of placement varied from two feet to 36 feet below the bottom of the refuse in the landfill trench (Fig. 6). The

Fig. 5.

Close-up of a pressure-vacuum lysimeter. Note the ceramic porous point and double copper access tubes.

lysimeter at 36 feet beneath the landfill trench bottom was 10 additional feet below ground surface. This project is in its third year and the suction lysimeters are still delivering several hundred milliliters on a regular sampling schedule. An incidental part of this investigation was evaluation of pressurevacuum lysimeters. This will be discussed later.

SOIL-WATER SAMPLING

;L'4 10'

-4,5' -91 13'

11

SL-3 SL-6 SL-2 SL-5 SL-1 ~-[-~ 5'-I: 5'-q~- 5'-.+-5',-*1 -4' -T'

3' -8'

l

-6'

Ji[ 10' ~ 12' -14.5'

-14.5'

17'

t~-13 5'

I~116"5' tJ-20'

-24'

i•LysimBelow eler Refuse ID~Depth

-29'

-36 ° 40'

Fig. 6. Longitudinal cross-section of lysimeter holes in bottom of landfill trench showing depths of lysimeter placement below refuse.

DESCRIPTION OF PRESSURE-VACUUM LYSIMETERS

The design and development of the type of soil-water sampler that was modified for use on The Pennsylvania State University projects was described by Wagner13). These samplers consisted of a porous ceramic cup having an outside diameter of 1.9 inches and an overall length of 2.5 inches (Fig. 7). The cup has a connecting lip around the top to allow it to be cemented to commercial plastic pipe to provide an air and water-tight seal. The top of the plastic pipe is plugged with a one-hole rubber stopper. A short length of copper tubing is inserted through the hole and a short length of thick-walled rubber tubing is connected to the copper tubing. A vacuum is applied to the assembly by means of a vacuum pump powered by a small internal combustion engine. A thumbscrew pinch clamp is tightened around the rubber tubing to maintain the vacuum after the pump is removed. In describing the function of the porous ceramic cup Wagner stated, "The cups, which will hold a fluid volume of about 100 ml, are similar in their properties to the porous cups used in soil-moisture tensiometers. When such a ceramic cup is in contact with moist soil, the pores in the wall of the cup will become filled with water due to capillary suction. The pore size is such that the water is held in these pores with a force

12

RICHARD R. PARIZEK AND BURKE E. LANE

To VacuUmpu"~-~--~~ mp ~ ~ / ~ Flosk-~ LJ/

CapillaryTube ~ RubberTubing

Rubber

StopperS.

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PlPaestic Pi AnyLength Porous ~ Ceramic ~ / B Cup ~=l~ Fig. 7.

Porous ceramic cup water-sampling assembly as described by Wagner ~8). Capillary tube is only inserted to recover water sample. No scale.

sufficient to cause the cup to become sealed against air pressure of at least 15 psi. A vacuum, therefore, may be drawn within the ceramic cup. In the event moisture in the soil is held at a tension of less than one atmosphere, water can be drawn through the pores of the ceramic wall and into the cup, due to suction developed by a vacuum within." To recover water samples a capillary tube is inserted down through the rubber and copper tubings and a vacuum is applied through an Erlenmeyer flask (Fig. 7), which captures the water as it is withdrawn. The porous ceramic cup assemblies are simply inserted down 2-inch diameter holes augered to depths of 6 to 36 inches. The soil was backfilled but not tamped around the units. This basic unit is being used by W. E. Sopper and L. T. Kardos at the waste water renovation and conservation research site1). Several disadvantages are apparent in these methods of construction, installation, and sample recovery. The method of installing the rubber stopper and associated tubings without cement may result in an inadequate air-tight seal because the friction seal between components may be broken during installation or backfilling, particularly in deeper holes, resulting in a rapid loss of vacuum or the inability to create a vacuum initially, improper or

SOIL-WATER SAMPLING

13

incomplete backfilling and tamping can result in voids around the porous tip caused by bridging of soil particles or stones in the native soil. These conditions will reduce the contact between the porous cup and the soil and will reduce the efficiency with which the device can withdraw moisture from the soil. Soil-water also can channel down the improperly filled drill-hole. The use of an internal combustion engine to power a vacuum pump limits the portability of sampling equipment and ease of sampling, and depth from which samples can be recovered using vacuum methods. The fragility of the equipment used to collect the water samples also restricts the ease with which samples are recovered, as does the length of capillary tubing necessary to recover samples from greater depths. This method of sample recovery also restricts the number of assemblies that can be installed in a small diameter hole (essentially) to one, because it is necessary to keep a straight access tube in which a capillary tube can be inserted. In many instances a more compact arrangement of "stacking" several samplers in one hole is more desirable and economical (Fig. 6). A hand-held vacuum pump which eliminates the bulky engine and vacuum pump set-up described above is used at Penn State. This pump is supplied by the Arthur A. Thomas Scientific Supply Company, Philadelphia, Pennsylvania. Modifications that were made to this prototype soil-water sampler are presented below. The improved pressure-vacuum lysimeter design and installation are shown on Fig. 8. It consists of the same soil-water assembly described above with several modifications. The rubber stopper is drilled to accommodate two 1-~-inch diameter copper tubes; one for use as a discharge tube and one to allow for evacuation and pressurization of the lysimeter. The two sections of copper tubing are epoxied into the cork to predetermined lengths; the longer section to within ½ inch of the bottom of the ceramic cup, and the shorter length only about two inches below the bottom of the cork. The longer section serves only as a discharge tube and the shorter section serves as both pressure and vacuum inlet. The length of tubing extending outside the lysimeter's cork is determined by the desired depth setting; extra lengths of tubing can be attached with swage-lock pressure fittings. After the first epoxy is cured, the cork and tubes are then epoxied into the open end of the plastic pipe. The tubes should be coded on their exposed ends so that the operator can determine which is the discharge tube after the hole has been backfilled. For additional protection against air leakage and to provide strength to the unit necessary during installation, the cork and the top of the lysimeter may be wrapped with many layers of black polyvinyl chloride (PVC) electrical tape. This gives a sturdy, rigid lysimeter with two coils of copper tubing attached for convenience in placement (Fig. 5).

14

RICHARD R. PARIZEK AND BURKE E. LANE /2-Way Pump / Plastic Tube and Clamp '~ Vocuum~ Port \

II

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Plastic Pipe'; 24" Long Tamped_ _ Backfill

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Super-Sil ~ PorousCup-%', 6" Hole-----~,: Bentonite :~ °*

Fig. 8. Cross-section of a typical pressure-vacuum lysimeter installation (compare with fig. 7). With the pressure pump attached to the lysimeter, the apparatus is ready to recover a water sample. No scale. A typical pressure-vacuum lysimeter installation is shown in Fig. 8. Placement holes are first drilled to the desired depth. They may be 4 to 6 inches in diameter depending upon the number of lysimeters to be placed in each hole. A plug of wet bentonite clay is placed in the bottom of the hole to isolate the lysimeter from the disturbed soil below it. This plug is optional. A layer of "Super-Sil", at least six inches deep, is placed on top of the bentonite. "Super-Sil" is the trade name for a commercially available, crushed, pure silica-sand of almost talcum powder consistency. This is used to provide a clean transmission medium for soil moisture moving under capillary pressure, to insure hydraulic contact of the adjacent soil medium with the porous tip, to fill uneven voids created during drilling, as well as to discourage clogging of the ceramic tip by colloids, organic matter, or soil particles. The lysimeter is placed in the hole to the desired depth, and "Super-Sil" is placed around it until the lysimeter is about half-buried. Native soil, free of pebbles and rocks, is backfilled and tamped with long metal rods. After the lysimeter is covered with about six inches of soil, a second plug of bentonite is deposited to further isolate the lysimeter and to guard against possible channeling of water down the drill hole. Backfilling is continued with native soil to the

SOIL-WATER SAMPLING

15

depth where it is desired to set the next lysimeter, at which point the above procedure is repeated. It was found that three lysimeters were the most that could be conveniently placed in any one six-inch diameter hole. If more than three were installed this led to difficulties in proper depth placement, prevented proper tamping of backfill material, added to the danger of crimping or tangling the copper tubing and to the risk of channeling soil water down the incompletely filled hole. Care was taken to accurately measure the depth of placement of each lysimeter. It was possible to set the lysimeters to within six inches of the desired depth even in 30-foot deep holes. After the lysimeters are placed, a short section of flexible tygon plastic tubing is secured over the end of each copper access tube with PVC electrical tape to allow thumbscrew pinch clamps to be used to seal the lysimeter between sampling periods, thereby maintaining the vacuum within the lysimeter. A very short length of copper tubing is then secured to the free end of the plastic tubing attached to the pressure-vacuum tube. The vacuum pump is attached to this tubing. The pump used in conjunction with these pressure-vacuum lysimeters is a two-way hand pump that can either deliver a back pressure or pull a vacuum. This pump was purchased from the Arthur Thomas Scientific Supply Company, Philadelphia, Pennsylvania. The pump is similar to a tire pump. It has a base on which the operator may stand while working the pump. A small vacuum gauge may be installed on the vacuum port of the pump by means of a tee-union. This enabled the operator to consistently apply a desired vacuum to all lysimeters (about 18 inches of mercury). A length of tygon tubing is secured to each of the pump's pressure and vacuum ports to allow the pump to be coupled to the access tubes of the lysimeters. The free ends of the pump's tubing are slipped over the short length of copper tubing that is secured to the pressure-vacuum tube of the lysimeter and is held securely by a small spring-loaded clamp. A typical pressure-vacuum lysimeter sampling sequence is as follows (Fig. 8): 1. The lysimeter's discharge tube is clamped shut and the vacuum side of the two-way pump is attached to the "in" tube. 2. A vacuum of approximately 18 inches of mercury is drawn and the "in" tubing is clamped shut. 3. To recover soil water samples, the pinch clamps are removed and the pressure side of the two-way pump is attached to the lysimeter's "in" tube. A few strokes of the hand pump generated enough pressure to force the water out of the lysimeter and into a collection bottle placed under the discharge tube. 4. After emptying the lysimeter the discharge tubing is clamped, the

16

RICHARD R. PARIZEK AND BURKE E. LANE

v a c u u m side o f t h e p u m p is a t t a c h e d t o t h e " i n " t u b e a n d t h e l y s i m e t e r is evacuated again to gather another sample.

PRESSURE-VACUUM LYSIMETER PERFORMANCE AND LIMITATIONS As stated earlier, an attempt was made to evaluate the performance

of

the lysimeters installed beneath the sanitary landfill trench. This was done b y c o n s i s t e n t l y a p p l y i n g t h e s a m e v a c u u m t o all l y s i m e t e r s a n d t h e n r e c o r d i n g a n d t a b u l a t i n g t h e v o l u m e s o f all r e c o v e r e d s a m p l e s . T a b l e 1 i l l u s t r a t e s t h e c o n s i s t e n c y w i t h w h i c h t h e l y s i m e t e r s p e r f o r m e d o v e r t h e first six m o n t h s TABLE 1. Sample volumes obtained from pressure-vacuum lysimeters for the period November 21, 1967 to May 13, 1968. All results in milliliters Date Sampled Depth (if)

1967 11-21

SL-1 6 SL-1 24 29 36

11-28

1968 12-11

2-4 2-18

850 430 600 850

NS 290 440 NS

800 585 740 800

860 630 790 825

3 10 13.5

125 830 950

No Samples NS 850 NS 850

. 475 775

SL-3

2 4 7

820 NS 875

NS 800 675

480 810 840

SL-4

4.5 9

790 820

NS 810

SL-5 12 16.5

820 775

SL-6

820 880 775

SL-2

Totals

8 10.5 14.5

11760

3-3

4-28

5-13

730 425 655 685

770 495 695 365

800 665 720 745

I0 460 645 710

NS NS

. 740 675

. . 800 795

710 750

700 730

755 790 840

750 675 NS

670 650 710

605 750 800

665 710 750

400 760 725

970 860

720 780

NS NS

615 600

710 740

680 685

675 675

NS 560

700 725

790 690

790 NS

775 . 30

825 280

700 660

700 650

NS 675 575

810 850 785

725 785 605

NS 730 750

690 770 NS

795 775 575

675 745 640

810 680 675

12075

11815

-

9420

10775

11500

10005

-

NS NS 700 NS

3-15

.

.

Note: Volumes of samples taken on 4-7-68 and 5-7-68 were not measured, but all were more than 500 ml. NS is No Sample taken on this date.

SOIL-WATER SAMPLING

17

of the water-sampling program. Later results are not tabulated because the format of the water-sampling program was changed and comparisons among all 16 lysimeters was not possible. The sample volumes obtained up to Spring, 1969 are still consistent with those given in Table 1. The efficiency of the lysimeters has not decreased. Lysimeter SL-2, at 3 feet was the only failure of 17 lysimeters originally installed. Its performance indicated that either one or both of its tubes had been pinched shut by the overburden of refuse or by the weight of the landfilling equipment. Difficulties in recovering samples occurred on two occasions: November 28, 1967 and February 18, 1968. Water froze in some of the exposed discharge tubes during very cold weather. At the waste water site a blow torch is used to melt ice within the copper tubes before recovering samples. This is not possible at the landfill site because more than 10 feet of tubing per station is exposed in an unheated sampling house. The average volume of soil water recovered from each lysimeter was greater than 700 ml during each sampling period. Soil-water samples of these volumes are sufficient to permit quantitative analysis of a number of chemical constituents. In the landfill study, for example, analyses for specific conductivity, chloride, total iron, ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, fluoride, BOD, and pH were run on each sample on a regular basis and analyses for calcium, sodium, potassium, and phosphate were run on a spot-check basis. The total volume of soil water recovered from all lysimeters per sampling period decreased gradually with time (Table 1). Soil water was gradually depleted from around the porous points because soil moisture was not replenished during the six-month period November 1, 1967 to May 1, 1968. Rainfall for the period was only 11 inches and refuse in the landfill absorbed all available water infiltrating into the area that was not consumed by evaporation or transpiration. In effect, the amount of soil water a pressure-vacuum lysimeter can extract with a given vacuum decreases over time as soil water is depleted, because what moisture remains in the soil is held more tightly by capillary forces. An evacuated lysimeter should extract soil moisture held by the soil with a tension up to, or equal to, the vacuum drawn on the lysimeter. The amount of water collected during a given period of time mainly depends upon the water content of the soil, all other factors such as water quality, grain size, porosity, and permeability being equal. A few simple precautions are necessary to insure that the lysimeters will work. It was found that merely screwing the pinch clamps finger-tight did not seal the plastic tubing sufficiently to maintain a vacuum for any length of time. The results shown in Table 1 for SL-3, 4 feet on November 21, 1967 and for SL-1, 6 feet on May 13, 1968 are due to improper sealing of the

18

RICHARD R. PARIZEK AND BURKE E. LANE

lysimeter tubes. All pinch clamps were tightened with pliers to eliminate this problem but occasionally a clamp that had not been positioned properly on the tubing would still lose its seal. When sealed properly these lysimeters would emit a hiss when opened after as long as three weeks, indicating that a vacuum was still operative and withdrawing water from the soil. The "suction" lysimeters described above were capable of recovering soil water samples from depths as great as 46 feet below ground surface. There appears to be no limit to the depth at which they can be placed, evacuated, and soil water pulled into the plastic chamber. The problem, however, is the recovery of samples from great depths. In the landfill study it was found that in order to recover the samples from a depth of 46 feet continuous pumping was necessary to force the water from the lysimeter, through the discharge tube, and into the collection bottle. Friction in the long, narrow tubes and the high lift (head difference) were the most probable causes for this difficulty. A slightly larger diameter discharge tube might help to reduce the friction. A portable, mechanized pump capable of delivering higher pressures could overcome the head but it would probably rupture the lysimeter assembly, if it were improperly assembled or excessive pressures were used. About 50 feet appears to be the maximum depth that these suction lysimeters can be set and still maintain ease and simplicity of operation, without requiring other modifications. One problem at these greater depths is to insure that holes are properly backfilled to avoid channeling. This would not be a problem if casing was left in place after drilling and only one or two lysimeters were placed at the bottom of the uncased part of the hole. Holes could be grouted shut to prevent channeling provided that the grout did not cause undesirable chemical changes within the soil water in the vicinity of the sampling point. When wash-boring methods of drilling are used in loose soils, drill holes may be oversized. These voids are hard to backfill and can facilitate channeling of soil water. Addition of "Super-Sil" adjacent to the porous point will at least insure adequate contact with the borehole walls. Other precautions are required, however, to prevent channeling about the "suction" lysimeters. Where 4 or more copper tubes are installed within a single hole, use of liberal amounts of dry bentonite or grout should minimize channeling along tubing clusters. The opening within the porous ceramic points are minute and serve to screen out suspended solids including most soil bacteria. Hence, soil water samples collected in this manner will not contain representative concentrations of bacteria nor will suspended solids or BOD values be truly representative. This may impose limitations in some studies. It has been suggested that suspended colloidal-sized particles soon would plug the ceramic points on the pressure-vacuum lysimeters. These devices

SOIL-WATER SAMPLING

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have been used successfully for 6 years on our Waste Water project with no apparent loss in efficiency. The ceramic points were buried in Super-Sil and this fine-grained pulverized quartz may have prevented the migration of suspended solids. Perched ground water, ground water, or water used in drilling may be encountered in some drill holes. Empty "suction" lysimeters will float in these holes and small diameter copper tubing is not rigid enough to be used to force the sampling devices to the desired depths. This difficulty can be eliminated by filling the reservoir chambers with water in advance. In some cases metal rods have been used to press the sampling tubes to the desired depth below water. The copper tubing is loosely taped to a metal rod with masking tape and the tip of the rod is allowed to rest on the top of the rubber stopper. After the lysimeter is pressed into place, the water soluble glue is allowed to soak for a while and the rod is pulled free. Super-Sil is first mixed with water to form a slurry and then poured into the holes containing water. The slurry will settle in time opposite the ceramic point and will not bridge in the hole as is the case when dry Super-Sil is used. The hole may be backfilled after the slurry is allowed to settle. In holes without standing water, both slurries and dry Super-Sil were used. Conclusions

1. The improved pressure-vacuum lysimeters described in this paper eliminate many problems that were inherent in the original design and installation procedure of a porous ceramic-cup soil-water sampling assembly, particularly when attempting to obtain samples from greater than a 6-foot depth. These improvements permit soil-water samples to be obtained from depths as great as 50 feet with equipment that can be carried by one man. These modifications are necessary if soil-water samples are required from deeper than the top-soil profile, the region in which the prototype soil water sampler was designed to operate. 2. One to six years of continuous operation under adverse field conditions on two research projects has demonstrated the reliability, the almost total lack of maintenance required, the ease and simplicity of installation and operation, and the efficiency that can be expected from these pressurevacuum lysimeters if care is taken in assembly and installation. 3. Pressure-vacuum lysimeters are not useful where soil bacteria, B.O.D. and suspended solids are to be analyzed for. These and other suspended materials are screened by the porous ceramic points. Laboratory tests can be run on lysimeters to determine what constituents are influenced when using this sampling procedure.

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RICHARD R. PARIZEK AND BURKE E. LANE

4. Pressure-vacuum lysimeters can be used to lecover soil water samples long after pan lysimeters fail to provide samples. The sampling period may be extended by at least two months over that for pan lysimeters. Eventually in the absence of new soil moisture, "suction" lysimeters will fail to yield soil-water samples but this will be at a water content well below field capacity. 5. Trench lysimeters are subject to flooding when interflow is excessive. Drainage facilities must be allowed for or frequent flooding may result. Only gravitational water and perched ground water may be sampled in this manner.

Acknowledgments This manuscript was prepared under support of the Mineral Conservation Section, College of Earth and Mineral Sciences, The Pennsylvania State University. The sampling devices were originally designed and used on the Waste Water Renovation and Conservation Project sponsored by The Pennsylvania State University. Details are herein included by permission of Dr. L. T. Kardos, project director. The deep pressure-vacuum lysimeter installations were also used on a project entitled "Site selection criteria for sanitary landfills in carbonate terrains" sponsored by the Mineral Conservation Section, College of Earth and Mineral Sciences and The Stream Quality Section, The Pennsylvania Department of Health. Various graduate students in the Department of Geology and Geophysics aided in installing the sampling facilities described above. Shamsul H. Siddiqui, Oscar Huh, Frank Mooreshead and John Bauer worked on the Sanitary Landfill project. Deep Pressure-Vacuum Lysimeters at the Waste Water project were installed with help from S. H. Siddiqui, Robert Williams and John Clark. The trench lysimeter was constructed with help from Richard E. Smith, S. H. Siddiqui, F. T. Caruccio and Phil Cohen presently or formerly with the same department.

Literature 1) R. R. Parizek, L. T. Kardos, W. E. Sopper, E. A. Myers, D. E. Davis, M. A. Farrell and J. B. Nesbitt, Waste Water Renovation and Conservation. Penn State Studies No. 23, The Penn. State Univ., University Park, Pa. (1967) 71 p. 2) B. E. Lane, Sanitary landfill leachate interactions with a carbonaterock derived soil in Central Pennsylvania. Master of Science Thesis, Department of Geology and Geophysics, The Pennsylvania State University (1969) 197 p. 3) L. N. Overrein, Lysimeter studies on tracer nitrogen in forest soils. I. Nitrogen losses by leaching and volatilization after addition of Urea-N 15. Soil Sci. 106, No. 4 (1968) 4) D. W. Cole and S. P. Gessel, Movement of elements through a forest soil as influenced

SOIL-WATER SAMPLING

5) 6) 7) 8)

9) 10) 11) 12)

13) 14)

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by tree removal and fertilizer additions. In: Forest-Soil Relationships in North America (Oregon State University Press, 1965) pp. 95-104 J. S. Joffe, Lysimeter studies: III. The movement and translocation of nitrogen and organic constituents in the profile of a podzolic soil. Soil Sci. 35 (1933) 401-411 N. H. J. Miller, The amount and composition of the drainage through unmanured and uncropped land. Barnfield, RothamsteA. J. Agric. Sci. 1 (1906) 377-399 M. Odelien and T. Vidme, Lysimeterforsok p~t As 1938-43. Meld. Norg. Landbrhoisk, 29 (1945) 1-90 H. Kohnke, F. R. Dreibelbis and J. M. Davidson, A survey and discussion of lysimeters and a bibliography on their construction and performance, U. S. Dept. Agr. Misc. Pub. 372 (1940) E. F. Wallihan, An improvement in lysimeter design. J. A. Soc. Agron. 32 (1940) 395-404 E. A. Colman, A laboratory study oflysimeter drainage under controlled soil moisture tension. Soil Sci. 62 (1946) 365-382 D. W. Cole, Alundum tension lysimeter. Soil Sci. 85 (1958) 293-296 D.W. Cole, S. P. Gessel and E. E. Held, Tension lysimeter studies of iron and moisture movement in glacial till and coral atoll soil. Soil Sci. Soc. Amer. Proc. 25 (1961) 321-325 G. H. Wagner, Use of Porous Ceramic Cups to Sample Soil Water Within the Profile. Soil Sci. 94 (1962) 379-386 D. W. Cole, A system for collecting, recording, and analyzing solutions in a forest soil. Am. Geophys. Union 49th ann. meetings (Abstract) p. 176, Washington, D. C. (1968)