Numerical evaluation of passive capillary samplers

European Geosciences Union.

1st General Assembly.

Nice, France, 25 - 30 April 2004

NUMERICAL EVALUATION OF PASSIVE CAPILLARY SAMPLERS

Jean-Guillaume LACAS*

_{, Marc VOLTZ}

_{, Philippe CATTAN}

INTRODUCTION

Ground surface

Sample extraction and pressurisation tubes

Unraveled fibers contacting soil

Installation cavity Wick support

Wick tubing

Sample collection bottle Hanging water-column [50cm-90cm] Collection depth [50cm-2m]

Typical pressure head profiles within a

fiberglass wick under differents flux

intensities.

In order to achieve a minimal disturbance of the native flow regime, the wick material (which determine a hydraulic conductivity), length and width and the contact area with the overlying soil have to be dimensioned so that the “soil pressure vs. flow” conditions of the wick match those of the soil. A mismatch induces converging or by-pass flows around the sampler, which lead to overestimation or underestimation of percolation fluxes.

-100 -80 -60 -40 -20 0 -1 -0,8 -0,6 -0,4 -0,2 0 Soil material

Wick system n°1 : well matched Wick system n°2 : too much resistive Wick system n°3 : too much conductive

Downward soil water flux (cm/h)

Pressure Head (cm

H2O)

Graphical illustration of different ‘‘hwickvs. qsoil‘‘ and ‘‘hsoilvs. qsoil‘‘ caracteristics : typical dimensioning work.

Passive capillary samplers, which sample water from the vadose zone via a hanging water column in a fiberglass wick, have shown potential to provide better estimates of actual soil percolation fluxes than alternative field methods. Unsaturated and saturated flows (water and solutes) are extracted continuously and without external vacuum generator from a non-disturbed soil

volume, through a significant area (typically 900cm2_).

PROBLEM DEFINITION - METHOD

The objective was to quantify the bias induced by such samplers on flux measurements. Two sources of bias were examined:

- deviations due to the theoretical assumptions (1D and permanent flow, exponential K-h relationship) underlying the above

analytical procedure, generally used for dimensioning the capillary wick sampler.

- uncertainties in the soil and wick hydrodynamic properties or in the dimensions of the wick sampler (collection area, wick section and wick length).

To that aim, we used the Hydrus 2D code (Simunek et al., 1999), solving the Richards equation for simulating two-dimensional unsaturated flow, to analyze the deviations of a soil-capillary wick system in comparison to an undisturbed soil.

1/ Evaluation of bias due to the analytical dimensioning procedure

2/ Evaluation of the uncertainty on flux measurement

RESULTS

CONCLUSIONS

•The analytical dimensioning method is relevant for soils which are well described by an exponential K-h relationship. But for most actual soils, this simple K-h model is not valid and complex relationships like Mualem-Van Genuchten do not permit analytical solutions of the Richards equation.

•The analytical dimensioning method is sufficient for large time-scale measurements only.

•Significant errors on the observed fluxes occur when the soil and wick properties do not exactly match due to 3D soil-wick interactions. This is the common case since the range of available characteristics of fiberglass soil-wicks is limited. •Finally, this work outlines the interest of a 2D numerical simulation approach when the use of passive capillary samplers is projected. The dimensioning can be done for a wide range of soil types owing to complex hydrodynamic models, for dynamic situations, and for a wide range of boundary conditions. The model can also quantify the bias on observed drainage fluxes with existing lysimeters, and in turn allows to remove the bias of the measured percolation fluxes.

•Nevertheless, given the uncertainty on hydraulic properties, flux measurement uncertainty appears to be important. •Attention should be taken at the wick length : the finer is the soil texture, the longer should be the wick in order to avoid bias.

References :

Simunek J., Sejna M. and VanGenuchten M. T., 1999. The HYDRUS-2D software package for simulating the two-dimensional movement of water, heat, and multiple solutes in variably-saturated media. Version 2.0, USDA.

Knutson, J.H. and Selker, J.S., 1994. Unsaturated hydraulic conductivities of fiberglass wicks and designing capillary wick pore-water samplers. Soil Sci Soc. Am. J., 58: 721-729.

Acknowledgments :

The authors thank for their financial support : CIRAD, region of Guadeloupe (France), Europe (“Assessment of water-pollution risks associated with agriculture in the French West Indies ; Management at the catchment scale” programme).

Photography of a sampler during installation under a banana tree.

Typical design of a passive capillary sampler.

Wicks

CIRAD. Philippe Cattan

W ick length (cm ) 0 20 40 60 80 100 -100 -80 -60 -40 -20 0 qs=0,001cm/h

Pressure Head (cm H2O)

qs=0,01cm/h

qs=0,1cm/h

qs=1cm/h

Succion exerted on the overlying soil by a 100cm long wick

1. CEMAGREF, UR Qualité des Eaux et Prévention des Pollutions, 3 bis quai Chauveau, 69336 Lyon, France 2. ENGEES, 1 Quai Koch, BP 1039F, 67070 Strasbourg, France 3. INRA, UMR L.I.S.A.H., 2 place Viala, 34060 Montpellier, France 4. CIRAD FLHOR, Station de Neuf Château, 97130 Capesterre Belle Eau, France Corresponding author* : Tel : +33 (0)4 72 20 86 05 Fax : +33 (0)4 78 47 78 75 E-mail : [email protected]

Knuston and Selker (1994) derived for 1D permanent flow analytical equations that relate wick and soil pressure heads to their hydraulic characteristics and to the percolating flux. They allow to choose the right wick characteristics for given soil and flow conditions. ( ) ( ) ( )h K (a h) K h a K h K with K q a h K A A q K A A q L a a h wick sat wick wick soil sat soil soil sat soil soil soil soil sat wick wick soil soil sat wick wick soil soil wick wick wick wick ⋅ = ⋅ = ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ − = ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + = exp exp ln 1 1 ) exp( ln 1 [ ] [ ]

[ ]

[ ]

Scheme of the soil cylinder with the wick sampler placed in the middle, modeled by a 2D-axisymetrical domain.

Boundary conditions used for « soil with a wick sampler » and « reference soil ».

Soil surface Symetry axis 3D cylinder 2D section Wick Soil material Wick material

Description of simulated configurations : both have been dimensioned analytically Soil type Ida Silt Loam Rehovot Sand Soil saturated

conductivity 1,5 cm/h 27,5 cm/h Wick type _{3/8-inch Med. Density}Amatex company Pepperell company_1/2-inch Wick saturated

conductivity 460 cm/h 1168 cm/h Wick section 0,74 cm2 1,65 cm2

Wick length 80 cm 45 cm

Collection area 225 cm2 75 cm2

Simulations were performed with a 11 day actual hyetograph

•There is no significant bias on the wick-extracted volumes during the 11 day long simulation performed with both soils, which are well described by an exponential conductivity. The small bias observed with Rehovot Sand is due to the fact that soil and wick properties couldn’t be perfectly matched.

•However, the instantaneous bias can be significant and vary strongly with time due to transitory effects.

•The wick length has a very large impact on extracted volumes if it’s too short, particularly for Ida Silt Loam •The typical uncertainties about the hydraulic properties of both soil

and wick materials can induce large deviations of wick-extracted volumes in comparison to the actual flux

Error due to measurement uncertainty on soil saturated conductivity

0 3000 6000 9000 0 2 4 6 8 10 Ks / Ks reference Cum u la ted f lux ( c m3

) Ida silt loam + AM 3/8 MD

Rehovot sand + PEP 1/2

Temporal variations of bias in the (Ida Silt Loam + Amatex 3/8in. Wick) system

0 80 160 240 0 50 100 150 200 250 300 350 Time (h) Fl ux ( c m 3 /h ) -0,5 0 0,5 1 1,5 Wick flow Referecne flow Bias B ias (W ic k f lo w /R ef er enc e f low )

Wick length impact on total extracted volumes

2000 4000 6000 8000 40 60 80 100 120 140 Wick length (cm) C u m u la te d v o lu m e s ( c m 3 )

Ida Silt Loam + AM3/8MD Rehovot Sand + PEP1/2 Comparison of cumulated fluxes in both wick and

reference systems during the 11days simulation y = 1,0629x R2 = 0,9986 y = 1,1973x R2 _{= 0,9914} 0 2000 4000 6000 0 2000 4000 6000

Cumulated reference flux calculated with the nodal velocity (cm3)

C u m u la te d wi ck-e x tr a c te d fl u x t h ro u g h th e s eep ag e l im it ( c m 3 )

Rehovot Sand + Pepperell 1/2 Ida Silt Loam + Amatex 3/8

Error due to measurement uncertainty on wick saturated conductivity

0 3000 6000 9000 0,0 0,5 1,0 1,5 2,0 2,5 Ks / Ks reference C u m u la te d fl u x ( c m 3

) Ida silt loam + AM 3/8 MD Rehovot sand + PEP 1/2