Soil water electrical conductivity determination based on the salinity index concept

Transcription

1 European Water 59: , E.W. Publication Soil water electrical conductivity determination baed on the alinity index concept G. Karga *, P. Mougiou, A. Petetidi and P. Kerkide Department of Natural Reource Development and Agricultural Engineering, Agricultural Univerity of Athen, Greece * karga@aua.gr Abtract: Key word: The dielectric device can provide imultaneouly and for the ame pot of oil, meaurement of the apparent oil dielectric permittivity (ε ) and bulk oil electrical conductivity (σ b ). For the prediction of the electrical conductivity of the oil olution (σ p ), the method baed on the concept of alinity index (X ) i propoed. In the tandard method X i defined a the partial derivative of the bulk oil electrical conductivity (σ b ) with repect to the apparent dielectric permittivity (ε ). The evaluation of thi approach i made by meaurement with a dielectric enor (i.e the WET) in different porou media at different level of electrical conductivity of the moitening KCl-olution (EC i ). From the reult it i hown that the relationhip between the σ b and ε i trongly linear for σ p and σ b value up to 7.4 and 1.74 ds m -1 repectively. The alinity index model performance at predicting σ p value i atifactory with RMSE value lying between ds.m -1 for the maximum electrical conductivity value of the moitening KCl-olution (EC i ). oil alinity, bulk electrical conductivity, apparent dielectric permittivity, dielectric enor 1. INTRODUCTION The oil alinity aement i mainly baed on meaurement of the electrical conductivity of the aturated pate extract, (EC e ) which ha been etablihed a the tandard method (U.S. Salinity Laboratory, 1954). However, the σ p i a better index of oil alinity than the EC e ince the plant root actually experience the olute concentration of the oil olution. The extraction of oil olution i often impractical and can only be implemented at quite high value of oil moiture (θ). To overcome thi difficulty in the determination of σ p indirect method have been propoed which are baed on the determination of both θ and σ b. Bulk oil electrical conductivity could be regarded a the combined effect of the electrical conductivity of the oil olution and olid phae, a well a, the interaction between thee. A direct conequence of the above i that σ b i related to σ p, θ and the oil characteritic (e.g. through poroity, clay content, inert or charged olid urface etc.). Until ome decade ago, σ b wa generally meaured by the Wenner device or the four electrode method (Corwin, 2002; Corwin and Hendrickx, 2002). The development of the dielectric enor, allowing imultaneou, automatic σ b and ε meaurement in the ame pot made the σ p etimation eaier and contributed ubtantially in the tudy of alt movement in oil. A number of model, in which σ b, θ and σ p or σ b, ε and σ p are related to each other, were propoed (Amente et al., 2000; Hilhort 2000; Malicki and Walczak, 1999; Mualem and Friedman, 1991; Rhoade et al., 1976). The relationhip σ b -ε ha been hown to be linear for ε >6.2 (Karga and Kerkide 2010, 2012). Malicki and Walczak (1999) introduced the term of oil alinity index (X ) decribing oil alinity tatu, a one of many application of the TDR technique. The alinity index wa defined a the partial derivative of σ b with repect to ε (equation 1) and appeared independent of the moiture content. The two variable from which the alinity index wa calculated were determined by TDR. X σ b = ε (1)

2 344 G. Karga et al. In practice, X i obtained through the appropriate expreion, (valid for σb>0.08 ds m -1 and ε >6.2) X σ b 0.08 = ε 6.2 (2) where σ b =0.08 ds.m -1 and ε =6.2 i the point where the line σ b (ε ;σ p ) converge. Salinity index i generally independent of θ, for the range θ>0.12 cm 3 cm -3 up-to aturation, where correponding ε value are uually larger than 6.2, but it ha been hown that thi i practically true for θ>0.2 cm 3 cm -3 (Wilczek et al., 2012). Malicki and Walczak (1999) reported that alinity index depend mainly on σ p and oil texture. X The σ p can be calculated for mineral oil if alinity index and the lope ( l = ) of the ECi alinity index and the electrical conductivity of the wetting olution (EC i ) are known. It ha been hown that l can be etimated from an empirical relationhip baed on and content. The propoed relationhip for σ p etimation i X σ p = = l σ 0.08 b ( ε 6.2)( S) (3) where S (and content) i expreed in on a ma bai. Wilczek et al. (2012) applied the model of Malicki and Walczak (1999) with data obtained by an FDR technique. In an attempt to minimize the effect of the bulk electrical conductivity on ε, they meaured the real part of the dielectric permittivity in an operating frequency range MHz. The WET dielectric enor ha the capacity to predict the water content through the ε, of the oil. It can alo meaure the σ b, through the imaginary part of the relative dielectric permittivity and the temperature of the oil (Delta T Device Ltd, 2007). The objective of the preent work i to evaluate the oil olution electrical conductivity σ p, predicted by the Malicki and Walczak model. To thi end, WET enor meaurement were made in four different oil uing four different level of the electrical conductivity of the moitening olution. 2. MATERIALS AND METHODS 2.1 WET enor All meaurement in the preent tudy were taken with a WET enor, a capacitance dielectric enor which operate at a frequency of 20 MHz. The enor conit of three metal rod, 6.8 cm long, 3 mm in diameter, and paced 1.5 cm apart. The volume ampled i cylindrical, 6.8 cm long, with a diameter of 10 cm for a total volume of about 500 ml (Delta-T Device Ltd., 2007). The central rod i covered by a coating material except at the tip, which act a a oil temperature meter. The enor wa connected to an HH2 moiture meter. The enor detect the change of the 20 MHz electromagnetic ignal and end thi information to the HH2, which calculate the capacitance, conductance and temperature of the material between the rod (i.e. the oil). The capacitance and conductance are ued to determine the ε and σ b uing built-in calibration file. In addition to oil temperature, the calculated output are θ and σ p, the latter baed on the model of Hilhort (2000). Soil water content i calculated from ε uing a imple regreion formula that relate θ to ε

3 European Water 59 (2017) 345 θ = a ε b (4) where a and b are fitting parameter. Since the WET enor output ε, the calibration conit of fitting the coefficient a and b, the value of which are known to vary with oil type. Texture baed value uggeted by the manufacturer are, for inorganic oil in general, a = and b = 0.178; for organic oil and and, a = and b = 0.167; and for clay, a = and b = (Delta- T Device Ltd., 2007). 2.2 Meaurement in oil Four porou material were ued that were air dried and put through a 2 mm ieve. The characteritic of oil are given in Table 1. The attained bulk denitie for the four oil (1, 2, 3 and 4) teted were: 1.44, 1.25,1.38, 1.52 g cm -3 repectively. Soil Sand Silt Table1. Phyicochemical characteritic of teted oil. Clay ph Organic matter CaCO 3 Cation Exchange Capacity (mmol c /kg) Saturation extract EC e (ds m -1 ) The dry oil ample were moitened with KCl-olution of known electrical conductivity (EC i =0.28, 1.2, 3 and 6 ds m -1 ) to predetermined moiture content, from dry until almot aturated with contant tep of Δθ=0.05 cm 3 cm -3. Thi could eaily be achieved ince the volume of the oil ample i etimated from it dry ma and it bulk denity and therefore the volume of oil olution to be gradually added i alo known. Moitening of the oil ample wa achieved by adding the predetermined volume of the olution and thoroughly mixing the ample for achieving uniform moiture a a whole. Thi wa repeated for each one of the KCl-olution electrical conductivity. For each oil type, around 30 oil column were prepared. After equilibrating for 24 h in air-tight condition the meaurement were taken with the WET enor been inerted vertically in the centre of the urface of each column. The θ wa calculated uing equation (4). σ p -value were calculated with the Malicki and Walczak model (equation 3). When reaching aturation, the oil ample after equilibrating for 24 h in air-tight condition, wa placed in a Buchner funnel where the oil olution wa extracted through the application of uction and σ p wa meaured by a conductivity meter. Optimal value for a and b could be determined for each oil by linear regreion between meaured θ (θ m ) and ε uing all θ m value for the four alinity level. 3. RESULTS AND DISCUSSION 3.1 Meaured dielectric propertie of oil Figure 1 illutrate the relationhip between ε and θ m for the four porou media and at four alinity (EC i ) level. Alo, the reult obtained by the Topp equation (Topp et al., 1980) are hown (olid line) a a reference. From thee it i obviou that ε value of the oil were larger than thoe of the Topp equation, epecially at larger θ m value. The different repone compared to Topp equation demontrated the need for individual calibration equation for each porou medium (Seyfried and Murdock, 2001).

4 346 G. Karga et al. Figure 1. The relationhip ε -θ m for the WET enor in 1,2,3 and 4 oil for variou alinity level (ds m -1 ) (ymbol). The relationhip according to Topp et al. (1980) i alo hown a a olid line. The relationhip between the θ m and the quare root of the apparent dielectric permittivity i trongly linear for all oil and alinity level (EC i ) (Karga et al., 2014). The lope of the linear relationhip were 0.09, 0.091, and and the intercept 0.173, 0.16, and for 1, 2, 3 and 4 oil with R 2 =0.982, 0.967, and 0.984, repectively. Thee value correponding to EC i =6 dsm -1. Maximum σ b value in the cae EC i =6 ds m -1 when the oil are aturated, are 1.63, 1.74, 1.56, and 1.38 ds m -1 for the 1, 2, 3 and 4 oil. The correponding value of the parameter α and b, however, do not coincide with thoe uggeted for inorganic oil by the manufacturer. For all porou media the α and b value, are lower. The very cloe to each other value, of α and b parameter of the four oil teted are due to their imilar particle ize ditribution, a i hown in Table Etimation of oil olution electrical conductivity (σ p ) In Table 2 the meaured value of σ p at the maximum θ m attained for every oil and EC i teted are preented. Maximum θ m value attained were 0.35, 0.45, 0.40 and 0.35 cm 3 cm -3 for 1, 2, 3 and 4 oil repectively. The difference between EC i and σ p i anticipated to be larger for low EC i value when the σ p value of the oil before adding the moitening olution i rather ignificant. More pecifically, the larget difference wa found in the oil 1, with EC i of 0.28 ds m -1 the σ p at the maximum θ m attained wa 3.65 ds m -1. Table 2. The value of σ p (ds m -1 ) at the maximum θ attained for the four ECi in the four oil teted. EC i (ds m -1 ) σ p (ds m -1 ) σ p (ds m -1 ) σ p (ds m -1 ) σ p (ds m -1 ) In order to tet the concept of alinity index introduced by Malicki and Walczak (1999) the σ b

5 European Water 59 (2017) 347 veru ε value were plotted for all moitening olution (Figure 2, left panel). The relationhip σ b -ε i valid for volumetric oil moiture θ>0.1 cm 3 cm -3 (Malicki and Walczak, 1999). The relationhip σ b =f(ε ; EC i ) i trongly linear (0.99<R 2 <0.999) for all oil and alinity level up to 6 ds m -1 and i independent of water content. Similar reult were obtained by Wilczek et al. (2012) uing the FDR technique. Figure 2. On the left: The relationhip between the σ b v ε for all porou media and alinity level. On the right: The relationhip between alinity index X and EC i for all porou media and alinity level. From the reult (Figure 2, left panel) it i apparent that the alinity index, with data obtained by the WET enor, depend on oil alinity. More pecifically in all four oil teted, the increae of EC i reulted in an increae of the alinity index and alo for the ame value of EC i there were mall difference in the value of the alinity index for the oil examined except for oil 1. Thee mall difference may be attributed to the light difference in the oil particle ize ditribution.

6 348 G. Karga et al. In Figure 2 (right panel) the relationhip between the alinity index and EC i, for the oil teted, i hown. From the reult one may notice that thi relationhip i trongly linear (R ). The lope of the linear relationhip have magnitude very different to thoe reported by Malicki and Walczak (1999) (their legend Figure 2) for imilar oil type. For example, the magnitude of the lope for the oil 1 (Sandy Loam) i , while the repective value of Malicki and Walczak (1999) for the loamy and i Thi difference can be attributed to the relatively low operating frequency of the WET enor compared to TDR. Figure 3. The relationhip between actual σ p value (EC w ) (ds m -1 ) (Vertical axi) and the predicted one (Horizontal axi) according to the model of Malicki and Walczak (open circle, equation 3) (ds m -1 ). Comparing the σ p of the Malicki-Walczak model (equation 3) to the actual σ p value (Figure 3) it appear that the model overetimate σ p up to a threhold of σ p value (around 4 ds m -1 ). Above thi threhold (4 ds m -1 ), the model in all cae underetimate σ p value. Thi underetimation become larger with increaing of σ p. More pecifically for oil 1 when meaured σ p. =6.01 ds m -1 RMSE wa 1.74 while for σ p. =7.6 it wa 2.04 ds m -1. Thi underetimation could be attributed to the large ε value predicted by the WET enor compared to TDR (Figure 1). Generally the Malicki-Walczak model perform better for θ higher than 0.25 cm 3 cm -3. Thu it appear that the WET repone to alinity i more reliable when the mobile water phae i dominant. 4. CONCLUSION It wa found that the meaured WET-ε value depend on oil type and are larger than thoe of the Topp equation. Alo the relationhip between the volumetric water content (θ m ) and the quare root of the apparent dielectric permittivity i trongly linear. The alinity index approach, a thi wa introduced by Malicki and Walczak, making ue of TDR data, i applicable, even when data are collected by the WET enor. The relationhip σ b = f(ε ; EC i ) i trongly linear for all oil and alinity level with maximum σ b around 2 ds m -1. The Malicki- Walczak model approache quite reliably σ p for σ p <4 ds m -1 while above thi value, it underetimate σ p.

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