TEMPORAL AND SPATIAL VARIATION OF SOIL WATER CONTENT MEASURED BY BOREHOLE GPR UNDER IRRIGATION AND DRAINAGE

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1 TEMPORAL AND SPATIAL VARIATION OF SOIL WATER CONTENT MEASURED BY BOREHOLE GPR UNDER IRRIGATION AND DRAINAGE L. W. Galagedara, G. W. Parkin Department of Land Resource Science, University of Guelph, Ontario, N1G 2W1, Canada J.D. Redman Sensors and Software Inc., Mississauga, Ontario, L4W 3R7, Canada A. L. Endres Department of Earth Science, University of Waterloo, Ontario, N2L 3G1, Canada ABSTRACT Borehole ground penetrating radar (GPR) was used to measure the temporal and spatial variability of soil water content under uniform wetting and drying conditions. Zero Offset Gather (ZOG) surveys were conducted before and during the uniform infiltration and the subsequent drainage experiments using the PulseEKKO 100 borehole system with 200 MHz antennas in horizontal access tubes. Time domain reflectometry (TDR) data were collected from 10 vertical probes installed at 0.1 m increments from 0.1 to 1.0 m below the ground surface. The TDR data were used as standard measures of soil water content to compare with the GPR estimated water content. The electromagnetic wave velocity along the survey profile at about 1.0 m below the ground surface was estimated using ZOG data by picking the arrival time of the first event. Volumetric water content was calculated using a standard empirical relationship between velocity and water content for each ZOG location. Measured higher soil water content zones are potentially preferential flow areas and were observed in consistent locations throughout both the wetting and drying experiments. The radius of influence of the borehole GPR measurements was about 0.5 m determined theoretically and by comparing GPR and TDR data. Keywords: water content; preferential flow; wetting front; borehole GPR; zero offset gather; electromagnetic waves; dielectric permittivity. INTRODUCTION Groundwater is one of the most important water resources as far as human and animal sustainability is concerned. This important resource is in danger of being contaminated from agricultural inputs such as fertilizer and pesticides, as well as waste disposal systems such as conventional septic systems and landfills. Many of the farms in Ontario use groundwater for domestic and animal water supply. A recent study shows that about 34% of wells have maximum number of coliform bacteria above the drinking water standard level (Goss et al., 1998). Both organic and inorganic contaminants could enter the groundwater system due to water flow through the soil matrix and via preferential water flow paths. For instance, highly variable and site specific unsaturated zone water movements have been observed by Yoder et al. (2001). Soil water content in the unsaturated zone plays an important role in determining the downward velocity of contaminants. Mapping of temporal and spatial variation of soil water content will therefore provide vital information on water flux and contaminant transport velocity within the vadose zone. No efficient method has been developed as yet to determine the location of preferential flow paths at the field scale. It is necessary to understand how preferential flow paths develop both in the vertical and horizontal direction in the landscape to minimize groundwater contamination. The ground wave velocity measured with surface GPR has been used to estimate soil water content (Chanzy et al., 1996, and Huisman et al., 2001) and soil water content profile (van Overmeeren et al., 1997). Vellidis et al. (1990) mapped the wetting front during uniform irrigation and observed good agreement between water content estimated with surface GPR and gravimetrically estimated water content. The borehole GPR technique has also been effectively used in estimating soil water content variations under conventional septic systems and wastewater disposal trenches (Parkin et al., 2000). In this study, we measured the temporal and spatial variation of soil water content under uniform irrigation and drainage. A similar study was conducted by Redman et al. (2000) under uniform and point sources of irrigation. This previous study lacked

2 independent measurements of the vertical water content profile measured to compare with the GPR data and water content variation during drainage. In the present study, GPR estimated water contents are compared to TDR measured water contents. Our experiment was conducted to achieve three objectives. (I) To estimate the temporal and spatial variation of soil water content under uniform irrigation and drainage with borehole GPR and to compare with TDR estimated water content. (II) To identify potential preferential flow zones within the experimental area. (III) To determine the zone of influence of borehole GPR in estimating the soil water content. METHODOLOGY Test Site The Cambridge Research Station ( ' N, ' W), of the University of Guelph, Ontario Canada, which has a welldrained sandy loam soil was selected for this experiment. The uniform irrigation and drainage experiment was conducted on a relatively flat 7.0 m x 4.0 m area in August 2000, where the length is oriented west to east and the width is oriented north to south (Fig. 1A). A previous GPR survey conducted at this site showed a 6.0 m-thick layer of sandy loam aquifer material overlying a sandy silt aquitard (Parkin et al., 2000). The ground water table was 1.34 m below the surface during the experiment. Two borehole GPR access tubes (PVC) of 6.0 m length and m diameter were installed horizontally at 2.0 m apart (one at 0 m N, 0~6.0 m E and the other at 2.0 m N, 0~6.0 m E) and 1.0 m below the surface (Fig. 1A and 1C). This arrangement provided a 2 m by 5.5 m horizontal plane at 1.0 m depth for the borehole GPR study (Fig 1A, 1B and 1C). Two plywood boxes of 2.0 x 0.5 x 1.0 m (L x W x H) size were installed at the west end of each GPR access tube for lowering the GPR antennas into the horizontal access tubes (Fig. 1C). A flexible tube was connected to each GPR access tube at the east end with the other end of the flexible tube open to the surface (Fig. 1C). Two ropes, which were marked at m intervals, were passed through the GPR access tubes and flexible tubes. One end of the rope was connected to each GPR antenna and the other end was accessible from the open end of the flexible pipe. Both transmitter and receiver antennas were pulled through the tubes using the rope. Uniform infiltration was provided using 28 drip tubes at m spacing laid over the entire area of 7.0 m x 4.0 m (Fig. 1). Each drip line is 7.0 m in length. The uniform irrigation was conducted for 43.7 hours at 0.02 m/hr and was followed by drainage for 8.0 hours. Borehole GPR A PulseEKKO 100 GPR system with 200 MHz borehole antennas was used. Zero Offset Gather (ZOG) surveys were conducted before and during the uniform irrigation and subsequent drainage experiments. In total, four background, five uniform irrigation and twelve drainage surveys were conducted during the experiment. Before each ZOG survey, five traces were recorded by holding both antennas vertically in the air at a 2.0 m separation. These data were used to determine the time zero for wave travel and then to calculate the true wave travel time of the first event for the subsurface measurements. For each ZOG survey, both transmitter and receiver antennas were inserted into the west end of the PVC access tube and, both antennas were moved through the access tubes at m increments by pulling on the ropes from the east end. For each location of the transmitter and receiver, a single trace was recorded, resulting in a total of 45 traces for each survey. Time Domain Reflectometry (TDR) Three TDR transects were placed at 0 m N, 1.0 m N and 2.0 m N in the experimental area (Fig. 1A and 1C). Each transect had two replicates of TDR wave-guides of 10 depths, where in each replicate, wave-guide depths ranged from 0.1 to 1.0 m in a 0.1 m increment. Sixty wave-guides in total were connected to a Tektronix 1502 C cable tester using four Dynamax multiplexers. Volumetric water contents were measured for background conditions and during the irrigation and drainage at 10 minute intervals. Volumetric Water Content from GPR data First arrival travel time of ZOG data for each transmitter and receiver location was used to calculate the electromagnetic wave velocity (V). The relative dielectric permmittivity (K a ) of the soil for each transmitter and receiver location was then calculated according to Davis and Annan (1989): c 2 K a := (1) V 2 where c is speed of light in free space (c = 3.0 x 10 8 m/s). The empirical relationship developed by Topp et al. (1980) was used to calculate the volumetric water content of the soil from the estimated relative dielectric permittivity values for GPR and for TDR measurements.

3 0 m N 1.0 m N 2.0 m N 0 m E Plywood Boxes Borehole GPR Access Tubes (at 1.0 m depth) 3.0 m E Vertical TDR Probes (2 replicates per each line) Irrigation Drip Lines 2.0 m Soil Surface 1.0 m 5.5 m E Flexible Pipes Manifold Access Tubes For Borehole GPR (A) (B) TDR probe lengths: 0.1m 0.5m 1.0m Soil Surface Plywood Box 1.0 m TDR Rep. 1 TDR Rep. 2 Flexible Pipe 0 m E Survey Direction (W to E) GPR Access 3.0 m E Tube 5.5 m E (C) Figure 1: Field Layout of the Uniform Irrigation and Drainage Experiment. (A) Plan View; (B) Cross Section; (C) Longitudinal Section.

4 RESULTS AND DISCUSSION Water Content Estimated with Borehole GPR Figure 2 shows water content measurement using borehole GPR during the uniform irrigation (Fig. 2A) and drainage (Fig. 2B) experiments. It is apparent in Fig. 2A that the wetting front has not arrived at the depth of influence of the borehole GPR during the initial 4.5 hr of uniform irrigation. Two relatively high water content zones were observed prior to irrigation at around 2.5 m E and 5.75 m E (circled in both graphs). During irrigation and drainage, these zones remained relatively high in soil water content. These wetter zones can be considered as potential preferential flow areas. Water Content (m 3 /m 3 ) Water Content (m 3 /m 3 ) Irrigation Drainage Figure 2: Water content estimated by borehole GPR during the uniform irrigation and drainage experiments (Legends show the elapsed time in hours in respective experiments. Circled zones are inferred preferential flow areas). Fig. 2A shows that steady state water content was achieved after about 27 hours of irrigation. In the drainage graph (Fig. 2B), water content decrease with time was very uniform at each measurement point. A B Comparison of GPR and TDR data Figure 3 compares the water content changes estimated with GPR and TDR methods during the irrigation (Fig 3A) and drainage (Fig. 3B) experiments. In both the wetting and drying experiments, calculated water content changes with ZOG data have much better agreement with calculated water content changes for greater TDR depths (0~0.7, 0~0.8, 0~0.9 and 0~1.0 m) than shallower depths (0~0.2, 0~0.4 and 0~0.5 m). Water Content Increase (m 3 /m 3 ) Water Content Decrease (m 3 /m 3 ) GPR TDR_0.2m TDR_0.4m TDR_0.6m TDR_0.7m TDR_0.8m TDR_0.9m TDR_1.0m GPR TDR_0.2m TDR_0.4m TDR_0.6m TDR_0.7m TDR_0.8m TDR_0.9m TDR_1.0m Figure 3: Water content changes calculated with GPR and TDR data during the wetting and drying experiments. Fig. 3A is for 43.7 hours of wetting and Fig. 3B is for 8.0 hours of drying. (Depths of TDR probes are given in respective legends). Water content estimated during drainage with GPR and for two different TDR depth intervals are plotted versus time in Figure 4. The same distance in west to east direction was selected for both GPR and TDR data. Good agreement was found between GPR and TDR data for greater TDR depths (0~0.9 m). For shallower TDR probes, the difference between GPR and TDR estimated water contents is greater, with the TDR values giving lower water contents. A B

5 Water Content (m 3 /m 3 ) Water Content (m 3 /m 3 ) 0.30 TDR at 0~0.4 m Depth GPR 4.3 m E Time in hours of Drainage TDR at 0~0.9 m Depth Figure 4: Water content estimated with GPR method and TDR at different depths during the drainage. Zone of Influence of the GPR Method TDR 4.3 m E GPR 5.3 m E TDR 5.3 m E Time in hours of Drainage The zone of influence for borehole GPR is often defined in terms of the Fresnel zone as a 3-D volume, referred to as the Fresnel volume (Cerveny and Soares, 1992). This definition of the zone of influence applies to monochromatic signals, but equivalent definitions exist for broadband signals if the bandwidth of the source wavelet is used as the frequency in the Fresnel zone calculation. The Fresnel volume is an elongated rotational ellipsoid with its foci at the locations of the transmitter and receiver. The Fresnel zone, a circular region, is the cross-section of the Fresnel volume in a plane perpendicular to the raypath. The maximum diameter of the Fresnel zone along the longest raypath is often given as the spatial resolution in tomography. The size of the Fresnel volume depends on the path length, the centre frequency or bandwidth of the transmitted pulse and the velocity within the medium. The Fresnel zone radius was calculated for the background, at the end of the irrigation and at the end of the drainage experiments. Exact distance between borehole radar access tubes from survey data were found to be 2.13 m and 1.95 m at m E and 5.75 m E, respectively with an average distance of 2.04 m. Calculated maximum, minimum and average first Fresnel zone radii for three different water contents from m E to 5.75 m E points at a m interval are given in Table 1. Table 1: Calculated first Fresnel zone radii in m (respective water contents are given in brackets in m 3 /m 3 ). Experiment Time Maximum Minimum Average Before Irrigation 0.57 (0.16) At the End of 0.50 Irrigation () At the end of 0.54 Drainage (0.18) 1 YL ( ) YL ( ) YYL ( ) YYL ( ) () 0.44 (0.19) 0.48 (0.13) 0.54 (0.13) 0.47 (0.22) 0.52 () LL,, L, L Figure 5: Zone of influence of GPR method for water content estimation (solid area for the background water content and the area inside the solid line for the water content at the end of irrigation experiment). The zones of influence for background conditions and at the end of the irrigation are shown in Figure 5. The zone of GPR influence is in the range of 0.4 ~ 0.5 m above and below the middle of the antennae depth. This depth range is consistent with the results of comparing TDR and GPR water content measurements in Figures 3 and 4. The Fresnel zone radius decreased about 12% during the irrigation and increased about 10% during the drainage. The changes in the Fresnel zone radius were due to a 76% increase in water content during irrigation and a 48% decrease in water content during drainage. By integrating over the average zone of influence (Fig. 5), the average longitudinal cross sectional area of the Fresnel volume either in vertical or horizontal plane was estimated to be 1.9 m

6 The zone of influence calculated in this way may not be adequate for the infiltration experiment where the transition from the wetter infiltration front to the relatively dry soil may be quite sharp. In this case, the velocity of the first arrival will be controlled by the high velocity zone sandwiched between the infiltration front and the saturated zone at a depth of about 1.3 m. This situation will persist until the infiltration front is close to the receiving and transmitting antennas at a depth of 1.0 m. CONCLUSION Spatial and temporal variations of soil water content estimated during irrigation and drainage experiments with borehole ground penetrating radar (GPR) in zero offset gather (ZOG) survey mode were measured. High water content zones detected with GPR, which are possible preferential flow areas, were very consistent in space during both wetting and drying conditions. It was also found that the water contents estimated with borehole GPR access tubes at 1.0 m depth, agreed much better with water contents measured using TDR within 0.5 m of the access tubes. The zone of influence of the borehole GPR method was calculated using the first Fresnel zone radius and this zone agreed well with the potential GPR sampling area estimated by comparing the GPR data to the TDR data. The longitudinal cross sectional areas of the Fresnel volume along the ray path (zones of influence) in vertical or horizontal plane for this site during the entire experiment were found to be 2.04, 1.73, and 1.94 m 2 for background, end of the irrigation and end of the drainage, respectively. Results show the potential ability of borehole GPR method for routine field applications such as mapping preferential flow patterns. Further experiments with TDR probes at deeper depths are recommended to support these results. ACKNOWLEDGEMENTS Authors wish to acknowledge Centre for Research in Earth and Space Technology (CRESTech), Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), and Natural Science and Engineering Research Council of Canada (NSERC) for providing the financial support. GPR instruments and advice provided by Dr. A.P. Annan of Sensors and Software Inc., Ontario was highly appreciated. REFERENCES Cerveny, V. and Soares, J.E.P., Fresnel volume ray tracing, Geophysics, Vol. 57, pp Chanzy, A., Tarussov, A., Judge, A. and Bonn, F., Soil Water Content Determination Using a Digital Ground-Penetrating Radar, Soil Science Society of America Journal, Vol. 60, pp Davis, J. L. and Annan, A. P., Ground Penetrating Radar for High Resolution Mapping of Soil and Rock Stratigraphy, Geophysical Prospecting, Vol. 37, pp Goss, M.J., Barry, D.A.J. and Rudolph, D.L., Contamination in Ontario Farmstead Domestic Wells and Its Association with Agriculture: 1. Results from Drinking Water Wells, Journal of Contaminant Hydrology, Vol. 32, pp Huisman, J.A., Sperl, C., Bouten, W. and Verstraten, J.M Soil Water Content Measurements at different Scales: Accuracy of Time Domain Reflectometry and Ground Penetrating Radar, Journal of Hydrology, Vol. 245, pp Parkin, G., Redman, D., von Bertoldi, P. and Zhang, Z., Measurement of Soil Water Content below a Wastewater Trench using Ground Penetrating Radar, Water Resources Research, Vol. 36, No. 8, pp Redman, D., Parkin, G., and Annan, A. P., Borehole GPR measurement of soil water content during an infiltration experiment, Proceedings on Ground Penetrating Radar Conference, Gold Coast, Australia, May 23-May26, 2000, p Topp, G. C., Davis, J. L. and Annan, A. P., Electromagnetic Determination of Soil Water Content: Measurements in Coaxial Transmission Lines, Water Resources Research, Vol. 16, No. 3, pp Van Overmeeren, R. A., Sariowan, S. V. and Gehrels, J. C., Ground Penetrating Radar for Determining Volumetric Soil Water Content; Results of Comparative Measurements at Two Test Sites, Journal of Hydrology, Vol. 97, pp Vellidis, G., Smith, M.C., Thomas, D.L. and Asmussen, L.E., Detecting Wetting Front Movement in a Sandy Soil with Ground Penetrating Radar, Transaction of American Society of Agricultural Engineers, Vol. 33, No. 6, pp Yoder, R.E., Freeland, R.S., Ammons, J.T. and Leonard, L.L., Mapping Agricultural Field with GPR and EMI to Identify Offsite Movement of Agrochemicals, Journal of Applied Geophysics, Vol. 47, pp