NS21B-01 INVITED
Mapping Soil Structure, Identification and Monitoring of Soil processes
As in other domains of earth exploration, geophysical surface analysis tools are very well adapted to the 3D mapping of soil structure. They generate well-sampled information which can be used for plotting large-scale soil maps as a guide for determining water and fertilizer requirements in precision agriculture, can be used to assist with the delineation of polluted areas, … In the case of small soil volumes they can also be used to localise cracks and preferential flow paths. Electrical measurement methods are the most suitable for the above applications because of the sensitivity of electrical conductivity to clay and water content, as well as to salinity. Dielectric permittivity exhibits the most direct relationship to free liquid water content, but GPR necessitates very short sampling intervals, and TDR measurements are limited to water content monitoring at one or several specific points. Electrical resistivity measurements have been successful in monitoring spatial soil characteristics, to follow both structural changes such as crack opening, and water displacements such as liquid uptake by plants. Self potential is sensitive to the presence of on-going redox biological activity, and 'streaming potential' is expected to provide a direct assessment of Darcy's velocity as do temperature variations. Indirectly, all of these parameters may help in the determination of hydraulic conductivity. Apart from short-term changes, on a daily to seasonal scale, long term changes such as pedogenesis processes on a secular scale and anthropogenic influences are revealed by variations in magnetic properties, which can be charted using both magnetic and electromagnetic prospection methods.
NS21B-02 INVITED
ECa-Directed Soil Sampling for Characterizing Spatial Variability: Monitoring Management- Induced Change
Characterizing spatial variability is an important consideration of any landscape-scale soil-related problem. Geospatial measurements of apparent soil electrical conductivity (ECa) are useful for characterizing spatial variability by directing soil sampling. The objective of this presentation is to discuss equipment, protocols, sampling designs, and a case study of an ECa survey to characterize spatial variability. Specifically, a preliminary spatio-temporal study of management-induced changes to soil quality will be demonstrated for a drainage water reuse study site. The spatio-temporal study used electromagnetic induction ECa data and a response surface sampling design to select 40 sites that reflected the spatial variability of soil properties (i.e., salinity, Na levels, Mo, and B) impacting the intended agricultural use of a saline-sodic field in California's San Joaquin Valley. Soil samples were collected in August 1999 and April 2002. Data from 1999 indicate the presence of high salinity, which increased with depth, high sodium adsorption ratio (SAR), which also increased with depth, and moderate to high B and Mo, which showed no specific trends with depth. The application of drainage water for 32 months resulted in leaching of B from the top 0.3 of soil, leaching of salinity from the top 0.6 m of soil, and leaching of Na and Mo from the top 1.2 m of soil. The leaching fraction over the time period from 1999-2002 was estimated to be 0.10. The level of salinity in the reused drainage water (i.e., 3-5 dS/m) allowed infiltration and leaching to occur even though high sodium and high expanding-lattice clay levels posed potential water flow problems. The leaching of salinity, Na, Mo, and B has resulted in increased forage yield and improved quality of those yields. Preliminary spatio-temporal analyses indicate at least short-term feasibility of drainage water reuse from the perspective of soil quality when the goal is forage production for grazing livestock. The implications of this research extend well beyond the provincial applications of assessing drainage water reuse in central California to the global potential of ECa-directed soil sampling for evaluating farm-induced management ramifications on soil and for characterizing soil spatial variability at field scales and larger spatial extents.
NS21B-03
Assessment Of Shallow Groundwater Connexions Between Streambed And Riverbanks: An Approach Combining Physico-chemical, Hydrological, Pedological and Geophysical Measurements
This study aims at assessing the connexions between streambed and riverbanks shallow groundwater. Indeed it influences major phenomenon such as soil loss linked to the collapse of riverbanks or the chemical and biological conditions of the riparian agro-ecosystems. These connexions can vary a lot at catchment scale depending on numerous factors such as topography, streambed shape (width, slope and tortuosity) and subsurface hydraulic properties. A multidisciplinary approach was carried out in November 2005 in Northern Lao P.D.R for a cultivated catchment located in mountainous environment. This approach combines (1) physico-chemical measurements both for surface and ground waters approximately every 20m along the stream; (2) Continuous Wave ElectroMagnetic (CWEM) measurements of apparent electrical conductivity at low induction number along the stream; (3) hydraulic gradient measurements both on left and right riverbanks along the stream; (4) High resolution Direct Current 2D Electrical Resistivity Tomography (ERT) with 0.4m inter-electrode spacing along ten 75m long transects perpendicular to the stream; (5) hand auger drilling to characterize soil cover for five of these transects. At catchment scale, water electrical conductivity and pH measurements allows to discriminate surface from ground waters. Hence, the measurements reveal numerous discrete groundwater inflows along streambed. The highest physico-chemical anomalies also correspond to CWEM apparent electrical conductivity anomalies. The calculation of the so-called "formation factor" allows to estimate the range of porosity variations within the saturated zone. The highest values are observed in the central part of the catchment where the stream flows in a swampy plain. At riverbank scale, measurements of hydraulic gradient close to the stream reveal that groundwater inflows and outflows can occur simultaneously respectively on right or on left riverbanks. ERT combined with soil observations allows to link these local asymmetries with land slide occurrence or with differential weathering of geological basement. This multi-disciplinary study provides a clearer understanding of hydrological and physico-chemical characteristics corresponding to variable connexions between stream and shallow ground waters thanks to a detailed assessment of the soil organisation. It also has a major perspective : parameters derived from geophysical surveys (geometry of soil and sediment layers, porosity estimation of saturated zones) are valuable for numeric coupled simulation of surface and ground water flows at riverbank scale as well as catchment scale.
NS21B-04
Short-period Changes of soil Thermal Parameters Determined from Subsurface Temperature data
Ground temperature measurements are used for several types of application (e.g., agronomy, hydrology) but their integration into physical processes is not straightforward. We present here over a year of temperature monitoring at 60 cm depth in a clayey soil with the objective of monitoring water content changes in the shallowest near-surface. We consider the relations between ground temperatures at depth and at the surface using diffusive analytical models (homogeneous half-space and two-layer models) and finite-difference numerical models. For several time windows, we have been able to invert for characteristic hydrogeophysical parameters, i.e. apparent thermal diffusivity for the half-space model and vadose layer thickness for the two- layer model. The inverted thermal diffusivities (1 to $4\times 10^{-7}$ $m^{2}/s$) are consistent with values found in the literature and can be translated into effective saturation. In addition, the time variations of the apparent thermal diffusivity and of the interface depth between non-saturated and saturated soil layers are correlated to seasonal changes of soil water content.
NS21B-05 INVITED
Non-Invasive Methods to Characterize Soil-Plant Interactions at Different Scales
Root water uptake is a dynamic and non-linear process, which interacts with the soil natural variability and boundary conditions to generate heterogeneous spatial distributions of soil water. Soil-root fluxes are spatially variable due to heterogeneous gradients and hydraulic connections between soil and roots. While 1-D effective representation of the root water uptake has been successfully applied to predict transpiration and average water content profiles, finer spatial characterization of the water distribution may be needed when dealing with solute transport. Indeed, root water uptake affects the water velocity field, which has an effect on solute velocity and dispersion. Although this variability originates from small-scale processes, these may still play an important role at larger scales. Therefore, in addition to investigate the variability of the soil hydraulic properties, experimental and numerical tools for characterizing root water uptake (and its effects on soil water distribution) from the pore to the field scales are needed to predict in a proper way the solute transport. Obviously, non-invasive and modeling techniques which are helpful to achieve this objective will evolve with the scale of interest. At the pore scale, soil structure and root-soil interface phenomena have to be investigated to understand the interactions between soil and roots. Magnetic resonance imaging may help to monitor water gradients and water content changes around roots while spectral induced polarization techniques may be used to characterize the structure of the pore space. At the column scale, complete root architecture of small plants and water content depletion around roots can be imaged by magnetic resonance. At that scale, models should explicitly take into account the three-dimensional gradient dependency of the root water uptake, to be able to predict solute transport. At larger scales however, simplified models, which implicitly take into account the heterogeneous root water uptake along roots, should be preferred given the complexity of the system. At such scales, electrical resistance tomography or ground-penetrating radar can be used to map the water content changes and derive effective parameters for predicting solute transport.
NS21B-06
How Does Hydrocarbon Decrease the Electrical Resisitivity of Soils?
The influence of low hydrocarbon (HC) concentrations (especially relevant for biological remediation) on the frequency-dependent electrical properties of soils is exemplarily investigated for n-hexadecane und phenanthrene. To this, spectral induced polarization (SIP) measurements on clean and contaminated model soils (sand and sand-kaolinite mixtures) are compared. The contamination with the non-conductive, non-polar HC resulted, in contrast to the expected increase, mostly in a decrease of the soil resistivity. Up to now, the degradation of HC was described as the only reason for a resistivity decrease associated with contaminations. The experimental results show that the resistivity decrease can be caused by HC itself. The hypothesis of HC (ab)sorption to the water immediately adjacent to the inner solid surface is adopted to explain the resistivity decrease. The HC (ab)sorption still requires a corresponding "desorption" of water molecules and ions from the near surface water. As a result, the resistivity of the bulk water and consequently of the whole rock is decreased. According to the introduced approach, HC sorption and hence the influence on the electrical properties depends on the size of the inner surface area and on the hydrophobicity of the HC. Therefore, the electrical properties of soils with small surface areas, such as sand, are less influenced by low HC concentrations. In contrast, the electrical behavior of soils with large inner surface areas can be appreciably changed by low HC contents.