Improved understanding of unsaturated flow and transport processes is limited by the lack of appropriate in situ measurement techniques. This study was conducted to determine whether two noninvasive ...cross-borehole geophysical methods combined could be used to estimate two important unsaturated zone transport parameters, namely the pore water velocity and longitudinal dispersivity. Cross-borehole electrical resistivity tomography and ground penetrating radar were used to estimate temporal and spatial variation of electrical resistivity and water content, respectively, during a 20-d forced infiltration experiment. The resulting one-dimensional profiles and two-dimensional images of moisture content and electrical resistivity were subsequently combined to estimate solute tracer concentration. The results were used to analyze the downward migration and vertical spreading of water and tracer mass. The two geophysical methods provided independent estimates of soil moisture content and electrical resistivity that were spatially and temporally consistent. The observed changes in moisture content and electrical resistivity were, as a first approximation, used in a one-dimensional moment analysis. The transport behavior was found to be very susceptible to layering of the subsurface. Even slight reductions in grain size apparently lead to flow barriers and associated lateral flow, resulting in tracer mass loss, reduced vertical pore water velocity, and increased longitudinal dispersivity. Synthetic data showed that the estimated unsaturated transport parameters (i.e., pore water velocity and longitudinal dispersivity) and the mass estimate were influenced by the selected electrical resistivity tomography inversion routine. In effect, an overprediction of all three parameters was observed.
Cross-borehole electrical resistance tomography (ERT) experiments utilize downhole electrodes, typically placed in a borehole filled with material of contrasting resistivity to the host rock. The ...circular geometry of the boreholes is three-dimensional, and inversion routines are typically two-dimensional. Because of the resistivity contrast, artifacts in the form of borehole inversion effects develop in the resistivity images. Other 3D effects resulting in inversion artifacts are shadow effects caused by use of a 2D code to invert data from a 3D body located outside the image plane. In both cases the inversion model misrepresents the spatial change in voltages as a corresponding spatial change in resistivity. Borehole inversion effects and shadow effects result as the forward solver attempts to resolve the discrepancy in the voltages for numerous electrode pairs into a meaningful resistivity distribution. Borehole inversion effects are shown to be related to the resistivity contrast between the borehole fill and the host medium, and to borehole diameter.
Borehole inversion effects do not materialize with small diameter boreholes (e.g. ⩽0.1
m) when the fill resistivity contrast is one order of magnitude or less; however, a borehole fill resistivity contrast of two orders of magnitude causes artifacts in the form of sheaths near the boreholes, and a conductive artifact between the boreholes. Larger diameter borehole (e.g. 0.2
m) induce significant borehole inversion effects with as little as one order of magnitude fill resistivity contrast. The general resistivity patterns are similar for the borehole inversion effects. However, the artifacts are amplified as resistivity contrast and/or borehole diameter increase. These results are significant because borehole inversion effects may mask a target heterogeneity or an artifact may be confused for an anomaly resulting in improper actions for site characterizations or remediation strategies. Suggestions of best practices for experimental design to prevent or minimize borehole inversion effects include: minimizing borehole diameter, minimizing borehole fill resistivity contrast, measuring and inputting the resistivity of the borehole fill/water into the model, and inverting changes in resistivity for time-lapse ERT data.
Shadow effects are shown to be related directly to the distance of the target heterogeneity out of the image plane. Our results show that shadow effects become insignificant when the target is between 3 and 5
m outside the 10-m wide image plane. Because the size of the target and resistivity contrast of the target to the host rock are site-specific variables, the actual distance at which shadow effects become insignificant will vary from site to site. To reduce misinterpretations by shadow effects reduce the dipole length of the four electrode measurements and supplement data from other well pairs and/or other methods.
Core Ideas
Vadose zone water is up to 3.1
×
10
12
m
3
in the Chinese Loess Plateau.
The deep vadose zone (below 5 m) stores 92.4% of the total vadose zone water.
Vadose zone water distribution is ...uneven, depending on loess thickness and precipitation.
Vadose zone water accounts for 42.1% of water resources in the Chinese Loess Plateau.
Water stored in the vadose (unsaturated) zone provides the majority of water required by plants and buffers water resources; thus, it is central to understanding ecological and hydrological processes in the Chinese Loess Plateau (CLP) with its thick loess deposits. We used multisource data on soil water content (SWC) and vadose zone thickness, combined with a spatial interpolation method, to quantify the vadose zone water and further deduce the water resource composition in the CLP. Vadose zone water is approximately 3.1 × 10
12
m
3
(±27.5%) in the CLP, 92.4% of which is stored in the deep vadose zone (>5 m and above the groundwater table). The water resources composition of the CLP comprises precipitation, river water, vadose zone water, and the saturated zone water (shallow groundwater), accounting for 2.1, 0.1, 42.1, and 55.7%, respectively. Although a large amount of water exists in the vadose zone, the SWCs in the upper (<5 m) and deep vadose zones are 47.4 and 65.3%, respectively, of the mean field capacity, both being at a low level. Our findings bridge the knowledge gaps on the deep vadose zone water and water resources composition in the CLP, providing the basis for decision‐making on balancing revegetation and water resources conservation.
•Soil-water carrying capacity of three revegetation species was determined.•Optimal plant cover (LAI) or biomass (NPP) varied across the Loess Plateau.•Climate, soil texture and elevation drive ...variations in LAI and NPP.•A revegetation threshold was recommended for future revegetation activities.
Re-vegetation is a necessary control measure of soil erosion in the Loess Plateau. However, excessive re-vegetation can aggravate soil water shortage, which can in turn threaten the health and services of restored ecosystems. An optimal plant cover or biomass (i.e., soil-water carrying capacity for vegetation, SWCCV) is important for regional water balance, soil protection and vegetation sustainability. The objective of this study was to determine the spatial distribution of SWCCV for three non-native tree (Robinia pseudoacaia), shrub (Caragana korshinskii) and grass (Medicago sativa) species used in the re-vegetation of the Loess Plateau. The dynamics of actual evapotranspiration (AET), net primary productivity (NPP) and leaf area index (LAI) were simulated using a modified Biome-BGC (Bio-Geochemical Cycles) model. Soil and physiological parameters required by the model were validated using field-observed AET for the three plant species at six sites in the study area. The validated model was used to simulate the dynamics of AET, NPP and LAI for the three plant species at 243 representative sites in the study area for the period 1961–2014. The results show that spatial distributions of mean AET, NPP and LAI generally increased from northwest to southeast, much the same as mean annual precipitation (MAP) gradient. In terms of maximum LAI, the ranges of optimal plant cover were 1.1–3.5 for R. pseudoacaia, 1.0–2.4 for C. korshinskii and 0.7–3.0 for M. sativa. The corresponding SWCCV, expressed as NPP were 202.4–616.5, 83.7–201.7 and 56.3–253.0 g C m−2 yr−1. MAP, mean annual temperature, soil texture and elevation were the main variables driving SWCCV under the plant species; explaining over 86% of the spatial variations in mean NPP in the study area. Further re-vegetation therefore needs careful reconsideration under the prevailing climatic, soil and topographic conditions. The results of the study provide a re-vegetation threshold to guide future re-vegetation activities and to ensure a sustainable eco-hydrological environment in the Loess Plateau.
The surface pattern of vegetation influences the composition and humification of peat laid down during the development of a bog, producing a subsurface hydrological structure that is expected to ...affect both the rate and pattern of water flow. Subsurface peat structures are routinely derived from the inspection of peat cores. However, logistical limits on the number of cores that can be collected means that the horizontal extent of these structures must be inferred. We consider whether subsurface patterns in peat physical properties can be mapped in detail over large areas with ground‐penetrating radar (GPR) and complex conductivity by comparing geophysical measurements with peat core data along a 36 m transect through different microhabitats at Caribou Bog, Maine. The geophysical methods show promise. Peat horizons produced radar reflections because of changes in the volumetric moisture content. Although these reflections could not be directly correlated with the peat core data, they were related to the depth‐averaged peat properties which varied markedly between the microhabitats. Well‐decomposed peat below a hollow was characterized by a discontinuous sequence of chaotic wavy reflections, while distinct layering of the peat below an area of hummocks coincided with a pattern of parallel planar reflections. The complex conductivity survey showed spatial variation in the real and imaginary conductivities which resulted from changes in the pore water conductivity; peat structures may also have influenced the spatial pattern in the complex conductivity. The GPR and complex conductivity surveys enabled the developmental history of the different microhabitats along the studied transect to be inferred.
The impact of riparian wetlands on the cycling, retention and export of nutrients from land to water varies according to local environmental conditions and is poorly resolved in catchment management ...approaches. To determine the role a specific wetland might play in a catchment mitigation strategy, an alternative approach is needed to the high-frequency and spatially detailed monitoring programme that would otherwise be needed. Here, we present a new approach using a combination of novel and well-established geochemical, geophysical and isotope ratio methods. This combined approach was developed and tested against a 2-year high-resolution sampling programme in a lowland permeable wetland in the Lambourn catchment, UK. The monitoring programme identified multiple pathways and water sources feeding into the wetland, generating large spatial and temporal variations in nutrient cycling, retention and export behaviours within the wetland. This complexity of contributing source areas and biogeochemical functions within the wetland were effectively identified using the new toolkit approach. We propose that this technique could be used to determine the likely net source/sink function of riparian wetlands prior to their incorporation into any catchment management plan, with relatively low resource implications when compared to a full high-frequency nutrient speciation and isotope geochemistry-based monitoring approach.
Frequency domain electromagnetic induction (EMI) methods have had a long history of qualitative mapping for environmental applications. More recently, the development of multi-coil and ...multi-frequency instruments is such that the focus has shifted toward inverting data to obtain quantitative models of electrical conductivity. However, whilst the collection of EMI data is relatively straightforward, inverse modeling is more complicated. Furthermore, although several commercial and open-source inversion codes exist, there is still a need for user-friendly software that can bring EMI inversion to a non-specialist audience. Here the open-source EMagPy software is presented as an intuitive approach to modeling EMI data. It comprises a graphical user interface (GUI) and a Python application programming interface (API) that is more suitable for specialized tasks. EMagPy implements both cumulative sensitivity and Maxwell-based forward operators and can model data for 1D and quasi-2D/3D cases using either deterministic or probabilistic methods. The EMagPy GUI has a logical ‘tab-based’ layout to lead the user through data importing, data filtering, inversion, and plotting of raw and inverted data. Additionally, a dedicated forward modeling tab is presented that allows the generation of synthetic data. In this publication, necessary considerations, and background, of EMI theory are described before EMagPy's capabilities are presented through a series of synthetic and field-based case studies. Firstly, the performance of cumulative sensitivity and Maxwell-based forward models, and the influence of measurement noise are assessed for synthetic cases. Then the importance of data calibration for a riparian wetland dataset, the ability to include a priori information for a river-borne survey, and the potential for monitoring soil moisture in a time-lapse example are all investigated. It is anticipated that EMagPy offers a user-friendly tool suitable for novice and experienced practitioners alike, and its intuitive nature means it can provide a useful tool for teaching purposes.
•EMagPy is an open-source Python API and GUI for 1D EMI modeling/inversion.•Both Maxwell-based and cumulative sensitivity forward models are implemented.•Inversion algorithms include deterministic and probabilistic methods.•The cumulative sensitivity function of McNeill (1980) performs well in low EC environments.•Capabilities of EMagPy are shown through case studies with real and synthetic data.
Characterization of the dynamics of moisture migration in the unsaturated zone of aquifers is essential if reliable estimates of the transport of pollutants threatening such aquifers are to be made. ...Electrical geophysical investigation techniques, such as ground‐penetrating radar, offer suitable methods for monitoring moisture content changes in the vadose zone. Moreover, these tools permit relatively large measurement scales, appropriate for hydrological models of unsaturated processes, and thus they offer a distinct advantage over conventional measurement approaches. Ground‐penetrating radar, when applied in transmission mode between boreholes, can provide high‐resolution information on lithological and hydrological features. The technique may be applied in tomographic mode and in a much simpler vertical profile mode. Both modes of measurement have been utilized using two boreholes 5 m apart located at a field site in the UK Sherwood Sandstone aquifer. Radar transmission measurements have been used to characterize the change in moisture content in unsaturated sandstone due to controlled water tracer injection. Continual monitoring of cross‐borehole radar measurements over an 18 month period has also permitted determination of travel times of natural loading to the system and has revealed the impact of subtle contrasts in lithology on changes in moisture content over time. The time series of inferred moisture contents show clearly wetting and drying fronts migrating at a rate of approximately 2 m month−1 throughout the sandstone.
Cross-borehole geophysical data can provide valuable information concerning hydrologic properties of the unsaturated zone. Such data are most often used sequentially, where images of soil physical ...properties are obtained through numerical inversion and then converted to hydrologic state properties using petrophysical relationships. If not accounted for, inversion artifacts are transferred to the resulting hydrologic images. We propose a framework in which multiple geophysical data sets can be incorporated using an integrated data fusion approach. The geophysical data collected are integrated in a forward modeling approach to evaluate a series of plausible hydrologic models. The approach permits an evaluation of the sensitivity of geophysical data for constraining hydrologic model parameters. We illustrate the approach using geophysical data collected during a dual water and solute tracer experiment in the unsaturated zone. Cross-borehole ground penetrating radar and electrical resistivity tomography, measuring electromagnetic travel time and electrical transfer resistances, respectively, were collected during a 20-d period. As a first approximation, one-dimensional flow was considered and three models (one, two, and five layers) of the subsurface were evaluated. The five-layered model was found to be the only model capable of mimicking the infiltration pattern satisfactorily. The results showed that only the hydraulic conductivity and one of the parameters (empirical parameter n) describing the soil moisture release curve for three of the five layers could be constrained by the data, illustrating the nonuniqueness of the problem. The data fusion approach proved, however, that application of multiple geophysical methods may reduce hydraulic parameter uncertainty.
Understanding the processes controlling recharge to aquifers is critical if accurate predictions are to be made on the fate of contaminants in the subsurface environment. In order to understand fully ...the hydrochemical mechanisms in the vadose zone it is essential that the dynamics of the hydrology can be suitably characterised. The correlation between moisture content and both bulk dielectric and resistivity properties of porous media is well established. Using suitably placed sensors in boreholes detailed depth profiles of dielectric and resistivity behaviour have been monitored over a period of two years at a Triassic Sherwood Sandstone aquifer field site at Hatfield, England. The borehole–borehole transmission radar and borehole resistivity profiles show a significant correlation. Through appropriate petrophysical relationships, derived from core samples, seasonal dynamics of the vadose zone are seen to illustrate the migration of wetting and drying fronts over the monitoring period. At a second field site in Eggborough, located 17km from Hatfield, similar temporal changes in moisture content in the sandstone were observed using borehole radar profiles. Travel times of seasonal wetting fronts through the sandstone at both sites appear to be approximately 2m per month. The retardation of this front propagation in the top 3m is also common to both sites, suggesting that pollutant transport may be principally controlled by near surface sediments. The results have important consequences to existing groundwater modelling programmes that are being utilised to predict transfer of agricultural chemicals through the vadose zone.
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