Characterising groundwater-dominated lowland catchments : the UK Lowland Catchment Research Programme (LOCAR)

This paper reports on a major UK initiative to address deficiencies in understanding the hydro-ecological response of groundwater-dominated lowland catchments. The scope and objectives of this national programme are introduced and focus on one of three sets of research basins – the Pang/Lambourn Chalk catchments, tributaries of the river Thames in southern England. The motivation for the research is the need to support integrated management of river systems that have high ecological value and are subject to pressures that include groundwater abstraction for water supply, diffuse pollution, and land use and climate change. An overview of the research programme is provided together with highlights of some current research findings concerning the hydrological functioning of these catchments. Despite the importance of the Chalk as a major UK aquifer, knowledge of the subsurface movement of water and solutes is poor. Solute transport in the dual porosity unsaturated zone depends on fracture/matrix interactions that are difficult to observe; current experimental and modelling research supports the predominance of matrix flow and suggests that slow migration of a time-history of decades of nutrient loading is occurring. Groundwater flows are complex; catchments vary seasonally and are ill-defined and karst features are locally important. Groundwater flow pathways are being investigated using natural and artificial geochemical tracers based on experimental borehole arrays; stream-aquifer interaction research is using a combination of geophysics, borehole array geochemistry and longitudinal profiles of stream flow and solutes. A complex picture of localised subsurface inflows, linked to geological controls and karst features, and significant longitudinal groundwater flow below the river channel is emerging. Management implications are discussed. Strategies to control surface application of nutrients are expected to have little effect on groundwater quality for several decades, and new modelling tools for decision support have been developed to represent these effects. Conventional modelling approaches are limited by the complexities of the subsurface system; catchment areas are difficult to define, hence tracking pollutant pathways to stream receptors is also problematic. Conventional distributed groundwater models have difficulty in capturing key aspects of the groundwater system. This raises important questions concerning the confidence that can be placed in models as routinely used for decision support and the level of knowledge required for catchmen

Assessing the dynamics of soil salinity with time-lapse inversion of electromagnetic data guided by hydrological modelling

Irrigated agriculture is threatened by soil salinity in numerous arid and semi-Arid areas of the world, chiefly caused by the use of highly salinity irrigation water, compounded by excessive evapotranspiration. Given this threat, efficient field assessment methods are needed to monitor the dynamics of soil salinity in salt-Affected irrigated lands and evaluate the performance of management strategies. In this study, we report on the results of an irrigation experiment with the main objective of evaluating time-lapse inversion of electromagnetic induction (EMI) data and hydrological modelling in field assessment of soil salinity dynamics. Four experimental plots were established and irrigated 12 times during a 2-month period, with water at four different salinity levels (1, 4, 8 and 12 dS m-1) using a drip irrigation system. Time-lapse apparent electrical conductivity (σa) data were collected four times during the experiment period using the CMD Mini-Explorer. Prior to inversion of time-lapse σa data, a numerical experiment was performed by 2D simulations of the water and solute infiltration and redistribution process in synthetic transects, generated by using the statistical distribution of the hydraulic properties in the study area. These simulations gave known spatio-Temporal distribution of water contents and solute concentrations and thus of bulk electrical conductivity (σb), which in turn were used to obtain known structures of apparent electrical conductivity, σa. These synthetic distributions were used for a preliminary understanding of how the physical context may influence the EMI-based σa readings carried out in the monitored transects as well as being used to optimize the smoothing parameter to be used in the inversion of σa readings. With this prior information at hand, we inverted the time-lapse field σa data and interpreted the results in terms of concentration distributions over time. The proposed approach, using preliminary hydrological simulations to understand the potential role of the variability of the physical system to be monitored by EMI, may actually allow for a better choice of the inversion parameters and interpretation of EMI readings, thus increasing the potentiality of using the electromagnetic induction technique for rapid and non-invasive investigation of spatio-Temporal variability in soil salinity over large areas.

3D-electrical resistivity tomography monitoring of salt transport in homogeneous and layered soil samples

Monitoring transport of dissolved substances in soil deposits is particularly relevant where safety is concerned, as in the case of geo-environmental barriers. Geophysical methods are very appealing, since they cover a wide domain, localising possible preferential flow paths and providing reliable links between geophysical quantities and hydrological variables. This paper describes a 3D laboratory application of Electrical Resistivity Tomography (ERT) used to monitor solute transport processes. Dissolution and transport tests on both homogeneous and heterogeneous samples were conducted in an instrumented oedometer cell. ERT was used to create maps of electrical conductivity of the monitored domain at different time intervals and to estimate concentration variations within the interstitial fluid. Comparisons with finite element simulations of the transport processes were performed to check the consistency of the results. Tests confirmed that the technique can monitor salt transport, infer the hydro-chemical behaviour of heterogeneous geomaterials and evaluate the performances of clay barrier

Analysis of time-lapse data error in complex conductivity imaging to alleviate anthropogenic noise for site characterization

Previous studies have demonstrated the potential benefits of the complex conductivity (CC) imaging over electrical resistivity tomography for an improved delineation of hydrocarbonimpacted sites and accompanying biogeochemical processes. However, time-lapse CC field applications are still rare, in particular for measurements performed near anthropogenic structures such as buried pipes or tanks, which are typically present at contaminated sites. To fill this gap, we have developed CC imaging (CCI) results for monitoring data collected in Trecate (northwest Italy), a site impacted by a crude oil spill. Initial imaging results reveal only a poor correlation with seasonal variations of the groundwater table at the site (approximately 6 m). However, it is not clear to which extend such results are affected by anthropogenic structures present at the site. To address this, we performed a detailed analysis of the misfit between direct and reciprocal time-lapse differences. Based on this analysis, we were able to discriminate spatial and temporal sources of systematic errors, with the latter commonly affecting measurements collected near anthropogenic structures. Following our approach, CC images reveal that temporal changes in the electrical properties correlate well with seasonal fluctuations in the groundwater level for areas free of contaminants, whereas contaminated areas exhibit a constant response over time characterized by a relatively high electrical conductivity and a negligible polarization effect. In accordance with a recent mechanistic model, such a response can be explained by the presence of immiscible fluids (oil and air) forming a continuous film through the micro and macropores, hindering the development of ion-selective membranes and membrane polarization. Our results demonstrate the applicability of CCI for an improved characterization of hydrocarboncontaminated areas, even in areas affected by cultural noise

Geoelectrical signatures of redox processes

Common redox potential sensors provide point measurements and are error prone. Destructive sampling can cause contact with air which will influence the redox state. Hence, assessing redox conditions in soil is challenging. Redox sensitive reactions like the degradation of organic contaminants will change water chemistry and geophysical properties. We explored geophysical techniques ERT and SIP to observe the degradation processes of Propylene Glycol (PG), a de-icing chemical used at airports. Two laboratory experiments were performed to monitor changes caused by PG degradation: 1) 3D ERT was measured, every week, in four sand tanks. One rich in oxides. 2) SIP was measured, every three days, in twenty-six oxides rich sand columns. In both experiments PG was applied just above the water table. ERT - When iron and manganese oxides were available, degradation reduced resistivity. When oxides were unavailable, the electrical resistivity increased, most likely due to methane production which reduced water saturation. SIP - An increase of the real conductivity was associated with the metal ions release but independent of the frequency. Also an increase of phase angle and imaginary conductivity at frequencies below 1Hz that precede the ion release was observed in the water

Geophysical characterisation of the groundwater-surface water interface

Interactions between groundwater (GW) and surface water (SW) have important implications for water quantity, water quality, and ecological health. The subsurface region proximal to SW bodies, the GW-SW interface, is crucial as it actively regulates the transfer of nutrients, contaminants, and water between GW systems and SW environments. However, geological, hydrological, and biogeochemical heterogeneity in the GW-SW interface makes it difficult to characterise with direct observations. Over the past two decades geophysics has been increasingly used to characterise spatial and temporal variability throughout the GW-SW interface. Geophysics is a powerful tool in evaluating structural heterogeneity, revealing zones of GW discharge, and monitoring hydrological processes. Geophysics should be used alongside traditional hydrological and biogeochemical methods to provide additional information about the subsurface. Further integration of commonly used geophysical techniques, and adoption of emerging techniques, has the potential to improve understanding of the properties and processes of the GW-SW interface, and ultimately the implications for water quality and environmental health

Influence of tree species and forest land use on soil hydraulic conductivity and implications for surface runoff generation

Forest planting is increasingly being incorporated into land management policies to mitigate diffuse pollution and localised flooding because forest soils are associated with enhanced hydraulic properties and lower surface runoff compared to soils under other vegetation types. Despite this, our understanding of the effects of different tree species and forest land use on soil hydraulic properties is limited. In this study we tested for the effects of two tree species, sycamore (Acer pseudoplatanus) and Scots pine (Pinus sylvestris), subject to contrasting land use systems, namely ungrazed forest and livestock grazed forest, on soil surface saturated hydraulic conductivity (Kfs) at a long term (23 year) experimental site in Scotland. Additionally these forest land use systems were compared to grazed pasture. Kfs was found to be significantly higher under ungrazed Scots pine forest (1239 mm hr− 1) than under ungrazed sycamore forest (379 mm hr− 1) and under both of these forest types than under pasture (32 mm hr− 1). However, this measure did not differ significantly between the sycamore and Scots pine grazed forest and pasture. It was inferred, from comparison of measured Kfs values with estimated maximum rainfall intensities for various return periods at the site, that surface runoff, as infiltration excess overland flow, would be generated in pasture and grazed forest by storms with a return period of at least 1 in 2 years, but that surface runoff is extremely rare in the ungrazed forests, regardless of tree species. We concluded that, although tree species with differing characteristics can create large differences in soil hydraulic properties, the influence of land use can mask the influence of trees. The choice of tree species may therefore be less important than forest land use for mitigating the effects of surface runoff

Limitations and considerations for electrical resistivity and induced polarization imaging of riverbed sediments:Observations from laboratory, field, and synthetic experiments

Characterization of riverbed sediments is important for understanding groundwater (GW) and surface water (SW) interactions, and their consequent implications for ecological and environmental health. There have been numerous studies using geoelectrical methods for GW-SW interaction studies; however, most applications have not focused on obtaining quantitative information. For instance, although numerous laboratory studies highlight the relationship between geoelectrical properties and relevant parameters (e.g. specific surface area, hydraulic conductivity, and cation exchange capacity), such relationships are not commonly applied to field-scale studies. Furthermore, in addition to the spatial resolution obstacles typically present when applying petrophysical models to field data, geoelectrical data from aquatic environments have complications arising from the presence of a conductive water column overlying a resistive bed. Inadequate consideration of these complications may further preclude the reliable use of such petrophysical models. In this work, laboratory measurements, synthetic modeling, and field measurements were conducted in a third-order river where the riverbed comprises alluvial gravel and underlying red sand. A strong relationship (R2 = 0.72) between imaginary conductivity and specific surface area was observed, and laboratory results were comparable to previous studies. It was demonstrated through synthetic modeling that river stage and channel width, regularization across the river-riverbed interface, and incorrect constraints of both the river conductivity and river stage can have varying influence on inverted geoelectrical images. Reliable geoelectrical images require a priori definition of river stage and conductivity, however inversion constraints using incorrect a priori values result in misleading artifacts. The conductivity image obtained from the field data in this work appeared to reflect the geoelectrical structure anticipated from the laboratory data; however, the phase angle image did not. Although this study focused on riverbed characterization, findings here demonstrate common pitfalls of inversion of aquatic-based geoelectrical data. Primarily, they highlight that synthetic modeling ought to be used to alleviate any uncertainty in the interpretation of geoelectrical models before predictions about GW-SW interactions can be made