Ecohydrologically important subsurface structures in peatlands revealed by ground-penetrating radar and complex conductivity surveys.

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

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

Characterization of reactive transport by 3-D electrical resistivity tomography (ERT) under unsaturated conditions

The leaching of nitrate from intensively used arable soil is of major concern in many countries. In this study, we show how time lapse electrical resistivity tomography (ERT) can be used to characterize spatially heterogeneous processes of ion production, consumption, and transport in soils. A controlled release fertilizer was introduced into an undisturbed soil core in a laboratory lysimeter and subjected to infiltration events. The production of ions resulting from processes associated with nitrification and their transport through the soil core was observed by time lapse ERT and analysis of seepage water samples from a multicompartment sampler. ERT images show development and propagation of a high-conductivity plume from the fertilizer source zone. Molar amounts of nitrate produced in and exported from the soil core could be well reproduced by time lapse ERT using a spatial moment analysis. Furthermore, we observed that several shape measures of local breakthrough-curves (BTCs) of seepage water conductivity and nitrate derived by effluent analyses and BTCs of bulk conductivity derived by ERT are highly correlated, indicating the preservation of spatial differences of the plume breakthrough in the ERT data. Also differences between nitrate breakthrough and a conservative tracer breakthrough can be observed by ERT. However, the estimation of target ion concentrations by ERT is error bound and the smoothing algorithm of the inversion masks spatial conductivity differences. This results in difficulties reproducing spatial differences of ion source functions and variances of travel times. Despite the observed limitations, we conclude that time lapse ERT can be qualitatively and quantitatively informative with respect to processes affecting the fate of nitrate in arable soils

Characterization of reactive transport by 3-D electrical resistivity tomography (ERT) under unsaturated conditions

The leaching of nitrate from intensively used arable soil is of major concern in many countries. In this study, we show how time lapse electrical resistivity tomography (ERT) can be used to characterize spatially heterogeneous processes of ion production, consumption, and transport in soils. A controlled release fertilizer was introduced into an undisturbed soil core in a laboratory lysimeter and subjected to infiltration events. The production of ions resulting from processes associated with nitrification and their transport through the soil core was observed by time lapse ERT and analysis of seepage water samples from a multicompartment sampler. ERT images show development and propagation of a high-conductivity plume from the fertilizer source zone. Molar amounts of nitrate produced in and exported from the soil core could be well reproduced by time lapse ERT using a spatial moment analysis. Furthermore, we observed that several shape measures of local breakthrough-curves (BTCs) of seepage water conductivity and nitrate derived by effluent analyses and BTCs of bulk conductivity derived by ERT are highly correlated, indicating the preservation of spatial differences of the plume breakthrough in the ERT data. Also differences between nitrate breakthrough and a conservative tracer breakthrough can be observed by ERT. However, the estimation of target ion concentrations by ERT is error bound and the smoothing algorithm of the inversion masks spatial conductivity differences. This results in difficulties reproducing spatial differences of ion source functions and variances of travel times. Despite the observed limitations, we conclude that time lapse ERT can be qualitatively and quantitatively informative with respect to processes affecting the fate of nitrate in arable soils

Predicting permeability from the characteristic relaxation time and intrinsic formation factor of complex conductivity spectra

Low-frequency quadrature conductivity spectra of siliclastic materials exhibit typically a characteristic relaxation time, which either corresponds to the peak frequency of the phase or the quadrature conductivity or a typical corner frequency, at which the quadrature conductivity starts to decrease rapidly toward lower frequencies. This characteristic relaxation time can be combined with the (intrinsic) formation factor and a diffusion coefficient to predict the permeability to flow of porous materials at saturation. The intrinsic formation factor can either be determined at several salinities using an electrical conductivity model or at a single salinity using a relationship between the surface and quadrature conductivities. The diffusion coefficient entering into the relationship between the permeability, the characteristic relaxation time, and the formation factor takes only two distinct values for isothermal conditions. For pure silica, the diffusion coefficient of cations, like sodium or potassium, in the Stern layer is equal to the diffusion coefficient of these ions in the bulk pore water, indicating weak sorption of these couterions. For clayey materials and clean sands and sandstones whose surface have been exposed to alumina (possibly iron), the diffusion coefficient of the cations in the Stern layer appears to be 350 times smaller than the diffusion coefficient of the same cations in the pore water. These values are consistent with the values of the ionic mobilities used to determine the amplitude of the low and high-frequency quadrature conductivities and surface conductivity. The database used to test the model comprises a total of 202 samples. Our analysis reveals that permeability prediction with the proposed model is usually within an order of magnitude from the measured value above 0.1 mD. We also discuss the relationship between the different time constants that have been considered in previous works as characteristic relaxation time, including the mean relaxation time obtained from a Debye decomposition of the spectra and the Cole-Cole time constant

A laboratory study to estimate pore geometric parameters of sandstones using complex conductivity and nuclear magnetic resonance for permeability prediction

We estimate parameters from the Katz and Thompson permeability model using laboratory complex electrical conductivity (CC) and nuclear magnetic resonance (NMR) data to build permeability models parameterized with geophysical measurements. We use the Katz and Thompson model based on the characteristic hydraulic length scale, determined from mercury injection capillary pressure estimates of pore throat size, and the intrinsic formation factor, determined from multi-salinity conductivity measurements, for this purpose. Two new permeability models are tested, one based on CC data and another that incorporates CC and NMR data. From measurements made on forty-five sandstone cores collected from fifteen different formations, we evaluate how well the CC relaxation time and the NMR transverse relaxation times compare to the characteristic hydraulic length scale and how well the formation factor estimated from CC parameters compares to the intrinsic formation factor. We find: (1) the NMR transverse relaxation time models the characteristic hydraulic length scale more accurately than the CC relaxation time (R2 of 0.69 and 0.39 and normalized root mean square errors (NRMSE) of 0.16 and 0.20, respectively); (2) the CC estimated formation factor is well correlated with the intrinsic formation factor (NRMSE=0.23). We demonstrate that that permeability estimates from the joint-NMR-CC model (NRMSE=0.13) compare favorably to estimates from the Katz and Thompson model (NRMSE=0.074). This model advances the capability of the Katz and Thompson model by employing parameters measureable in the field giving it the potential to more accurately estimate permeability using geophysical measurements than are currently possible

A highly anomalous Red-winged Blackbird (Agelaius phoeniceus) song

Red-winged Blackbird (Agelaius phoeniceus) is a highly vocal species with a repertoire of similar, yet acoustically distinct songs. These songs may be altered drastically if, as a nestling, the male goes deaf or becomes acoustically isolated. In deaf Red-winged Blackbirds, these dramatic song alterations may present as songs bearing slight resemblance to the introductory phrase of their normal song. Here, we present a Red-winged Blackbird song observed in Ottawa, Ontario, Canada, that is far outside any normal variation in Red-winged Blackbird songs. Given the individual’s age and the consistency of the anomalous song, it is possible that this is a deaf bird