Derivation of lowland riparian wetland deposit architecture using geophysical image analysis and interface detection

Abstract

For groundwater-surface water interactions to be understood in complex wetland settings, the architecture of the underlying deposits requires investigation at a spatial resolution sufficient to characterize significant hydraulic pathways. Discrete intrusive sampling using conventional approaches provides insufficient sample density and can be difficult to deploy on soft ground. Here a noninvasive geophysical imaging approach combining three-dimensional electrical resistivity tomography (ERT) and the novel application of gradient and isosurface-based edge detectors is considered as a means of illuminating wetland deposit architecture. The performance of three edge detectors were compared and evaluated against ground truth data, using a lowland riparian wetland demonstration site. Isosurface-based methods correlated well with intrusive data and were useful for defining the geometries of key geological interfaces (i.e., peat/gravels and gravels/ Chalk). The use of gradient detectors approach was unsuccessful, indicating that the assumption that the steepest resistivity gradient coincides with the associated geological interface can be incorrect. These findings are relevant to the application of this approach in settings with a broadly layered geology with strata of contrasting resistivities. In addition, ERT revealed substantial structures in the gravels related to the depositional environment (i.e., braided fluvial system) and a complex distribution of low-permeability putty Chalk at the bedrock surface-with implications for preferential flow and variable exchange between river and groundwater systems. These results demonstrate that a combined approach using ERT and edge detectors can provide valuable information to support targeted monitoring and inform hydrological modeling of wetlands.

Discrete wetland groundwater discharges revealed with a three-dimensional temperature model and botanical indicators (Boxford, UK)

“…The alluvial cover overlying the gravels ranges up to 2.7 m thick, mostly consisting of peat, with intermixed occurrences of silt, sand and gravel (Allen, et al, 2010). Within the wetland, these deposits are typically around a metre thick (Chambers, et al, 2014).…”

“…The low permeability 'putty' chalk is considered to act as a confining layer to the Chalk aquifer, and its uneven distribution has implications for exchange between groundwater and surface water (Chambers, et al, 2014). Studies at a nearby site 100 m upstream of the Observatory, part of the Lowland Catchment Research (LOCAR) programme (Wheater, et al, 2007), have indicated variable hydraulic connection between the Chalk, gravels and surface water (Abesser, et al, 2008;Allen, et al, 2010;Lapworth, et al, 2009).…”

Current uses of ground penetrating radar in groundwater-dependent ecosystems research

“…In the absence of precipitation, many wetlands and seasonal-water ecosystems reveal their dependence on shallow groundwater for the sustenance of riparian flow, river base-flow, and adequate moisture levels in soil-root and unsaturated zones (Fan et al, 2013;Chambers et al, 2014;Uhlemann et al, 2016). Most of GDE are currently threatened by a combination of global climate forces that extend drought periods and local human-induced pressures such as land-use changes and pollution (Yuce, 2006;Ribeiro, 2009;Kløve et al, 2011;Silva et al, 2012).…”

Modelling groundwater/surface water interaction in a managed riparian chalk valley wetland

“…Where deviations occur, they fall into two groups: locations where the model underestimates levels (3G, 6G–9G and 12G) and locations where levels are overestimated towards the end of the simulation period (1G, 2G and 5G). The ERT survey revealed significant braided structures in the gravels (Chambers et al ., ). These suggest large differences in gravel porosity across the site, which would cause localized and depth‐dependent variations in hydraulic conductivity, as could quantities of reworked chalk in lower levels (Allen et al ., ).…”