Implications of Peat Soil Conceptualization for Groundwater Exfiltration in Numerical Modeling: A Study on a Hypothetical Peatland Hillslope

Abstract

Fully integrated physically based hydrological modeling is an essential method for increasing hydrological understanding of groundwater‐surface water (GW‐SW) interactions in peatlands and for predicting anthropogenic impacts on these unique ecosystems. Modeling studies represent peat soil in a simplistic manner, as a homogeneous layer of uniform thickness, but field measurements consistently show pronounced spatial variability in peatlands. This study evaluated uncertainty in groundwater levels and exfiltration fluxes associated with the simplified representation of the peat soil layer. For transferability of the results, impacts of selected topographical and hydrogeological conceptual models on GW‐SW exchange fluxes were simulated in a hypothetical hillslope representing a typical aquifer‐mire transect. The results showed that peat soil layer geometry defined the simulated spatial GW‐SW exchange patterns and groundwater flow paths, whereas total groundwater exfiltration flux to the hillslope and groundwater level in the peatland were only subtly altered by different conceptual peat soil geometry models. GW‐SW interactions were further explored using different scenarios and dimensionless parameters for peat hydraulic conductivity and hillslope‐peatland system slope. The results indicated that accurate representation of physical peat soil properties and landscape topography is important when the main objective is to model spatial GW‐SW exchange. Groundwater level in virtual peatland was not greatly affected by groundwater drawdown in an adjacent aquifer, but the magnitude and spatial distribution of GW‐SW interactions was significantly altered. This means that commonly used groundwater depth observations near peat‐mineral soil interfaces and within peatlands may not be a suitable indicator for monitoring the hydrological state of groundwater‐dependent peatland ecosystems.

Aquifer-Peatland Hydrological Connectivity and Controlling Factors in Boreal Peatlands

“…The larger proportion of inflows from groundwater at Cheinu is similar to that of Quillet et al (2017) for slope peatlands. A similar result is also reported by Autio et al (2020) who simulated larger groundwater flow into peatlands when the slope contrast between the peatland and its immediate uphill surroundings was pronounced. The contrast in inflow sources at the two peatlands could also be due to their position within the watershed, with Cheinu being located close to the regional outlet and thus accumulating more flow from upgradient areas.…”

“…This impact is transferred to the peatlands through modifications in hydraulic gradients in the peatland vicinity (upgradient and downgradient) which results in lower (or higher) groundwater inflows and outflows. Autio et al (2020) have also reported that long-term groundwater drawdown (such as that caused by a recharge reduction) may not be reflected in peatland water levels, due to changes in the proportion of surface and groundwater flows and velocities.…”

“…Sensitivity of aquifer-peatland connectivity to recharge has been addressed in the scientific literature with groundwater flow models that explicitly represent the hydrogeological characteristics of both the geological media and the organic deposits (e.g., Bourgault et al, 2014;Levison et al, 2014;Rossi et al, 2014;Thompson et al, 2015;Li et al, 2019). Autio et al (2020) summarize studies that have used integrated surface-subsurface flow models including peatlands to study this connectivity. These authors report that peat properties and landscape topography are crucial parameters in models aimed at quantifying interactions between surface and subsurface reservoirs.…”

Numerical Modeling of Nitrate Removal in Anoxic Groundwater during River Flooding of Riparian Zones

“…A low-permeable clay layer delimits the sand aquifer. The two-layer peat-sand conceptual model is similar to other Danish (Brüsch and Nilsson 1993;Hoffmann et al 2006;Karan et al 2013;Jensen et al 2017;Steiness et al 2019), German (Lischeid et al 2007), Finnish (Rossi et al 2012), United States (e.g., Böhlke et al 2002;Puckett et al 2002), and Canadian riparian zones (Cey et al 1999;Devito et al 2000;Vidon and Hill 2004a;Vidon and Hill 2004b;Craig et al 2010) and other conceptual riparian models (Gold et al 2001;Dahl et al 2007;Autio et al 2020). Typically, the peat layer and aquifer thicknesses are 1 to 3 m and from a few meters to tens of meter, respectively.…”

“…Our conceptual model falls within these ranges. Recently, Autio et al (2020) performed a numerical study showing the importance of a correct specification of the top peat layer on simulated exfiltration to the surface.…”