Quantifying spatial groundwater dependence in peatlands through a distributed isotope mass balance approach

Abstract

The unique biodiversity and plant composition of peatlands rely on a mix of different water sources: precipitation, runoff, and groundwater (GW). Methods used to delineate areas of ecosystem groundwater dependence, such as vegetation mapping and solute tracer studies, are indirect and lack the potential to assess temporal changes in hydrology, information needed in GW management. This paper outlines a new methodology for mapping groundwater‐dependent areas (GDAs) in peatlands using a 2H and 18O isotope mass balance method. The approach reconstructs the initial isotopic composition of the peat pore water in the uppermost peat layer before its modification by evaporation. It was assumed that pore water in this layer subject to evaporation is a two‐component mixture consisting of GW and precipitation input from the month preceding the sampling period. A Bayesian Monte Carlo isotope mixing model was applied to calculate the proportions of GW and rainwater in the sampled pore water and to assess uncertainties. The approach revealed large spatial variability in the contribution of GW to the pore water present in the top layer of peatland, covering the range from approximately 0 to 100%. Results show that the current GW protection zones determined by Finnish legislation do not cover the GDAs in peatlands and highlight a need for better classification of groundwater‐dependent ecosystems and conceptualization of aquifer‐ecosystem interactions. Our approach offers an efficient tool for mapping GDAs and quantifying the contribution of GW to peatland pore water. However, more studies are needed to test the method for different peatland types.

Characterizing spatial and temporal variation in 18O and 2H content of New Zealand river water for better understanding of hydrologic processes

“…Since periods of high flow tend to be isotopically similar to recent precipitation (von Freyberg et al, 2018), we compared flow-weighted and unweighted river water lines. Furthermore, we also calculated Local River Water Lines (LRWL) representing local precipitation and evaporation processes, which are influenced by local climate, geology, and landcover (Isokangas et al, 2017) for each sampling site. Where multiple sites were located within the same connected river network, we compared LRWLs for upstream and downstream sites.…”

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

“…However, recent findings about kinetic fractionation in the water pools and tracks of an extended drainage network in a raised bog within the Bruntland Burn showed that evaporation can have a fractionating effect on the stable isotopes of peatland waters, despite the relatively low energy available (Sprenger et al, 2017b). While such isotopic fractionation of open waters in peatlands of the northern latitude were found by others (Carrer et al, 2016;Gibson et al, 2000;Isokangas et al, 2017), the potential fractionation dynamics of the water in the upper soil layer in these cold regions have not previously been studied.…”

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

“…These techniques, applicable for peatland sites, include conventional direct hydrological measurements (use of piezometers, slug tests, boreholes, etc.) (Johansen et al, 2011; Rossi et al, 2012), thermal imaging and other remote sensing techniques (Bechtold et al, 2018; Briggs et al, 2016; Hare et al, 2015, 2017), geophysical measurements (Lowry et al, 2009; McLachlan et al, 2017), and use of natural tracers, including stable water isotopes (Isokangas et al, 2017; Levy et al, 2014, 2016; Négrel et al, 2010). These methods provide valuable information on the current state of GW‐SW connectivity but are of little use in predicting how the system will react to changing conditions, such as groundwater abstraction, changes in land use, or climate change.…”

“…However, there is no rigorous framework on how to represent peat soils in fully integrated GW‐SW models. Peat soils have highly heterogeneous properties (Baird et al, 2008; Beckwith et al, 2003; Holden & Burt, 2003; Isokangas et al, 2017; Lewis et al, 2012; Päivänen, 1973; Rosa & Larocque, 2008) and are thus difficult to represent in numerical models. Field studies have shown that factors such as preferential flow paths (Holden & Burt, 2002; Lowry et al, 2009), hydraulic conductivity (Drexler et al, 1999; Hare et al, 2017), and abrupt changes in peat thickness (Hare et al, 2017; Lowry et al, 2009) may affect the location of groundwater discharge.…”