GROUNDWATER EXFILTRATION TO PEATLANDS: A MODELLING STUDY ON A HYPOTHETICAL PEATLAND HILLSLOPE AND METHODS FOR SPATIAL MONITORING

In several areas around boreal regions glaciofluvial sand and gravel formations as eskers are important source of potable water. Same aquifers can also be important to sustain groundwater (GW) dependent mires within their surrounding areas. To better understand this groundwater-surface water (GW-SW) interaction and the possible impacts of abstraction, modelling and in situ studies were conducted. Generally, 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. The modelling study evaluated uncertainty in GW 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 were simulated in a hypothetical hillslope representing a typical aquifer‐mire transect.

The modelling study results showed that peat soil layer geometry defined the simulated spatial GW‐SW exchange patterns and GW flow paths. However, total GW exfiltration flux to the hillslope and GW level in the peatland were only subtly altered by different conceptual peat soil geometry models. 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. GW level in the virtual peatland was not greatly affected by GW drawdown in an adjacent aquifer, but the magnitude and spatial distribution of GW‐SW interactions was significantly altered. For this reason, additional methods for monitoring the peatlands spatially for GW-SW interactions would be needed for enhanced management. Thermal imagery and stable water isotopes have showed the potential to pinpoint the crucial areas within the mire areas in case site studies in Northern Finland where abstraction is planned. Thermal imagery e.g. with a drone shows the cold GW seepage spots on the surface of mires. Stable water isotope analyses (¹⁸O and ²H mass balance) of the peat pore water can separate GW and precipitation ratios prior to modifications by evaporation. This method has the potential to map the spatial variability of the groundwater dependence of a mire in details.