Effects of subsurface water infiltration systems on land movement dynamics in Dutch peat meadows

Abstract. Large-scale drainage and cultivation of peat soils over the last centuries, occurring worldwide, has resulted in substantial CO₂ emission and land subsidence caused by peat decomposition by microbial activity, shrinkage and soil compaction. In addition, seasonal reversible vertical soil movement is caused by shrink and swell in the unsaturated zone and by poroelastic deformation in the saturated zone. To reduce CO₂ emission and land subsidence in drained peat soils, subsurface water infiltration systems (WIS) are expected to be a suitable measure. In this study, effects of WIS on seasonal vertical soil movements are evaluated, based on field measurements from five locations in Dutch peat meadows, for the years 2021 and 2022. For one of these locations, a 4-years timeseries was available, allowing to make a first estimate of the rate of multi-year land subsidence. At each study location, vertical soil movement has been measured using spirit levelling and extensometers, both in a parcel with a WIS and in a nearby reference parcel without any measure. Phreatic groundwater level fluctuations are found to induce soil volume decreases and increases in both the saturated and the unsaturated zone, which cause vertical land movement dynamics of up to 10 cm in the dry summer of 2022 at a location with a relatively thick (6 m) peat layer. Poroelastic deformation processes in the deeper saturated soil contribute substantially to surface level movement. In peat meadows, subsurface water infiltration systems, if correctly applied, reduce seasonal vertical soil movements while (potentially) reducing soils’ resilience to drought-induced volume losses. Seasonal vertical soil surface dynamics are about an order of magnitude higher than longer term (years to decades) land subsidence rates, which are commonly in the order of mm yr^-1 in the Dutch drained peat areas. Therefore, multi-year data series are needed to filter out variations in seasonal dynamics, which are mainly introduced by annual variations in weather conditions, and more accurately estimate land subsidence.