Development of integrated water quality model for evaluating the peatland rehabilitation measures in Ireland

There is increasingly demanding effort across Europe to restore degraded peatlands through rewetting techniques. While the measures of restoration by rewetting can effectively encourage the growth of peat-forming mosses, there have been concerns about peatland water quality problems due to the interactions between raised water table level and the release of nutrients such as ammonium (NH₄⁺), nitrate (NO₃^-) and phosphate (PO₄^3-), which may pose a significant risk to water quality in receiving water bodies. Studies have shown that drained peatlands have high concentration of DOC, NH₄⁺,  NO₃^-,  PO₄^3-  and DON and rewetting is capable of reducing the concentration to near natural levels, however for NH₄⁺ and PO₄^3- studies have recorded elevated concentrations after full rewetting (WTD within 0 – 10cm) and less under partially rewetting ( WTD 20cm to surface) for which some researchers have suggested partial rewetting to curb internal release of nutrient on rewetted peatland.

To date, there is no peatland-specific water quality model. Although some existing water quality models have been applied to peatlands, the complex adaptive nature of peatland is oversimplified, e.g. complex interaction between fluctuating water table levels and biogeochemical processes that affect nutrients concentration are either neglected or not explicitly represented in the model. To address this knowledge gap, the objective of this study is to develop an integrated water quality model for peatlands, considering the advection-dispersion processes of solute transport, biogeochemical processes and related environmental factors that affect the evolution and variation of nutrients.

The solutions of the system of governing partial differential equations were implemented by using the finite volume method (FVM). While the overall solver was based on an explicit scheme, e.g. by using the Euler’s forward method, a second-order central differencing scheme was applied to discretise the dispersion term. In order to make the solver stable, the advection term was discretised using the second-order upwind total variation diminishing (TVD) method. The model was verified by comparing the numerical modelling results of a 1D benchmark problem with related analytical solutions, getting a coefficient of determination of R² > 0.99. The 2D version of the water quality model has been coupled with DigiBog_Hydro model which simulates groundwater flow processes. This integrated water quality model will be applied to some selected bogs in Ireland which are under rehabilitation to investigate the hydrological and water quality responses to related restoration methods such as drain blocking and bunds creation. The results of integrated modelling will be compared to the experimental results of water quality measurements of related peatlands.