Impacts of wildfire and drought on hydrological connectivity and solute dynamics in a temperate blanket peat catchment

Intact peatlands provide crucial ecosystem services, regulating discharge by retaining water and providing high water quality by retaining solutes. These services can become compromised when peatlands become degraded by natural disturbances such as wildfire and drought. Such disturbances in traditionally non-fire prone regions will likely become more frequent and severe under future climates, potentially impacting downstream water quality. Understanding how fire and drought alter hydrological and biogeochemical processes in these regions is necessary for future risk assessment.

The 2018 Saddleworth moorland wildfire (England) offered a unique opportunity to study the combined impacts of severe wildfire and drought on stream water quality fed from a peatland-dominated catchment in a traditionally non-fire prone region (i.e., northern Europe). Capitalising on this event, our study aimed to (1) quantify stream chemistry changes and (2) understand patterns of element mobilisation and transport within the disturbed catchment. We evaluated concentration-discharge (C-Q) responses for nine variables (dissolved organic carbon, sulphate, Na, Ca, Pb, Zn, Al, Cu and turbidity) in five post-fire storm events over a nine-month period. C-Q responses were considered together with hysteresis and flushing indices (HI and FI, respectively) to further describe solute dynamics within storms.

Highest average concentrations of nutrients and base cations occurred in the storms immediately following the wildfire (~0 – 3 months post-fire) and average concentrations decreased into the autumn and spring (~3 – 9 months post-fire). In contrast, average metal concentrations began increasing in autumn and into the spring storms, coinciding with the timing of catchment re-wetting. Element behaviour patterns inferred from C-Q responses and HI/FI indices suggest rapid mobilisation and flushing of nutrients and base cations following the wildfire, and a shift to dilution behaviours in the spring storms. This shift indicates a change from surface transport and an exhaustion of readily available burnt materials. Metals consistently displayed delayed mobilisation, where concentrations peaked after the discharge peak, indicating a within-peat or distal headwater sources.

Our results suggest that seasonal re-wetting and rejuvenated hydrologic connectivity of the catchment following extreme drought was a dominating factor controlling source zone activation, mobilisation and transport of solutes in our catchment. Additionally, water quality impacts appeared to be limited to the first ~3 months following the wildfire, suggesting certain aspects of wildfire impacts in temperate peatlands may be short-lived. Our results contribute to defining potential water quality risks in drought and wildfire disturbed peat catchments under future climates.