Shallow groundwater–surface water interactions in pond–peatland complexes along a Boreal Plains topographic gradient

Shallow pond–peatland complexes are important water resources and waterbird habitat on the Boreal Plains of Canada, and are potentially threatened by accelerated resource development. We examined two shallow pond–peatland complexes located in contrasting topographic positions within a clay-rich till region: one on a moraine and one on a lowland clay plain, to evaluate the influence of landscape position on wetland groundwater–surface water interactions, hydrologic function, and potential response to disturbance. Hydrometric and geochemical measurements indicated that precipitation and evaporation dominated the annual water balance of both ponds. Forested mineral uplands adjacent to the pond–peatland complexes contributed no runoff inputs. Small recharge rates (<5 mm year−1) from the ponds through low-permeable clay to the underlying aquifer were measured at both topographic locations. Shallow groundwater fluxes to/from the pond were controlled by water storage in and exchange with the adjacent riparian peatlands. Shallow groundwater fluxes may have contributed as much as 23% of inputs and outputs, and influenced pond chemistry and permanence. Shallow groundwater exchange between the pond and adjacent peatlands varied seasonally and differed between the moraine and lowland complex. Groundwater flow reversals occurred around the entire perimeter of the moraine pond; recharge from the moraine pond to the peatland during dry periods was off-set by discharge to the pond from the peatland during wet periods. In contrast, shallow groundwater ‘flow-through’ conditions were observed at the lowland pond for most of the study; however, during a high-rainfall event, a groundwater flow reversal within the outflow peatland initiated discharge back into the pond. These results suggest that the hydrologic regimes of some pond–peatland complexes in clay-rich till within the Boreal Plains will be more sensitive to local-scale disturbances that impact flow dynamics and storage of near-shore peatlands, rather than larger-scale disturbances of mineral upland regions.

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