The Effect of Access Road Construction on the Hydrology of Wetlands in Rock Barren Landscapes

Abstract

Dense networks of access roads can be found across the Canadian landscape. Though necessary for natural resource and mineral exploration projects, access roads are linear disturbances that can alter hydrological processes operating within the landscape. While this has previously been studied in many landscapes, research has not been conducted in wetland-dominated depressional landscapes of the Precambrian shield. As such, four wetlands were instrumented with paired monitoring wells and piezometer nests to assess hydrological change upstream and downstream of the road cut-through. In all four wetlands, the road obstructed the movement of lateral flow, resulting in ponding upstream as water was discharged to the surface. Downstream of the road, the wetland experienced a lowered water table due to reduced water inputs, especially during drought conditions. The difference between the upstream and downstream water table position (∆WT) was largest when the culvert was placed 20 cm above the surface, indicating that large water inputs and prolonged flooded conditions was required for water to be permitted downstream. Conversely, the ∆WT was smallest when the culvert was embedded 50% into the subsurface, confirming a previous suggestion (Phillips, 1997) that partial burial is the ideal culvert placement to maintain drainage patterns. In wetlands with comparable culvert placements (perched on the moss surface), the ∆WT was smallest in the wetland that received not only water input from lateral flow, but from groundwater discharge and overland flow as well. This suggests that multiple water sources are important to provide water to the bisected unit downstream of the road cut-through. Furthermore, wetlands with a groundwater connection and deeper depression depths were capable of maintaining a water table during drought conditions, and as such were not subjected to long-term aerobic conditions that can result in peat degradation. In general, the ∆WT was higher in the wetland during the fall rewetting than during the drought. Provided that culvert design was standardized throughout the road network, wetlands that received a greater contribution of overland flow would be at a greater risk for flooding and subsequently hydrological change. As such, a GIS model was created to assess the relative flooding potential of wetlands using criteria (catchment area, proportion of rock cover, stream order, surface water connection) that represents the first-order controls on runoff in Precambrian shield landscapes (T3 template; Buttle, 2006). The model output was evaluated using field data, where wetlands were ranked based on its water table position during the winter period when snowmelt was assumed to occur. The model was capable of assessing the hydroperiod of different wetland types, where the highest and lowest flooding potential was associated with marshes and bogs, respectively. A higher flooding potential was also associated with deeper depressions, which have been shown to have a higher hydroperiod (Didemus, 2016). The flooding potential of wetlands was variable throughout the landscape, and was not correlated with a particular wetland metric (i.e. wetland type, wetland area). The model results suggest that wetlands can be assessed based on flooding potential, in conjunction with other traditionally used wetland metrics. As such, an understanding of the hydrological function of wetlands, and proper selection of culvert design and wetlands for road crossing can be completed in order to minimize hydrological change associated with the construction of access roads.