Silt and phosphorus availability in unpaved road materials from forested watersheds in Canada, and implications for drinking water resources
Creators: Humeny, Erin C.
Publisher: name: University of Alberta Library
Year: 2024

<pre><code>Many communities in Canada rely on surface drinking water supplies that are located within or downstream of forests. The typically clean, clear waters flowing from forests are sensitive to disturbances that can degrade source water quality. Unpaved access roads are one of the most widespread disturbances in forested watersheds, and they are known to increase the amount of sediment and nutrients delivered to streams. The goal of this thesis was to improve understanding, in a drinking water source quality and treatment context, of the hazards associated with unpaved access roads in forested watersheds. This study focused on quantifying two potential hazards to drinking water resources: silt (particles &lt;63 μm, in this thesis) and phosphorus (P), specifically bioaccessible P. Silt and P availability, along with the mobilization of silt from road-related sediment sources, reflects what could be transported in runoff either directly to surface water supplies or to their contributing areas upstream.
 Watershed-scale median silt contents of road surface materials ranged from 7–55%. Cutslope sample sizes were limited; however, cutslope silt contents were typically higher than those of road surface materials from the same site (p = 0.01). The silt contents of road surface materials were variable, with ranges as high as 50–74% within watersheds, and “hotspots” of high silt availability were common. The median silt content of sediments mobilized in runoff from unpaved road surfaces was 95%, and sediments contained, on average, 3.6 times more silt than road surface materials. These results were consistent with selective erosion. Phosphorus was ubiquitous in the road-related materials sampled, with concentrations ranging from 97–6016 mg/kg. However, across all road surface material and cutslope samples, bioaccessible P comprised only 12% to 17% of total P. Rather, most of the P in road-related materials (median = 82% and 72% for road surface materials and cutslopes, respectively) consisted of apatite P, which is not bioaccessible. These results indicate that, in many cases, total P is not a reliable indicator of the water quality hazard associated with P inputs from unpaved access roads. As with silt contents, bioaccessible P concentrations in road surface materials varied among watersheds (by up to ca. 200 mg/kg) and within watersheds (by up to 200–300 mg/kg). “Hotspots” of high P availability also occurred within some watersheds. The occurrence of “hotspots” of high silt and P availability represent elevated water quality hazards, particularly where they overlap, due to the increased potential for bioaccessible P transport associated with the selective erosion of silt.
 This study shows that unpaved access roads are potential source of silt and P (particularly bioaccessible P), provides new information on the amount and types of P in unpaved access road-related materials, and highlights the opportunity to preventively manage hazards associated with unpaved access roads.
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