Accumulation and Composition of Gully Pot Sediments under Varying Anthropogenic Activities

Abstract: Gully pots (GPs) are an integral urban drainage component, contributing to draining runoff from urban surfaces and reducing the load of solids and associated contaminants on downstream piped systems and receiving waters. Over time, the silting-up of GPs leads to an impaired hydraulic condition, increasing the risk of GP blockage-induced flood events and reducing solids retention performance. These challenges are typically addressed through GP emptying programmes. However, current GP maintenance strategies lack clear aims and a robust evidence-base, against which their effectiveness can be benchmarked. The overall aim of this doctoral thesis is to enhance understanding of the factors influencing physicochemical properties and accumulation of solids in GPs, thereby laying the groundwork for further assessment and optimisation of GP maintenance programmes. The study encompasses various anthropogenic factors such as land use types, seasonal activities, traffic conditions, environmental regulatory measures, GP design and catchment characteristics. Research reported in this thesis includes both desk-based modelling (e.g. a study on sediment scour behaviours), and the results of extensive field sampling campaign (sediments from a total of 87 GPs were assessed for dry masses, particle size distribution (PSD) and a range of conventional and emerging urban and traffic-related substances).The considerable variations in physicochemical properties, mass accumulation rates of sediments and associated contaminants in GPs question the utility of the prevalent generalised maintenance strategy. Although GPs in trafficked roads showed significantly higher solids mass accumulation rates compared to those in non-trafficked catchments, this factor alone could not fully explain observed variations. For example, whilst selected typically traffic-sourced contaminants such as 4-tert-octylphenol (OP) and heavy-molecular-weight polycyclic aromatic hydrocarbons (PAH-H) exhibited significantly higher concentrations in GP sediments from trafficked roads, further typically- traffic related pollutants (e.g. aliphatic hydrocarbon > C16-C35, Zn, 4-nonylphenol (NP) and monobutyltin (MBT)) were reported for commercial and car parking land-use types. In terms of temporal dynamics, winter-spring road safety measures did not consistently result in significantly higher solids mass accumulation rates compared to summer-autumn accumulation data. However, where present, a greater influence of winter-spring road safety measures was identified under certain scenarios. For example, GPs along straight roads with adjacent pedestrian/bicycle paths showed an up-to 130-fold increase in solids accumulation rates following winter-spring accumulation, largely due to the practice of using coarse traction grits in this road feature type. Synergistic impacts of winter-spring road safety measures and road layout features were also observed. The amplified tyre and road wear by the combined factors of studded tyres, traction grits and frequent braking at roundabouts likely contributed to e.g., significantly higher mass fraction of <63 µm sediments and concentrations of Zn, Cd, OP, selected PAHs and phthalates in corresponding GPs following winter-spring accumulation. Additionally, significant temporal changes in basic parameters of GP sediments such as pH, electrical conductivity (EC) and total organic carbon (TOC) were identified, indicating substance phase changes (e.g. adsorption/desorption) at solid-liquid interfaces should be anticipated, and the impact of these considered when assessing GP sediment management needs.This research also contributes to addressing this need through an extensive characterisation of GP sediments for a broad list of contaminants. The comparison of sediment substance concentrations with Predicted No-Effect Concentrations (PNECs) or (where available) Norwegian guideline values indicated that 31 substances in at least one investigated GPs exceeded threshold values. This demonstrates GPs can accumulate contaminated sediments and underscores the need to revisit the role of GPs in mitigating runoff quality (and quantity) as part of an integrated urban stormwater management plan.