Michael E. Bartkow

Potential denitrification rates were measured using the acetylene block method, in sediments collected from streams in the sub-tropical, south-east Queensland region of Australia. Our aim was to estimate how much nitrogen could be removed from lotic systems by denitrification at the regional scale. Denitrification measured at 65 sites in August and September from a catchment of 22700 km2 was extrapolated to all streams and rivers in the region based on the sediment area available for denitrification. Denitrification rates ranged between 4 and 950 µmol N m–2 h–1, with most sites having rates below 150 µmol N m–2 h–1. Based on these results, the current study estimates that a total of 305 t of nitrogen could be denitrified per year from all streams and rivers in the region, representing 6% of the total annual nitrogen load from surrounding land use. During baseflow conditions, when nitrogen loads to streams are low, the proportion of nitrogen removed through denitrification would be substantially higher, in some cases removing 100% of the nitrogen load. It is proposed that denitrification is an important process maintaining low concentrations of dissolved inorganic nitrogen under baseflow conditions and is therefore likely to enhance nitrogen limitation of primary production in this region.

Polyethylene passive sampling devices (PSDs) were deployed to investigate how passive samplers of multiple surface area-to-volume ratios could be used to characterize uptake kinetics for polyaromatic hydrocarbons (PAHs). Theoretically, uptake profiles for different thickness PSDs of the same surface area should show the following: where uptake is linear, the amount of compound accumulated in the different PSDs will be the same and where equilibrium is approached, the amount accumulated by the different PSDs will be proportional to sampler thickness. Polyethylene sheets of the same surface area and approximately 100 and 200 μm thickness were collected after 30, 60, and 90 days of exposure along with samples from a codeployed high volume sampler. Twelve priority pollutant PAHs could be routinely quantified in replicate PSDs. Overall, reproducibility between replicate PSDs was satisfactory, with normalized differences rarely exceeding 25%. The smallest analytes quantified, fluorene, phenanthrene, and anthracene, were shown to approach equilibrium during the deployment period, whereas uptake for fluoranthene and pyrene moved into the curvilinear stage. For most of the larger molecular weight PAHs such as indeno[1,2,3-cd]pyrene, uptake could be described using a linear uptake model. Preliminary sampling rates for the compounds which remained in the linear stage of uptake ranged between 0.5 and 1.5 m³ d^-1 dm^-2. Sampler to air partition coefficients were estimated for PAHs which approached equilibrium and predicted for some of the other compounds. Results suggest that a single deployment of PSDs with multiple surface area-to-volume ratios can be sufficient to determine whether uptake was linear or approaching equilibrium for a range of PAHs.