Search

Discharge of treated sewage effluent to rivers can degrade aquatic ecosystem quality, interacting with multiple stressors in the wider catchment. In predominantly rural catchments, the river reach influence of point source effluents is unknown relative to complex background pressures. We examined water column, sediment and biofilm biogeochemical water quality parameters along river transects (200 m upstream to 1 km downstream) during summer at five wastewater treatment works (WWTW) in Scotland. Treated sewage effluent (subset, n = 3) pollutant concentrations varied between sites. Downstream concentration profiles of water and sediment biogeochemical parameters showed complex spatial changes. A hypothesised point source signature of elevated concentrations of pollution immediately downstream of WWTW then a decaying pollution 'plume' did not commonly occur. Instead, elevated soluble reactive phosphorus (SRP), ammonium and coliforms (maximum 0.23 mgP/l, 0.33 mgN/l and >2 × 10 6 MPN/100 ml) occurred immediately downstream of two WWTW, whereas some downstream pollutant concentrations decreased. Microbial substrate respiration responses only differed 1 km downstream. Significantly greater concentrations of sediment metal occurred >500 m downstream, likely due to the redeposition of historic contaminated sediments. Significantly lowered chlorophyll-a downstream of one WWTW coincided with elevated metals, despite water SRP and sediment P increases. Overall, stress caused to microbes and algae by effluent contaminants outweighed the subsidy effect of WWTW nutrients. We observed variable effluent flows to the rivers limited localised pollution downstream of WWTW and overall influence of arable land cover on river water quality. Together, this challenges views of consistently discharging point sources impacting low dilution sensitive rivers in summer contrasting with 'diffuse' sources. Thus, river water column and benthic compartments are altered at varying scales by point sou

Pathways for replacing chemical fertilisers (reliant on rock P resources) with alternative P-bearing materials require assessment of soil processes, crop nutrient acquisition and potential pollution consequences. We examined bioenergy waste materials, individually and as combined ash and anaerobic digestate in terms of plant P availability and mobility. We compared initial effects on mixing of amendments with a test soil and effects after 6-weeks pot trials, with and without wheat growth, against soil and chemical fertiliser controls. Chemical extractions, 31P NMR spectroscopic determination of P forms and phytase-labile P assays examined processes of P release. 31P NMR analysis revealed that ash comprised dominantly inorganic orthophosphate P with inherent low P solubility. Initial ash mixing with soils increased solution pH, soluble P in water (pure ash alone) and in citrate (ash alone and in blends). Digestate comprised a diverse array of orthophosphate and organically-complexed P forms, similar to the test soil P compositions, with limited P solubility on initial mixing. Following no plant incubations high water-soluble P with ash additions remained but all effects on citrate-soluble P were normalised. Incubations with plants increased water-soluble P in digestate only and blended amendment treatments relative to initial mixing. When comparing to chemical fertiliser the digestate plus ash blends led to smaller water-soluble P, but equal P in above-ground biomass after incubation. The ash-digestate C:N:P ratios and P form diversity appeared to promote microbial regulation of plant P availability versus potential leaching. The results suggest that the initial days-months are important periods for amendment interactions with soils during plant establishment and the lag before strong growth when system P mobility may induce polluted runoff. Biogeochemical P solubility controls require further study across differing soils and timescales to inform management of bioene