AbstractPhosphorus (P) is a key element for crop production and often a key nutrient limiting biological production to waterbodies in agricultural catchments. Although a suite of regulations on the concentration of P in rivers exist under the EU Water Framework Directive and Habitats Directive, there is a limited understanding on the background concentration of P that is derived from the weathering of natural P-bearing minerals within the catchment. This project investigates this issue in the headwaters of the Hampshire Avon catchment in southern England where P concentrations currently breach P standards for Good Ecological Status under the EU WFD and at which adverse effects on ecology are unlikely under the EU Habitats Directive. Drilled core samples were recovered from farmland and woodland sites in the Vale of Pewsey, which is underlain by the Upper Greensand (UGS) aquifer, where the headwaters of the Upper Avon are drained.
Chemical analysis of UGS bulk rock and porewater samples indicate that solid P concentration ranges between 0-0.8 wt.%, and soluble reactive P and total dissolved P concentrations range from below the limit of detection (1 mg/L in a small number of samples. Mineralogical analysis of bulk UGS samples showed that the most abundant natural P-bearing mineral form is carbonate fluorapatite (~0.1 wt.% on average) and is commonly present in the form of P nodules. These are found to accumulate in four UGS horizons and when accessible by water they dissolve very slowly contributing a very little P flux to the river. By contrast, the most abundant anthropogenic P-bearing mineral is microcrystalline hydroxyapatite (~0.01 wt.% on average). Mineralogical evidence and geochemical modelling show that it is highly reactive and can rapidly dissolve, migrate downward, and re-precipitate many times as it moves from surface to the aquifer below. Additionally, high P concentrations in the porewaters are never co-incident with P nodule horizons, suggesting that dissolved porewater P concentrations are mainly controlled by the anthropogenic surficial P inputs and the dissolution of hydroxyapatite, rather than the weathering of carbonate fluorapatite nodules in the UGS bedrock.
Overall, this study suggests that natural P concentrations in the UGS are very low, and probably lower than the lowest P thresholds already included in the EU Habitats Directive for this type of river. The majority of P in the porewaters in the UGS is likely to be associated with anthropogenic activity in the Vale of Pewsey, with P flux to the aquifer passing through several phases of precipitation and dissolution in the form of hydroxyapatite, which acts as a transient phase, as P migrates vertically downward to the water table. Finally, this study demonstrates that the UGS aquifer behaves as a restraining filter for P. It is neither an unreactive medium, nor retains P, but instead it attenuates the P transfer continuum linking land to streams.
|Date of Award
|21 Jan 2021
|Penny J Johnes (Supervisor), Heather L Buss (Supervisor) & Daren Gooddy (Supervisor)