The water quality of the Great Ouse

The results of a 1-year detailed water quality study of the Great Ouse are presented. Being recharged to an important degree from groundwater sources within the carbonate bearing bedrock, the waters are enriched in base cations and they are of relatively high alkalinity (approx. 4000 microEq./l) and pH (approximately 8). Many chemical constituents such as sodium, chloride, boron, nitrate and soluble reactive phosphorus vary over time in response to changing flow conditions. For most determinands, concentrations decrease with increasing flow in response to dilution of point and groundwater sources by rainfall. However, for barium and nitrate, concentrations increase with flow and this is indicative of increased runoff from the agriculturally impacted soils. While this feature is certainly expected for nitrate due to fertiliser application, the pattern for barium is at face value unexpected as it would normally be expected to behave like other divalent base cations such as calcium, coming from aquifer leaching. However, it seems that agricultural disturbance of the land can also lead to enhanced runoff: a feature now becoming apparent in several UK studies. The concentrations of micro-organic herbicides vary over time in response to the different times of application and only isoproturon shows a clear response to varying flow conditions. With regards to biological processes, this is manifest by changes in pH, dissolved carbon dioxide and silica. At most times of the year the waters are oversaturated with respect to atmospheric CO2 (EpCO2) by a factor of approximately 6 and at these times pH is approximately 7.7 and silica concentrations are approximately 4 mg-Si/l. However, in the early spring period pH increases to values over 8, EpCO2 declines to about the atmospheric levels and silica declines to approximately 1 mg-Si/l. This change probably reflects the dynamic diatom blooms and decays common in this river.