River water quality in the Humber catchment: an introduction using GIS-based mapping and analysis

The regional water quality of the Humber catchment was mapped for key inorganic chemical determinands using a GIS system and an extensive Environment Agency and LOIS monitoring database. The resultant maps revealed the major factors affecting the general characteristics of regional water quality. Sewage inputs from industrial and domestic sources account for the high concentration of many determinands in urban areas. The concentrations of particulate components increase in tidal zones because of sediment trapping and tidal re-suspension effects. Some determinands also exhibit localized high concentrations related to coal mine drainage, soil pollution caused by past ore mining, bedrock geology, the agricultural use of fertilizers and the ingression of seawater into the estuary.

Pollution regimes and variability in river water quality across the Humber catchment: interrogation and mapping of an extensive and highly heterogeneous spatial dataset

Water quality inter-relationships based on an extensive Environment Agency database are used to identify seven key types of clean and polluted riverine environments within the Humber catchment. These types were selected on the basis of linear separations between average concentrations of ammonium, chloride, dissolved nickel and chromium and acid-available particulate nickel and chromium. However, a more detailed analysis of determinand and flow relationships at each type locality revealed complex patterns due to the highly variable nature of pollutant sources at the local scale. The information presented contrasts systems at a larger scale, where simpler integrated features are observed, and which can be represented by two-component mixing models. Scale and hydrology seem to be important features in determining the simplicity or complexity of determinand inter-relationships, and the implications of these findings are discussed.

The water quality of the River Wear, north-east England

A 1-year detailed study of water quality in the River Wear in combination with longer (decadal) term Environment Agency data (Harmonised Monitoring Scheme) show the influences of historic lead-zinc and coal mining and sewage inputs. The water quality for many determinands, such as sodium, chloride, boron, nitrate, and soluble reactive phosphorus, varies seasonally due to changing flow conditions. For most dissolved determinands, concentrations decrease with increasing flow in response to dilution of point and diffuse sources by rainfall. However, concentrations increase with increasing flow for dissolved organic carbon, aluminium, lead, iron, yttrium, and the lanthanides and actinides. This increase probably reflects two processes. Firstly, trace element-enriched runoff occurs from the acidic moorland areas of the catchment when it wets up. Secondly, at high flows, increased production of transition metals bearing microparticulate material occurs; these pass through conventional filters used to separate dissolved from particulate materials. For the particulate components, iron, manganese, aluminium, and some trace transition metals are present above the analytical detection limits and concentrations increase with increasing flow in response to increasing suspended sediment levels. However, particulate metal concentrations are relatively low for the Wear compared to the other eastern UK rivers, even though historic lead-zinc mining activity in the upper portions of the Wear catchment has led to extensive spoil contamination of the land. This feature probably reflects the lack of a significant flood plain area, where trace contaminants can accumulate within the sediments. The importance of inputs of pumped water from former coalmines is highlighted; increased manganese and dissolved carbon dioxide concentrations and reduced pH result. Examination of the Harmonised Monitoring Scheme data indicate no clear long-term changes in water quality for all but one determinand, although analysis is hampered by changes in sampling frequency for all determinands and changes in detection limits for certain trace metals. The only clear long-term change is a reduction in the micro-organic insecticide, aldrin. Nonetheless, the long-term data, when plotted as a function of month, follow the same pattern as the LOIS data. For aldrin, the higher values during the early period of record occurred in the latter half of each year, presumably as a consequence of seasonal application and enhanced leaching during the autumn.

Patterns in trace element chemistry in the freshwater tidal reaches of the River Trent

The major, minor and trace element chemistries at two freshwater tidal sites on the River Trent are described and compared with the non-tidal river, to demonstrate the variability in chemical processes in a major chemically and hydrologically active part of the river system, which is often overlooked in freshwater hydrochemical studies. The study shows a chemical gradient along which concentrations increase downstream for suspended sediments and acid-available particulate (AAP; > 0.45 microm) trace element fractions and also the 'dissolved' (< 0.45 microm) fractions of those trace elements with low-solubility phases. The study highlights the importance of tidally induced suspended-sediment dynamics and the generation of microparticulates for trace element transport within the tidal reaches. The microparticulate and particulate fractions are associated with both anthropogenic and lithogenous sources, and the concentrations of these two fractions co-vary according to a two-component mixing model involving riverine and tidal endmembers, with the tidal endmember exhibiting a strong lithogenous component. In many cases the AAP trace element concentrations are highly linearly correlated. At the downstream tidal site, suspended sediment concentrations were particularly high and ranged from 44 to over 24,000 mg/l as a result of tidally induced sediment mobilisation. As a consequence of this, particulate metal concentrations are especially high, and here, for the first time within the east coast studies of UK rivers, particulate fractions were measurable for trace elements such as boron and molybdenum, previously reported above detection limits only in dissolved form. It is demonstrated that the microparticulate components are not simply related to suspended sediment concentrations or to the bulk composition of the AAP fraction. Rather, microparticulate generation is related to a more complex pattern, probably linked to hydrodynamic factors involving sediment resuspension and microparticulate transport mechanisms.

The water quality of the River Trent: from the lower non-tidal reaches to the freshwater tidal zone

The water quality of a major river system, the River Trent, which flows into the Humber estuary and drains large centres of population and industry in central England, is examined in terms of trace element, nutrient, major-ion, suspended sediment and carbon concentrations and set within the context of the transition from non-tidal to the tidal freshwater reaches. Detailed investigation of the water quality variability in the non-tidal river Trent has revealed mainly simple patterns in dissolved chemical concentrations, controlled largely by hydrology and the mixing of baseflow and stormflow endmember chemistries. However, silicon, carbon and nitrate show much more complex behaviour, as concentrations of these chemical determinands are regulated by in-stream biological processes. Major increases in concentrations of suspended solids and acid-available particulate (AAP) trace elements are shown to be linked to tidal mobilization of sediment and the location of the 'turbidity maximum'. Reductions in the concentrations of AAP trace elements per gram of suspended sediment suggest the mobilization of a different sediment type with a lower trace element content, which may result from: (i) a secondary sediment source; or (ii) mobilization of coarser-grained sediment under the stronger tidal hydrodynamic forces. Increased correlations for AAP-trace element interrelationships downstream suggest increasing simplicity of mixing relationships.

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.