Hydrology and the ecological quality of Scottish river ecosystems

Hydrology is a primary control on the ecological quality of river systems, through its influence on flow, channel geomorphology, water quality and habitat availability. Scottish rivers are widely perceived to be of high ecological quality, with abundant flow volumes and high water quality. However, historical and current river flow regulations, and land use change have altered the physical and chemical characteristics of Scottish rivers, with adverse consequences for aquatic biota. Baseline hydrological, geomorphological and water quality conditions in Scottish rivers are thus summarised. The impacts of river regulation and land use change on the hydrology, geomorphology and water quality of Scottish rivers are then discussed. Consequences of these changes for aquatic habitat are examined, with particular reference to the economically significant salmonid species (Salmo salar and Salmo trutta). Policy and management issues relating to the future ecological quality of Scottish rivers are reviewed. These include the impacts of climate change on ecological quality, the calculation and implementation of ecologically acceptable flows, and river restoration and best management practices within integrated catchment planning.     

Nitrate concentrations in river waters of the upper Thames and its tributaries

The spatial and temporal patterns of in-stream nitrate concentrations for the upper Thames and selected tributaries are described in relation to point and diffuse sources for these rural catchments. The rivers associated with catchments dominated by permeable (Cretaceous Chalk) bedrock show a smaller range in nitrate concentrations than those associated with clay and mixed sedimentary bedrock of lower permeability. The differences reflect the contrasting nature of water storage within the catchments and the influence of point and diffuse sources of nitrate. Nitrate concentrations often increase in a gradual way as a function of flow for the rivers draining the permeable catchments, although there is usually a minor dip in nitrate concentrations at low to intermediate flow due to (1) within-river uptake of nitrate during the spring and the summer when biological activity is particularly high and (2) a seasonal fall in the water table and a change in preferential flow-pathway in the Chalk. There is also a decrease in the average nitrate concentration downstream for the Kennet where average concentrations decrease from around 35 to 25 mg NO(3) l(-1). For the lower permeability catchments, when point source inputs are not of major significance, nitrate concentrations in the rivers increase strongly with increasing flow and level off and in some cases then decline at higher flows. When point source inputs are important, the initial increase in nitrate concentrations do not always occur and there can even be an initial dilution, since the dilution of point sources of nitrate will be lowest under low-flow conditions. For the only two tributaries of the Thames which we have monitored for over 5 years (the Pang and the Kennet), nitrate concentrations have increased over time. For the main stem of the Thames, which was also monitored for over 5 years, there is no clear increase over time. As the Pang and the Kennet river water is mainly supplied from the Chalk, the increasing nitrate concentrations over time clearly reflect increasing nitrate concentrations within the groundwater. It primarily reflects long-term trends for agricultural fertilizer inputs and significant aquifer storage and long water residence times. The results are discussed in terms of hydrogeochemical processes and the Water Framework Directive and are compared with data from other eastern UK rivers. The importance of diffuse sources of nitrate contamination is highlighted. On a flow weighted basis, the average diffuse component of nitrate is around 95% for the Thames Basin rivers draining Chalk and for the corresponding rivers draining less permeable strata, there is a more significant but not major point source component (at least in terms of flux); the average diffuse component is 79% in this case. These data fit well with earlier assessments of agricultural sources to UK surface waters. Under baseflow conditions the diffuse sources remain dominant for the Chalk fed Thames Basin rivers, but point sources can be dominant for the low permeability cases. On a proportionate basis, the Thames Basin rivers are similar to the rural rivers of the Tweed and Humber Basins in terms of percentage diffuse components although the lower intensity agriculture occurring for the rivers monitored means that the average nitrate concentrations are lower for the rural rivers of central and northern England and the borders with Scotland: the Humber and Tweed.     

The water quality of the River Thame in the Thames Basin of south/south-eastern England

The water quality of the River Thame, a tributary of the River Thames in the Thames basin, is described in relation to point and diffuse contaminant inputs and runoff from permeable and impermeable bedrock geology with their own characteristic water quality. The data is examined to see if the market town of Aylesbury in the upper part of the catchment influences water quality. Previous studies highlighted the influence of Aylesbury sewage treatment works (STW) on soluble reactive phosphorus (SRP) concentrations in the river before and after phosphorus (P) stripping at the STW. Variations in water quality along the river are described and the study indicates that, apart from SRP, water quality determinants seem to be relatively unaffected by Aylesbury. The Thame water quality is compared with other catchment typologies and it is very similar to that of the main stem of the Thames even though the Thames is mainly Chalk groundwater fed. Differences in water quality largely link to the amount of STW effluent within the rivers and to the endmember compositions of the groundwater and near surface water sources.     

Modelling the dispersion of radionuclides following short duration releases to rivers: Part 1. Water and sediment

This paper evaluates and generalises state-of-the-art approaches for modelling short duration liquid discharges of radionuclides ((3)H, (14)C, (60)Co, (134)Cs, (137)Cs, (65)Zn, (89)Sr, (90)Sr, (125)I, (131)I, (241)Am, isotopes of Pu and U) to rivers. An advection-dispersion model was parameterised and used to predict the concentrations of radionuclides in the river environment, i.e. in river water, river bed sediment and fish (Part II of this paper covers uptake to fish). The coupled transport and bio-uptake model was used to predict the concentrations of radionuclides in the River Thames, UK, and one of its tributaries as a result of hypothetical short duration discharges. A simplified version of this model was developed and presented as "look-up" graphs. The influence of various environmental parameters on model output was evaluated by sensitivity analysis. Time-integrated water and sediment concentrations and maximum sediment concentrations may be predicted for all rivers on the basis of the river volumetric flow rate only. Maximum concentration in water is, however, also dependent on other river characteristics. For this latter case, generalised modelling approaches are tested for use in situations where detailed hydrological and dispersion data are not available.     

The river landscapes of London and the Thames catchment: their status and future

Recent river landscape assessments of the eastern part of the Thames catchment have enabled a better understanding of their landscape status and value. These rivers have formed the focal point for human settlement and have been extensively affected by their use. Flood defence through channelization and surfacing of the watercourses has been highly detrimental to their landscape quality. Flooding has itself been exacerbated by catchment urbanization and by land drainage systems. The issues affecting the landscape planning and design of river systems are examined and suggestions made as to how they can be addressed.     

Radio-iodine in thyroid glands of swans, farm animals and humans, also in algae and river water from the Thames Valley, England

A highly sensitive counting system has been used to measure radio-iodine in environmental samples from the Thames Valley. Iodine-125 and occasionally iodine-131 have been found in the thyroid glands of most of the swans that have died on the River Thames, the River Wey and the Grand Union Canal, and in algae and water samples from the Thames and many of its tributaries. The presence of this activity is ascribed to the waste discarded into the drainage system by hospitals and research laboratories, reaching the rivers via the effluent from sewage treatment works. The Thames is used as a source of drinking water, particularly in London and its western approaches. Weed and water samples collected from river water abstraction points, reservoirs, tap water supplies, and animal water troughs fed from this supply all contained low levels of iodine-125. The drinking water route can account for the iodine-125 found in the thyroids of farm animals from west Surrey and in a few people living in London. The amounts found constitute a trivial radiation dose to man and animals as they are far below the acceptable limit of exposure for man.     

Streambed Sediment as a Barrier to Groundwater Pollution: Insights from Fieldwork and Modelling in the River Thames Basin

Integrated fieldwork and modelling has been undertaken to assess the vulnerability of riverside wells to pollution when river water is contaminated. Results highlight the importance of streambed sediments as a barrier to groundwater pollution by river water. The bed of the River Thames is lined with brown and grey organic-rich silts of low hydraulic conductivity (about 0.002 m/d). A 3-D model for solute transport in stream-aquifer systems was developed and applied to two riverside sites in the middle Thames valley. Model results indicate that well-water quality would only breach EC limits if the river was heavily polluted for a week or even a month (clearly an unlikely scenario). Sensitivity analyses showed the models to be far more sensitive to streambed parameters than to aquifer parameters. Thus it would be more sensible to focus fieldwork on the measurement of streambed properties than on the more usually measured aquifer parameters.     

A comparison of two river quality models

Two deterministic river quality models are evaluated and compared by applying them to the R. Blackwater. The first model, Qual-II is a general purpose universal model developed by the United States Environment Protection Agency and the second a version of the Water Research Centre river quality model (Blackwater model) developed specifically for the R. Blackwater by Thames Water Authority but is no longer in use. After model calibration, the general Qual-II model produced a closer prediction than the Blackwater model, when compared with observed data. The main reason for the differences in the model results was considered to be the treatment of non-point input sources. The results suggest that if modelling resources are limited, the Qual-II model can be applied rather than a river specific model.     

The equitable approach to pollution control management in the thames estuary

Wood & Cockburn (IAWPR Conference, Vienna, 1979) outlined the theory of an equitable approach to pollution control. In order to use this approach, the pollution budget for the stretch of river must be defined, and this budget must be distributed equitably among the dischargers. The equitable approach assumes that all dischargers have an equal right to a share of the budget proportional to the potential load which would have been discharged were no treatment given. This paper extends the theory into practice and discusses some of the advantages and problems in the Thames Estuary. The dissolved oxygen pollution budget, based on quality objectives for the Thames, is defined, and is distributed equitably amongst the dischargers. In setting the equitable allowances it is clear that other restraints prevent the fresh water rivers from using their full allowance, and this becomes available for other dischargers, further, it is difficult to properly ascribe allowances for non-point discharges and storm discharges. Because the setting of the pollution budget, and the equitable approach use two different bases, they are not necessarily compatible, and therefore in the Thames a purely equitable approach would result in some failure to meet the objectives. A method for overcoming the problem is set out in the paper. When compatibility between equitability and the quality objectives has been achieved, required discharge standards can be calculated.     

An advanced modelling tool for simulating complex river systems

The present paper describes MOHID River Network (MRN), a 1D hydrodynamic model for river networks as part of MOHID Water Modelling System, which is a modular system for the simulation of water bodies (hydrodynamics and water constituents). MRN is capable of simulating water quality in the aquatic and benthic phase and its development was especially focused on the reproduction of processes occurring in temporary river networks (flush events, pools formation, and transmission losses). Further, unlike many other models, it allows the quantification of settled materials at the channel bed also over periods when the river falls dry. These features are very important to secure mass conservation in highly varying flows of temporary rivers. The water quality models existing in MOHID are base on well-known ecological models, such as WASP and ERSEM, the latter allowing explicit parameterization of C, N, P, Si, and O cycles. MRN can be coupled to the basin model, MOHID Land, with computes runoff and porous media transport, allowing for the dynamic exchange of water and materials between the river and surroundings, or it can be used as a standalone model, receiving discharges at any specified nodes (ASCII files of time series with arbitrary time step). These features account for spatial gradients in precipitation which can be significant in Mediterranean-like basins. An interface has been already developed for SWAT basin model.     

Decreasing boron concentrations in UK rivers: Insights into reductions in detergent formulations since the 1990s and within-catchment storage issues

The changing patterns of riverine boron concentration are examined for the Thames catchment in southern/southeastern England using data from 1997 to 2007. Boron concentrations are related to an independent marker for sewage effluent, sodium. The results show that boron concentrations in the main river channels have declined with time especially under baseflow conditions when sewage effluent dilution potential is at its lowest. While boron concentrations have reduced, especially under low-flow conditions, this does not fully translate to a corresponding reduction in boron flux and it seems that the “within-catchment” supplies of boron to the river are contaminated by urban sources. The estimated boron reduction in the effluent input to the river based on the changes in river chemistry is typically around 60% and this figure matches with an initial survey of more limited data for the industrial north of England. Data for effluent concentrations at eight sewage treatment works within the Kennet also indicate substantial reductions in boron concentrations: 80% reduction occurred between 2001 and 2008. For the more contaminated rivers there are issues of localised rather than catchment-wide sources and uncertainties over the extent and nature of water/boron stores. Atmospheric sources average around 32 to 61% for the cleaner and 4 to 14% for the more polluted parts.The substantial decreases in the boron concentrations correspond extremely well with the timing and extent of European wide trends for reductions in the industrial and domestic usage of boron-bearing compounds. It clearly indicates that such reductions have translated into lower average and peak concentrations of boron in the river although the full extent of these reductions has probably not yet occurred due to localised stores that are still to deplete.     

Modelling of BOD-DO dynamics in an ice-covered river in Northern China

BOD-DO dynamics in a river in northern China during a period of sub-zero temperatures have been investigated and a mathematical model which describes the dynamics has been proposed. The model parameters: deoxygenation rate k₁, reaeration rate k₂, BOD sedimentation—resuspension rate k₃ were determined by a parameter estimation method using the field data collected on Tu Men River. Validation of the model shows that BOD-DO dynamics in the river during that season are well described by the model. The model offers a tool for understanding the natural purification process in the rivers in northern China under ice-cover conditions and also for control of water quality in those rivers.     

Impact of combined sewer overflow on urban river hydrodynamic modelling: a case study of the Chicago waterway

Combined sewer overflow (CSO) can be a critical inflow source for urban rivers during storm events. This paper presents a case study of the Chicago waterway. A three-dimensional (3D) river hydrodynamic model was developed and integrated with an urban rainfall-runoff model using the Open Modelling Interface (OpenMI). Both the effects of CSO discharge on river and river water levels on CSO outlets were considered by the integrated model. A historical storm, which was similar to a 100-year return period rain event, was simulated and compared with field measurements. This study highlights the necessity of quantifying CSO for hydraulic modelling of urban rivers under extreme storm event conditions, and shows that an integrated hydrologic and hydraulic approach can be used to address this challenge. The 3D river hydrodynamic model can deal with the complex hydrodynamics at river confluences and provide better hydrodynamic results for water quality modelling in the future.     

A model for nitrate-nitrogen transport and denitrification in the river Thames

A mathematical model describing the mass flow, including denitrification, of nitrate in the main river Thames has been developed. A three year period, including severe drought and flooding, was used to test the model. Good agreement was found between the nitrate concentrations observed at several sites on the river Thames and those calculated by the model. The model be used to establish the sensitivity of nitrate levels in the Thames above intakes to changes in tributary inputs which could be affected by various management strategies.     

Shrimps and whitebait in the polluted Thames estuary

This paper records the seasonal incidence of shrimps, sprats and herrings, the most abundant nektonic species, off West Thurrock in the Thames estuary during 1967–70. It also deals with their age classes and their diet. The object of monitoring the shrimp fish fauna is to establish a quantitative base line against which the effects of future developments for the river, such as the erection of a barrier, can ultimately be measured. It can also serve as an index to the effects of the strike of public employees of sewage plants on the state of the river, and monitoring for this purpose continues and will be reported later.     

Factors Governing Macrophyte Status in Hampshire Chalk Streams: Implications for Catchment Management

Severe droughts in the late 1980s and 1990s have highlighted the fragility of many aquatic environments in southern England; in particular, declining river flows may be having an adverse affect on macrophytes in the chalk-fed streams of the Rivers Test and Itchen. Detailed macrophyte survey data which were collected from representative sites in both rivers during 1991–1995 are interpreted in the context of changing hydroclimatic, water quality and in-stream channel conditions. Multivariate statistical analyses support the assertion that river flow is the most significant parameter governing observed changes in macrophyte cover. During low flow years, filamentous algae have increased at most sites to the detriment of Ranunculus spp. These findings have catchment-management implications which should be addressed if the long-term integrity of such aquatic ecosystems is to be safeguarded.     

Modeling and Monte Carlo simulation of TCDD transport in a river

A one-dimensional water quality model to assess the long-term fate of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in three compartments (water, sediment, fish) of a river has been developed using the literature data on various model parameters. The transient deterministic model with constant or nonrandom parameters is solved numerically by the method of orthogonal collocation, while an analytical solution is developed for the steady-state model. The impact of uncertainty in several model parameters has been studied by means of Monte Carlo simulations assuming that the uncertain parameters are uncorrelated and can be modeled by three probability distributions (uniform, normal and lognormal). For the case of a high TCDD discharge into a small, shallow river, we find that the maximum TCDD contents of water and fish are well below the prescribed safe limits. We also find that the effects of uncertainty on water quality metrics are quite complex or nonintuitive and can be substantial. This is especially true for TCDD in fish, which can be higher by as much as 50-70% than the deterministic predictions, if the parameter uncertainties follow uniform distributions.     

Calcite saturation in eastern UK rivers

Calcite saturation in eastern UK rivers is assessed in relation to the potential kinetic inhibition of calcite precipitation. Two well established inhibitors are considered: soluble reactive phosphorus (SRP, i.e. inorganic monomeric phosphorus); and dissolved organic carbon (DOC). The rivers show a range of calcite saturation levels from approximately 1 hundredth to approximately 100-fold. The greatest range occurs for the northernmost river considered, the Tweed, where the waters range from highly unsaturated to highly oversaturated. The lowest range occurs for the most southerly rivers (the Great Ouse and the Thames) where the waters are consistently oversaturated with respect to calcite. The contrasting patterns relate to a greater diversity of water quality within the northern regions. Thus, during the winter, the main waters are derived from the upland areas with acidic soils and low weathering rates. During the summer baseflow periods, groundwater inputs are more important and high photosynthesis results in particularly high pHs and calcite oversaturation. In contrast, for the southern rivers, the main source of water during both the summer baseflow and the winter highflow periods comes from calcium carbonate rich aquifer sources. Statistical analysis of pH vs. the logarithm of the calcite saturation index for each river indicates strong linear features with individual gradients of approximately 1. This linearity results from an autocorrelation (the logarithm of the saturation index is calculated from the pH) and this indicates that calcite solubility controls are not operative in any of the rivers examined. A comparison of calcite saturation levels and SRP and DOC concentrations show a pattern inconsistent with kinetic hindrance, although some structure is observed, probably due to the mixing reactions between point and diffuse sources of water with contrasting chemistry.     

Orthophosphate Concentrations in the River Thames

Observed concentrations of orthophosphate in the River Thames, from its source to its tidal limit, exceed 0.1 mg P/1 for most of its length; reasons for annual and seasonal variations are explained. By using data routinely collected by the Environment Agency throughout the Thames catchment, the orthophosphate load in the river, derived from agricultural sources and sewage-treatment works, is estimated.
A water quality model, TOMCAT, has been adapted to simulate observed data and used to estimate river concentrations if orthophosphate loads from sewage-treatment works are reduced.     

Statutory Water Quality Objectives and River Water Quality

This paper summarizes some of the key issues which have been considered in developing proposals for a scheme of classifying river quality for the purpose of setting water quality objectives on a statutory basis. Particular attention is given to (a) the relationship between use-related and general water quality targets, (b) the role of biological assessment, and (c) the requirements of EC directives. Following widespread consultation on the proposals, it was concluded that a clear distinction should be made between sets of 'use classes'which would serve to set statutory water quality objectives (SWQOs) and a general quality assessment scheme which would be applied independently to assess trends and general progress over a period of time. Once the classification scheme has been established by the Secretaries of State through regulations, SWQOs will be introduced to river stretches in a number of 'pilot'catchments across England and Wales, following local consultation. The setting of SWQOs will provide the focus for local decision-making on what the appropriate level of water quality should be – taking into account the desired uses of rivers, what will be required to achieve them, and the associated costs.