The concentration-discharge slope as a tool for water quality management

Recent technological breakthroughs of optical sensors and analysers have enabled matching the water quality measurement interval to the time scales of stream flow changes and led to an improved understanding of spatially and temporally heterogeneous sources and delivery pathways for many solutes and particulates. This new ability to match the chemograph with the hydrograph has promoted renewed interest in the concentration-discharge (c-q) relationship and its value in characterizing catchment storage, time lags and legacy effects for both weathering products and anthropogenic pollutants. In this paper we evaluated the stream c-q relationships for a number of water quality determinands (phosphorus, suspended sediments, nitrogen) in intensively managed agricultural catchments based on both high-frequency (sub-hourly) and long-term low-frequency (fortnightly-monthly) routine monitoring data. We used resampled high-frequency data to test the uncertainty in water quality parameters (e.g. mean, 95th percentile and load) derived from low-frequency sub-datasets. We showed that the uncertainty in water quality parameters increases with reduced sampling frequency as a function of the c-q slope. We also showed that different sources and delivery pathways control c-q relationship for different solutes and particulates. Secondly, we evaluated the variation in c-q slopes derived from the long-term low-frequency data for different determinands and catchments and showed strong chemostatic behaviour for phosphorus and nitrogen due to saturation and agricultural legacy effects. The c-q slope analysis can provide an effective tool to evaluate the current monitoring networks and the effectiveness of water management interventions. This research highlights how improved understanding of solute and particulate dynamics obtained with optical sensors and analysers can be used to understand patterns in long-term water quality time series, reduce the uncertainty in the monitoring data and to manage eutrophication in agricultural catchments.

Unravelling organic matter and nutrient biogeochemistry in groundwater-fed rivers under baseflow conditions: Uncertainty in in situ high-frequency analysis

In agricultural catchments, diffuse nutrient fluxes (mainly nitrogen N and phosphorus P), are observed to pollute receiving waters and cause eutrophication. Organic matter (OM) is important in mediating biogeochemical processes in freshwaters. Time series of the variation in nutrient and OM loads give insights into flux processes and their impact on biogeochemistry but are costly to maintain and challenging to analyse for elements that are highly reactive in the environment. We evaluated the capacity of the automated monitoring to capture typically low baseflow concentrations of the reactive forms of nutrients and OM: total reactive phosphorus (TRP), nitrate nitrogen (NO₃-N) and tryptophan-like fluorescence (TLF). We compared the performance of in situ monitoring (wet chemistry analyser, UV-vis and fluorescence sensors) and automated grab sampling without instantaneous analysis using autosamplers. We found that automatic grab sampling shows storage transformations for TRP and TLF and do not reproduce the diurnal concentration pattern captured by the in situ analysers. The in situ TRP and fluorescence analysers respond to temperature variation and the relationship is concentration-dependent. Accurate detection of low P concentrations is particularly challenging due to large errors associated with both the in situ and autosampler measurements. Aquatic systems can be very sensitive to even low concentrations of P typical of baseflow conditions. Understanding transformations and measurement variability in reactive forms of nutrients and OM associated with in situ analysis is of great importance for understanding in-stream biogeochemical functioning and establishing robust monitoring protocols.

Understanding nutrient biogeochemistry in agricultural catchments: the challenge of appropriate monitoring frequencies

We evaluate different frequencies of riverine nutrient concentration measurement to interpret diffuse pollution in agricultural catchments. We focus on three nutrient fractions, nitrate-nitrogen (NO3-N), total reactive phosphorus (TRP) and total phosphorus (TP) observed using conventional remote laboratory-based, low-frequency sampling and automated, in situ high-frequency monitoring. We demonstrate the value of low-frequency routine nutrient monitoring in providing long-term data on changes in surface water and groundwater nutrient concentrations. By contrast, automated high-frequency nutrient observations provide insight into the fine temporal structure of nutrient dynamics in response to a full spectrum of flow dynamics. We found good agreement between concurrent in situ and laboratory-based determinations for nitrate-nitrogen (Pearson's R = 0.93, p < 0.01). For phosphorus fractions: TP (R = 0.84, p < 0.01) and TRP (R = 0.79, p < 0.01) the relationships were poorer due to the underestimation of P fractions observed in situ and storage-related changes of grab samples. A detailed comparison between concurrent nutrient data obtained by the hourly in situ automated monitoring and weekly-to-fortnightly grab sampling reveals a significant information loss at the extreme range of nutrient concentration for low-frequency sampling.

Establishing relative release kinetics of faecal indicator organisms from different faecal matrices

AIMS

A laboratory assay for comparative characterization of various faecal matrices with respect to faecal indicator organism (FIO) release using, artificial rain water.

METHODS AND RESULTS

Fresh sheep and beef-cattle faeces, dairy cattle slurry and beef cattle farm yard manure (FYM) were collected from commercial units in south-west England and applied to 20 randomized 1 m(2) plots established on permanent grassland. Representative samples from each faecal matrix (n = 5) were collected on four occasions over 16 days. One gram of each sample was transferred to a sterile vial to which 9 ml of standard local rain was carefully pipetted. The vial was then rotated through 360 degrees, 20 times in 60 s to 'simulate' a standardized interaction of the faecal material with rainfall, providing an assay of comparative release potential. Appropriate decimal dilutions were prepared from the eluent. Following agitation, with a sterile spatula, the remaining faecal material and eluent in the vials were vortex mixed for 60 s before decimal dilutions were prepared from the resulting mixture, providing a quantitative assessment of the total FIO in the sample from which percentage release could be determined. Bacterial concentrations were enumerated in duplicate by membrane filtration following standard methods for FIO. Significant differences in release kinetics of Escherichia coli and enterococci from each of the faecal matrices were determined.

CONCLUSIONS

Differences in release from each faecal substrate and between FIO type (E. coli and intestinal enterococci) were observed in this laboratory study. The order of release of E. coli from the faecal matrices (greatest to least, expressed as a percentage of the total present) was dairy cattle slurry > beef cattle FYM > beef-cattle faeces > sheep faeces. For intestinal enterococci the order of percentage release was dairy cattle slurry > beef-cattle faeces > beef cattle FYM > sheep faeces.

SIGNIFICANCE AND IMPACT OF THE STUDY

This laboratory-based method provides the first data on the relative release kinetics of FIO from different faecal matrices in rain water. This is fundamental information needed to parameterize laboratory-based microbial models and inform approaches to field and catchment risk assessment.

The release of phosphorus to porewater and surface water from river riparian sediments

Sediments can be both a source and a sink of dissolved phosphorus (P) in surface water and shallow groundwater. Using laboratory mesocosms, we studied the influence of flooding with deionized water and simulated river water on P release to solution using sediment columns taken from a riparian wetland. The mesocosm incubation results showed that rather than retaining nutrients, sediments in the riparian zone may be a significant source of P. Concentrations of dissolved P in porewater reached more than 3 mg L(-1) and in surface water over 0.8 mg L(-1) within a month of sediment inundation. The reductive dissolution of P-bearing iron (Fe) oxides was the likely mechanism responsible for P release. Dissolved P to Fe molar ratios in anaerobic samples were approximately 0.45 when columns were flooded with water that simulated the chemistry of the adjacent river. This suggests there was insufficient Fe in the anaerobic samples to precipitate all P if the solutions were oxygenated or transported to an aerobic environment. If the anaerobic wetland solutions were delivered to oxygenated rivers and streams adjacent to the riparian zone, the equilibrium concentration of P in these systems could rise. The timing of P release was inversely related to the nitrate (NO3-) concentration in floodwater. This indicates that in riparian zones receiving low nitrate loads, or where NO3- loads are being progressively reduced, the risk of dissolved P release may increase. These findings present particular challenges for restoration and management in riparian areas.

Field drains as a route of rapid nutrient export from agricultural land receiving biosolids

We report research on the environmental risk of incidental nutrient transfers from land to water for biosolids amended soils. We show that subsurface (drainflow) pathways of P transport may result in significant concentrations, up to 10 mg total P l(-1), in the drainage network of an arable catchment when a P source (recent biosolids application) coincides with a significant and active transport pathway (rainfall event). However, the high P concentrations were short-lived, with drainage ditch total P concentrations returning to pre-storm concentrations within a few days of the storm event. In the case of the drainflow concentrations reported here, the results are unusual in that they describe an 'incidental event' for a groundwater catchment where such events might normally be expected to be rare owing to the capacity of the hydrological system to attenuate nutrient fluxes for highly adsorbed elements such as P. Consequently, there is a potential risk of P transfers to shallow groundwater systems. We suggest that the findings are not specific to biosolids-alone, which is a highly regulated industry, but that similar results may be anticipated had livestock waste or mineral fertilizer been applied, although the magnitude of losses may differ. The risk appears to be more one of timing and the availability of a rapid transport pathway than of P source.

Phosphorus dynamics observed through increasing scales in a nested headwater-to-river channel study

The hypothesis that the dynamic patterns of phosphorus (P) transport at plot scale are similar to the patterns that could be observed quasi-simultaneously (i.e., approximately at the same time) at a river basin scale, in terms of inputs and dilution of P, across a range of rainfall and runoff conditions, was tested. From this information, it was hoped to be able to make some simple inferences about the connectivity or mass flux of P transport between the different scales of observation. An intensive study using 30-m2 plots, 1-ha plots and nested river channel sites ranging in scale from 20 ha up to a maximum of 834 km2 in the River Taw basin, South West England, UK, was conducted with three campaigns under differing flow conditions: (1) a zero rainfall base flow period, (2) a 10-mm rainfall residual flow period, and (3) a 42-mm rainfall storm flow period. The mass flux of total P ranged from 49 kg during base flow to 4 tonnes during the storm period at the largest 834 km2 scale. During base flow conditions, total phosphorus (TP) concentrations from diffuse sources were low (26 microg L-1 in the upper catchment) and reactive P forms dominated the fractions filtered <0.45 microm. During storm flow, concentrations of TP increased at all scales within the drainage basin, to a maximum of 500 microg L-1 and were sufficient to override the effect of any point source inputs of P. Unreactive (i.e., mostly 'organic') forms of P dominated the fractions that were >0.45 microm during residual flows and storm flows. Spatially normalised discharge apparently decreased with increasing scale, most notably during storm flow conditions and this implies that there is some storage of water through the catchment and in part may reflect varying velocities of water in different pathways. Most attenuation and dilution of P appeared to occur at larger scales, whilst the hydrological connectivity between source areas and receiving waters was greatest at smaller scales (<20 ha), and during the highest flows. The importance of diffuse agricultural sources in contributing to P export through the basin was dominated by dynamic temporal changes in hydrological activity, with a 'piston pushing' effect of particulate associated P through the basin as it wets up in response to rainfall input. We concluded that the processes of P transfer are different at different scales. However, some uncertainties of spatial heterogeneity around the catchment underlie the difficulties in dealing with scale and thus more data and studies of this nature are required.

A tiered risk-based approach for predicting diffuse and point source phosphorus losses in agricultural areas

Implementation of the European Union Water Framework Directive requires an assessment of the pressures from human activity, which, combined with information on the sensitivity of the receiving waterbody to the pressures, will identify those water bodies at risk of failing to meet the Directive's environmental objectives. Part of the process of undertaking the risk assessment for lakes is an assessment of diffuse agricultural phosphorus (P) pressures. Three approaches of increasing sophistication were developed for this purpose: a basic 'risk screening' approach (tier 1) applicable to all lakes in Great Britain (GB) and based on export coefficients for different land cover classes and animal types; the Pressure Delivery Risk Screening Matrix approach (tier 2) that differentiated between pressures in surface water and groundwater river basins; and the Phosphorus Indicators Tool (PIT), a simple model of locational risk and P delivery potential (tier 3). Application of the three approaches to a range of lake catchments in England demonstrated that a tiered risk assessment approach was appropriate which was tailored to the quality of the available data. A step-wise procedure was developed whereby if the tier 1 and 2 approaches showed a catchment to be at high risk of failing to meet the Directive's environmental objectives with regard to P, it was justifiable to undertake a more detailed assessment using the tier 3 approach. The tier 1 approach was applied to all lakes in GB greater than 1 ha in size on the assumption that the boundary between the good/moderate status classes under the Water Framework Directive guidelines represented a doubling of the total P (TP) reference conditions. The initial outputs suggested that 51% of lakes in GB are predicted to not meet the TP targets identified for high or good status and must, therefore, be considered at risk. There were regional differences in numbers of lakes at risk. Scotland appeared to have the fewest sites at risk (18%); England the most (88%), with Wales having an intermediate percentage (56%). A comparison of P pressures on freshwaters using the tier 2 approach with other pressures on waterbodies (e.g. nitrate, sediment) in GB is shown as risk maps on the Environment Agency website at: . The tier 3 approach was applied to data-rich catchments and identified at the 1 km(2) areas of relatively high risk of P delivery to water.

The phosphorus transfer continuum: linking source to impact with an interdisciplinary and multi-scaled approach

This critical review introduces a template that links phosphorus (P) sources and mobilisation processes to the delivery of P to receiving waters where deleterious impact is of concern. It therefore serves as a key introductory paper in this special issue. The entire process is described in terms of a 'P transfer continuum' to emphasise the interdisciplinary and inter-scale nature of the problem. Most knowledge to date is derived from mechanistic studies on the sources and mobilisation of P using controlled experiments that have formed the basis for mitigation strategies aimed at minimising transfer from agricultural fields. However, our ability to extrapolate this information to larger scales is limited by a poor knowledge base while new conceptual advances in the areas of complex systems and fractal dynamics indicate the limitations of past theoretical frameworks. This is compounded by the conceptual and physical separation of scientists working at different scales within the terrestrial and aquatic sciences. Multi-scaled approaches are urgently required to integrate different disciplines and provide a platform to develop mechanistic modelling frameworks, collect new data and identify critical research questions.