Modelling metaldehyde in catchments: a River Thames case-study

Spatially resolved environmental fate models: A review

Spatially resolved environmental models are important tools to introduce and highlight the spatial variability of the real world into modeling. Although various spatial models have been developed so far, yet the development and evaluation of these models remain a challenging task due to several difficulties related to model setup, computational cost, and obtaining high-resolution input data (e.g., monitoring and emission data). For example, atmospheric transport models can be used when high resolution predicted concentrations in atmospheric compartments are required, while spatial multimedia fate models may be preferred for regulatory risk assessment, life cycle impact assessment of chemicals, or when the partitioning of chemical substances in a multimedia environment is considered. The goal of this paper is to review and compare different spatially resolved environmental models, according to their spatial, temporal and chemical domains, with a closer insight into spatial multimedia fate models, to achieve a better understanding of their strengths and limitations. This review also points out several requirements for further improvement of existing models as well as for their integration.

Trends in metaldehyde concentrations and fluxes in a lowland, semi-agricultural catchment in the UK (2008-2018)

Metaldehyde, a widely used molluscicide, is one of the most commonly detected pesticides in aquatic environments in the UK. In this study, metaldehyde concentrations and fluxes in stream water over a ten-year period (2008-2018) are reported for the River Colne catchment (Essex, southeast England), and the influence of hydrological conditions and application regimes are assessed. In general, peaks in metaldehyde concentration in river water occasionally exceeded 0.25 μg L^-1, and concentrations did not typically exceed the European Union Drinking Water Directive (EU DWD) regulatory limit of 0.1 μg L^-1. Metaldehyde concentration peaks displayed a seasonal pattern. Metaldehyde concentrations during periods when the molluscicide was not applied to agricultural land (January, July) and during the spring-summer application period (February to June) were generally low (0.01-0.03 μg L^-1). Peaks in metaldehyde concentration mainly occurred during the autumn-winter application season (August to December), and were typically associated with high intensity hydrological regimes (daily rainfall ≥10 mm; stream flow up to 18 m³ s^-1). Where metaldehyde concentrations exceeded the EU DWD regulatory limit, this was short-lived. The annual flux at the top of the Colne catchment (0.2-0.6 kg a^-1) tended to be lower than in the middle of the catchment (0.3-1.4 kg a^-1), with maximum flux values observed at the bottom of the catchment (0.5-25.8 kg a^-1). Metaldehyde losses from point of application to surface water varied between 0.01 and 0.25%, with a maximum of 1.18% (2012). Annual flux was primarily controlled by the annual precipitation and stream flow (R² = 0.9) rather than annual metaldehyde use (kg active applied). Precipitation explained 37% and 81% of variability in metaldehyde concentration and flux, respectively. Annual ranges in metaldehyde concentration were greater in the years 2012 and 2014 with an overall reduction in the range of metaldehyde concentrations evident over the period 2015-2018. It is the expectation that metaldehyde concentrations in stream water will continue to decrease following the withdrawal of metaldehyde for outdoor use in the UK from March 2022.

Listening to Slugs: Acceptability and Consumption of Molluscicide Pellets by the Grey Field Slug, Deroceras reticulatum

Gastropod damage to crop plants has a significant economic impact on agricultural and horticultural industries worldwide, with the Grey Field Slug (Deroceras reticulatum (Müller)) considered the main mollusc pest in the United Kingdom and in many other temperate areas. The prevailing form of crop protection is pellets containing the active ingredient, metaldehyde. Metaldehyde can cause paralysis and death in the mollusc, depending on the amount ingested. The paralysing effects may result in reduced pellet consumption. A greater understanding of metaldehyde consumption may reveal an area that can be manipulated using novel molluscicide formulations. Novel pellet types included commercial metaldehyde pellets coated so that metaldehyde is released more slowly. In both laboratory and arena trials, an audio sensor was used to record individual slugs feeding on a variety of pellet types, including commercially available toxic pellets (metaldehyde and ferric phosphate) and novel metaldehyde formulations. The sensor was used to record the length of each bite and the total number of bites. There was no significant difference in the length of bites between pellet types in laboratory trials. Novel pellets were not consumed more than commercial pellet types. Commercial pellet types did not differ in consumption.

Impact of dams and climate change on suspended sediment flux to the Mekong delta

The livelihoods of millions of people living in the world's deltas are deeply interconnected with the sediment dynamics of these deltas. In particular a sustainable supply of fluvial sediments from upstream is critical for ensuring the fertility of delta soils and for promoting sediment deposition that can offset rising sea levels. Yet, in many large river catchments this supply of sediment is being threatened by the planned construction of large dams. In this study, we apply the INCA hydrological and sediment model to the Mekong River catchment in South East Asia. The aim is to assess the impact of several large dams (both existing and planned) on the suspended sediment fluxes of the river. We force the INCA model with a climate model to assess the interplay of changing climate and sediment trapping caused by dam construction. The results show that historical sediment flux declines are mostly caused by dams built in PR China and that sediment trapping will increase in the future due to the construction of new dams in PDR Lao and Cambodia. If all dams that are currently planned for the next two decades are built, they will induce a decline of suspended sediment flux of 50% (47-53% 90% confidence interval (90%CI)) compared to current levels (99 Mt/year at the delta apex), with potentially damaging consequences for local livelihoods and ecosystems.

Modelling heavy metals in the Buriganga River System, Dhaka, Bangladesh: Impacts of tannery pollution control

Heavy metal pollution from tanneries is a global problem in many rapidly developing economies. Effluent discharges into rivers cause serious problems for water quality, damaging ecology and threatening the livelihoods of people, especially in developing urban centres which often have a high concentration of factories. The industry intensive capital area of Bangladesh is impacted with high levels of metals pollution in rivers in the Greater Dhaka Watershed. Sampling and modelling studies have been undertaken to assess pollution in the Buriganga River System in Dhaka. The process based, dynamic model INCA (Integrated Catchments) model has been used to simulate metals along the Buriganga River System in Central Dhaka. Observed and simulated metals concentrations are high, and the model shows that the proposed transfer of the tannery industry upstream helps to reduce the pollution significantly downstream. However, moving the industry upstream may be counterproductive as it is discharged into the upper reaches of the river. This will create pollution upstream unless the newly constructed effluent treatment system can operate at a high level.

Integrated catchment management for reducing pesticide levels in water: Engaging with stakeholders in East Anglia to tackle metaldehyde

In the agriculture intensive eastern region of England, plant protection products are widely applied to protect crops such as wheat and oilseed rape from pests and diseases, thus creating a risk of reaching nearby water courses through surface runoff. The EU Drinking Water Directive sets a stringent limit of 0.1 μg/l and 0.5 μg/l for individual and total pesticides respectively in treated potable water. However, peak metaldehyde levels have been persistently detected in raw water and reducing them to these limits has proven challenging and costly, in particular when using conventional treatment. In line with the EU Water Framework Directive, a more suitable approach and one adopted by the local water company, Anglian Water Services Ltd., would require moving towards mitigating pollution at source, preferably through participative action with multiple stakeholders in the agricultural industry. Initial findings demonstrate the potential of product substitution for reducing metaldehyde levels in surface waters. Reviewing Anglian Water's "Slug it Out" trial, we discuss key learnings derived from their experiences and make recommendations about the potential of the catchment approach to address the wider pesticide challenge.

Comparison of different monitoring methods for the measurement of metaldehyde in surface waters

Metaldehyde is recognised as an emerging contaminant. It is a powerful molluscicide and is the active compound in many types of slug pellets used for the protection of crops. The application of pellets to land generally takes place between August and December when slugs thrive. Due to its high use and physico-chemical properties, metaldehyde can be present in the aquatic environment at concentrations above the EU Drinking Water Directive limit of 100 ng L^-1 for a single pesticide. Such high concentrations are problematic when these waters are used in the production of drinking water. Being able to effectively monitor this pollutant of concern is important. We compared four different monitoring techniques (spot and automated bottle sampling, on-line gas chromatography/mass spectrometry (GC/MS) and passive sampling) to estimate the concentration of metaldehyde. Trials were undertaken in the Mimmshall Brook catchment (Hertfordshire, UK) and in a feed in a drinking water treatment plant for differing periods between 17th October and 31st December 2017. This period coincided with the agricultural application of metaldehyde. Overall, there was a good agreement between the concentrations measured by the four techniques, each providing complementary information. The highest resolution data was obtained using the on-line GC/MS. During the study, there was a large exceedance (500 ng L^-1) of metaldehyde that entered the treatment plant; but this was not related to rainfall in the area. Each monitoring method had its own advantages and disadvantages for monitoring investigations, particularly in terms of cost and turn-a-round time of data.

Simulating climate change and socio-economic change impacts on flows and water quality in the Mahanadi River system, India

Delta systems formed by the deposition of sediments at the mouths of large catchments are vulnerable to sea level rise and other climate change impacts. Deltas often have some of the highest population densities in the world and the Mahanadi Delta in India is one of these, with a population of 39 million. The Mahanadi River is a major river in East Central India and flows through Chattisgarh and Orissa states before discharging into the Bay of Bengal. This study uses an Integrated Catchment Model (INCA) to simulate flow dynamics and water quality (nitrogen and phosphorus) and to analyze the impacts of climate change and socio-economic drivers in the Mahanadi River system. Future flows affected by large population growth, effluent discharge increases and changes in irrigation water demand from changing land uses are assessed under shared socio-economic pathways (SSPs). Model results indicate a significant increase in monsoon flows under the future climates at 2050s (2041-2060) and 2090s (2079-2098) which greatly enhances flood potential. The water availability under low flow conditions will be worsened because of increased water demand from population growth and increased irrigation in the future. Decreased concentrations of nitrogen and phosphorus are expected due to increased flow hence dilution. Socio-economic scenarios have a significant impact on water quality but less impact on the river flow. For example, higher population growth, increased sewage treatment discharges, land use change and enhanced atmospheric deposition would result in the deterioration of water quality, while the upgrade of the sewage treatment works lead to improved water quality. In summary, socio-economic scenarios would change future water quality of the Mahanadi River and alter nutrient fluxes transported into the delta region. This study has serious implications for people's livelihoods in the deltaic area and could impact coastal and Bay of Bengal water ecology.

Modelling the effects of climate and land-use change on the hydrochemistry and ecology of the River Wye (Wales)

Interactions between climate change and land use change might have substantial effects on aquatic ecosystems, but are still poorly understood. Using the Welsh River Wye as a case study, we linked models of water quality (Integrated Catchment - INCA) and climate (GFDL - Geophysical Fluid Dynamics Laboratory and IPSL - Institut Pierre Simon Laplace) under greenhouse gas scenarios (RCP4.5 and RCP8.5) to drive a bespoke ecosystem model that simulated the responses of aquatic organisms. The potential effects of economic and social development were also investigated using scenarios from the EU MARS project (Managing Aquatic Ecosystems and Water Resources under Multiple Stress). Longitudinal position along the river mediated response to increasing anthropogenic pressures. Upland locations appeared particularly sensitive to nutrient enrichment or potential re-acidification compared to lowland environments which are already eutrophic. These results can guide attempts to mitigate future impacts and reiterate the need for sensitive land management in upland, temperate environments which are likely to become increasingly important to water supply and biodiversity conservation as the effects of climate change intensify.