Browsing by Author "Cassidy, Rachel"

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Open Access
A national-scale high-resolution runoff risk and channel network mapping workflow for diffuse pollution management
(Elsevier, 2024-08-07) Service, Thomas; Cassidy, Rachel; Atcheson, Kevin; Farrow, Luke; Harrison, Taylor; Jack, Paddy; Jordan, Phil; Environmental Protection
Managing diffuse pollution from agricultural land requires a spatially explicit risk assessment that can be applied over large areas. Major components of such assessments are the precise definition of both channel networks that often originate as small channels and streams, and Hydrologically Sensitive Areas (HSAs) of storm runoff that occur on land surfaces. Challenges relate to regions of complex topography and land use patterns, particularly those which have been heavily modified by arterial drainage. In this study, a national scale, transferrable workflow and analysis were developed using a specifically commissioned LiDAR survey. Research on the first half of Northern Ireland (6927 km2) is reported where field-edge drain to major river channels were mapped from 1 m (16 points per metre) digital terrain models, and in-field HSAs were defined across over 400,000 fields with a median field size of 0.86 ha. Manual drainage mapping supplemented with a novel automated drainage channel correction process resulted in an unparalleled high-resolution national drainage network with 37,320 km of channels, increasing mapped channel density from 0.9 km km−2 to 5.5 km km−2. The HSAs were based on a Soil Topographic Index (STI) system using hillslope and contributing area models combined with soil hydraulic characteristics. In all, 249 km2 of runoff risk HSAs were identified by extracting the top 95th percentile of the modelled STI as the areas with the highest propensity to generate in-field runoff. At field and individual farm scale these targeted risk maps of diffuse pollution were delivered to over 13,000 farmers and form part of the nationwide Soil Nutrient Health Scheme programme.
Open Access
Approaches to herbicide (MCPA) pollution mitigation in drinking water source catchments using enhanced space and time monitoring
(Elsevier, 2020-10-08) Morton, Phoebe; Cassidy, Rachel; Floyd, Stewart D.; Doody, Donnacha; McRoberts, W. Colin; Jordan, Philip; Environmental Protection
Freshwater occurrences of the selective acid herbicide 2-methyl-4-chloro-phenoxyacetic acid (MCPA) are an ongoing regulatory and financial issue for water utility industries as the number and magnitude of detections increase, particularly in surface water catchments. Assessments for mitigating pesticide pollution in catchments used as drinking water sources require a combination of catchment-based and water treatment solutions, but approaches are limited by a lack of empirical data. In this study, an enhanced spatial (11 locations) and temporal (7-hourly to daily sampling) monitoring approach was employed to address these issues in an exemplar surface water source catchment (384 km2). The spatial sampling revealed that MCPA was widespread, with occurrences above the 0.1 μg L−1 threshold for a single pesticide being highly positively correlated to sub-catchments with higher proportions of ‘Improved Grassland’ land use (r = 0.84). These data provide a strong foundation for targeting catchment-based mitigation solutions and also add to the debate on the ecosystems services provided by such catchments. Additionally, of the 999 temporal samples taken over 12 months from the catchment outlet, 25% were above the drinking water threshold of 0.1 μg L−1. This prevalence of high concentrations presents costly problems for source water treatment. Using these data, abstraction shutdowns were simulated for five scenarios using hydrometeorological data to explore the potential to avoid intake of high MCPA concentrations. The scenarios stopped abstraction for 4.2–9.3% of the April–October period and reduced intake of water containing over 0.1 μg L−1 of MCPA by 16–31%. This represents an important development for real-time proxy assessments for water abstraction in the absence of more direct pesticide monitoring data.
Open Access
Assessments of Composite and Discrete Sampling Approaches for Water Quality Monitoring
(Springer, 2018-04-12) Cassidy, Rachel; Jordan, Phil; Bechmann, Marianne; Kronvang, Brian; Kyllmar, Katarina; Shore, Mairead; Environmental Protection
Achieving an operational compromise between spatial coverage and temporal resolution in national scale river water quality monitoring is a major challenge for regulatory authorities, particularly where chemical concentrations are hydrologically dependent. The efficacy of flow-weighted composite sampling (FWCS) approaches for total phosphorus (TP) sampling (n = 26–52 analysed samples per year), previously applied in monitoring programmes in Norway, Sweden and Denmark, and which account for low to high flow discharges, was assessed by repeated simulated sampling on high resolution TP data. These data were collected in three research catchments in Ireland over the period 2010–13 covering a base-flow index range of 0.38 to 0.69. Comparisons of load estimates were also made with discrete (set time interval) daily and sub-daily sampling approaches (n = 365 to >1200 analysed samples per year). For all years and all sites a proxy of the Norwegian sampling approach, which is based on re-forecasting discharge for each 2-week deployment, proved most stable (median TP load estimates of 87–98%). Danish and Swedish approaches, using long-term flow records to set a flow constant, were only slightly less effective (median load estimates of 64–102% and 80–96%, respectively). Though TP load estimates over repeated iterations were more accurate using the discrete approaches, particularly the 24/7 approach (one sample every 7 h in a 24 bottle sampler - median % load estimates of 93–100%), composite load estimates were more stable, due to the integration of multiple small samples (n = 100–588) over a deployment.
Open Access
A carrying capacity framework for soil phosphorus and hydrological sensitivity from farm to catchment scales
(Elsevier, 2019-06-04) Cassidy, Rachel; Thomas, Ian A.; Higgins, Alex J.; Bailey, John S.; Jordan, Phil; Environmental Protection
Agricultural fieldswith above optimumsoil phosphorus (P) are considered to pose risks to water quality and especially when those areas are coincident with hydrologically sensitive areas (HSAs) that focus surface runoff pathways. This is a challenge tomanage in areas of agricultural intensity in surfacewater dominated catchments where water quality targets have to be met. In this study, a soil P survey of 13 sub-catchments and 7693 fields was undertaken in a 220 km2 catchment. HSAs were also determined as the top 25th percentile risk froma runoff routingmodel that used a LiDAR digital elevation model and soil hydraulic conductivity properties. Distributions of these spatial data were compared with river soluble reactive phosphorus (SRP) concentration measured fortnightly over one year. The results showed that 41% of fields exceeded the agronomic optimumfor soil P across the sub-catchments.When compared with the available water quality data, the results indicated that the high soil P carrying capacity area of the sub-catchmentswas 15%. Combining high soil P and HSA, the carrying capacity area of the sub-catchmentswas 1.5%. The opportunities to redistribute these riskswere analysed on fields with below optimum soil P and where HSA risk was also minimal. These ranged from 0.4% to 13.8% of sub-catchment areas and this limited potential, unlikely to fully reduce the P pressure to over-supplied fields, would need to be considered alongside addressing this over-supply and also with targeted HSA interception measures.
Open Access
Evaluating Groundwater Nitrate Status across the River Ythan Catchment (Scotland) following Two Decades of Nitrate Vulnerable Zone Designation
(MDPI, 2023-04-18) Johnson, Hamish; Simpson, Emma May; Troldborg, Mads; Ofterdinger, Ulrich; Cassidy, Rachel; Soulsby, Chris; Comte , Jean-Christophe; Environmental Protection
Diffuse agricultural pollution is one of the greatest challenges to achieving good chemical and ecological status of Scotland’s water bodies. The River Ythan in Aberdeenshire was designated a Nitrate Vulnerable Zone (NVZ) in the year 2000, due to the eutrophication of the Ythan Estuary and rising nitrate trends in Private Water Supply (PWS) groundwater abstractions. The third River Basin Management Plan (RBMP) for Scotland reported the Ellon groundwater body of the River Ythan catchment to be of poor chemical status as of 2021 with respect to nitrate, and forecasted groundwater recovery beyond 2027. Following two decades of NVZ designation, we investigated the drivers of groundwater nitrate across the River Ythan catchment through an analysis of long-term (2009–2018) groundwater quality monitoring data collected by the Scottish Environmental Protection Agency (SEPA) and a recent synoptic groundwater nitrate sampling survey of PWSs. Groundwater nitrate was found to remain elevated across the catchment area, and appeared to be highly sensitive to agricultural practices and meteorological forcing, indicating a high sensitivity of groundwater quality to environmental change. Further hydrogeological characterisation is recommended to better understand the effects of agricultural practices on groundwater quality, and to facilitate achievement of future RBMP goals under a changing climate.
Open Access
Evaluation of Chemcatcher® passive samplers for pesticide monitoring using high-frequency catchment scale data
(Elsevier, 2022-09-30) Farrow, Luke; Morton, Phoebe; Cassidy, Rachel; Floyd, Stewart D.; McRoberts, W. Colin; Doody, Donnacha; Environmental Protection
Passive samplers (PS) have been proposed as an enhanced water quality monitoring solution in rivers, but their performance against high-frequency data over the longer term has not been widely explored. This study compared the performance of Chemcatcher® passive sampling (PS) devices with high-frequency sampling (HFS: 7-hourly to daily) in two dynamic rivers over 16 months. The evaluation was based on the acid herbicides MCPA (2-methyl-4-chlorophenoxyacetic acid), mecoprop-P, fluroxypyr and triclopyr. The impact of river discharge parameters on Chemcatcher® device performance was also explored. Mixed effects modelling showed that time-weighted mean concentration (TWMC) and flow-weighted mean concentration (FWMC) values obtained by the HFS approach were both significantly higher (p < 0.001) than TWMC values determined from PS regardless of river or pesticide. Modelling also showed that TWMCPS values were more similar to TWMCHFS than FWMCHFS values. However, further testing revealed that MCPA TWMC values from HFS and PS were not significantly different (p > 0.05). There was little indication that river flow parameters altered PS performance—some minor effects were not significant or consistent. Despite this, the PS recovery of very low concentrations indicated that Chemcatcher® devices may be used to evaluate the presence/absence and magnitude of acid herbicides in hydrologically dynamic rivers in synoptic type surveys where space and time coverage is required. However, a period of calibration of the devices in each river would be necessary if they were intended to provide a quantitative review of pesticide concentration as compared with HFS approaches.
Open Access
Muddying the waters: Impacts of a bogflow on carbon transport and water quality
(Elsevier, 2024-02-09) Morton, Phoebe; Hunter, William; Cassidy, Rachel; Doody, Donnacha; Atcheson, Kevin; Jordan, Phil; Environmental Protection; Fisheries and Aquatic Ecosystems
Landslides of peat have been recorded throughout Britain and Ireland for centuries. Whilst these events are not uncommon, land degradation can amplify their magnitude and frequency and, crucially, their immediate impacts are rarely documented. A 20,000 m3 bogflow event that occurred on land undergoing development in the Irish border area in November 2020 was monitored at high frequency in the major receiving river system (384 km2). Samples collected every seven hours over a 28 day period at a site 37 km downstream were analysed for suspended sediment (SS), particulate organic carbon (POC) and dissolved organic carbon (DOC and UV-derived fractions), synchronous with hydrometeorological data and turbidity. There was no impact of the bogflow on DOC concentrations or loads. However, concentrations of SS and POC in the first samples after the bogflow were 825 mg/L and 346 mg C/L, respectively, and fish kill was estimated at 100 %. Analysis of detrended SS and POC loads suggested the main impacts of the bogflow on water quality lasted just eight days. Over this period, an additional 1318 t of SS and 608 t of POC were transported as far as the monitoring point, equating to 325 % more SS and 925 % more POC than would have been expected otherwise under the same river flow conditions. The carbon loss and water quality impacts were short lived, but nevertheless severe, and highlight the vulnerability of peatlands and the risks when these environments are inappropriately managed.
Open Access
Perspectives on water quality monitoring approaches for behavioural change research
(Frontiers Media, 2022-07-01) Jordan, Phil; Cassidy, Rachel; Environmental Protection
This review considers enhanced approaches to river water quality monitoring in north-western Europe following a series of study visits (11 sites in 7 countries). Based on the evidence gathered, options were identified and evaluated for their suitability to deliver specific water quality monitoring objectives and with a focus on effecting behavioral change. Monitoring programs were diverse, ranging from enhanced grab sampling and laboratory analysis to sub-hourly sampling of multiple parameters and nutrients in autonomous high-specification, bank-side or mobile laboratories. Only one program out of all the cases evaluated could readily identify influences that had produced behavioral change among stakeholders. This was principally because the other programs were focused on top-down policy change or surveillance rather than specifically focused on influencing behavior. Nevertheless, program researchers were clear that stakeholder engagement potential was very high and that the sites acted as important focus points for discussion on water quality issues, and so part of a suite of tools that might ultimately change behavior. This identifies a space where water quality monitoring solutions could be adapted for behavioral change research.
Open Access
Quantifying MCPA load pathways at catchment scale using high temporal resolution data
(Elsevier, 2022-05-24) Atcheson, Kevin; Mellander, Per-Erik; Cassidy, Rachel; Cook, Sally; Floyd, Stewart; McRoberts, Colin; Morton, Phoebe; Jordan, Phil; Environmental Protection
Detection of the agricultural acid herbicide MCPA (2-methyl-4-chlorophenoxyacetic acid) in drinking water source catchments is of growing concern, with economic and environmental implications for water utilities and wider ecosystem services. MCPA is poorly adsorbed to soil and highly mobile in water, but hydrological pathway processes are relatively unknown at the catchment scale and limited by coarse resolution data. This understanding is required to target mitigation measures and to provide a framework to monitor their effectiveness. To address this knowledge gap, this study reports findings from river discharge and synchronous MCPA concentration datasets (continuous 7 hour and with additional hourly sampling during storm events) collected over a 7 month herbicide spraying season. The study was undertaken in a surface (source) water catchment (384 km2—of which 154 km2 is agricultural land use) in the cross-border area of Ireland. Combined into loads, and using two pathway separation techniques, the MCPA data were apportioned into event and baseload components and the former was further separated to quantify a quickflow (QF) and other event pathways. Based on the 7 hourly dataset, 85.2 kg (0.22 kg km􀀀 2 by catchment area, or 0.55 kg km􀀀 2 by agricultural area) of MCPA was exported from the catchment in 7 months. Of this load, 87.7 % was transported via event flow pathways with 72.0 % transported via surface dominated (QF) pathways. Approximately 12 % of the MCPA load was transported via deep baseflows, indicating a persistence in this delayed pathway, and this was the primary pathway condition monitored in a weekly regulatory sampling programme. However, overall, the data indicated a dominant acute, storm dependent process of incidental MCPA loss during the spraying season. Reducing use and/or implementing extensive surface pathway disconnection measures are the mitigation options with greatest potential, the success of which can only be assessed using high temporal resolution monitoring techniques.
Open Access
Quantifying nutrient and sediment erosion at riverbank cattle access points using fine-scale geo-spatial data
(Elsevier, 2023-10-14) Scott, Alison; Cassidy, Rachel; Arnscheidt, Joerg; Rogers, David; Jordan, Phil; Environmental Protection
Unrestricted cattle access to the riparian zone can exacerbate riverbank erosion in grazed grassland catchments. Knowledge gaps include the magnitude of erosion and other environmental pressures at cattle access points. This study aimed to address this by using two high resolution geo-spatial methods; 1) aerial photogrammetry and 2) terrestrial laser scanning to measure cumulative, seasonal, and annual erosion rates at nine unmitigated cattle access points in Northern Ireland. Total, fine sediment and total phosphorus exports were determined through bulk density and deep soil core sampling campaigns of exposed bank faces. Accumulated erosion was estimated using method 1) at 1.0 – 49.5 t and 0.51 – 16.64 kg for total sediment and total phosphorus, respectively. Using method 2) median annual export coefficients of 0.19 – 0.21 t m−1 and 0.065 – 0.087 kg m−1 (normalised to streambank length) were determined for total sediment and total phosphorus transfers respectively and these mostly occurred during the grazing season (median 84% for both sediment and total phosphorus). In terms of livestock pressures, these annual exports equate to 0.34 – 0.40 t LU-1 yr−1 and 0.103 – 0.111 kg LU-1 yr−1 for total sediment and total phosphorus, respectively (1.19–1.89 LU ha−1). The conventional measure of protective fencing is likely to prevent such transfers to rivers. Scaling a nationwide agri-environment scheme over six years which installed 2,493 km of riparian fencing (and assuming from this study that 1.9 % of all riparian field boundaries had cattle access impact), this measure potentially saved 9,047–9,999 t yr−1 and 3,095 – 4,143 kg yr−1 of total sediment and total phosphorus, respectively, from entering water courses.
Open Access
Reducing MCPA herbicide pollution at catchment scale using an agri-environmental scheme
(Elsevier, 2022-05-20) Cassidy, Rachel; Jordan, Phil; Farrow, Luke; Floyd, Stewart; McRoberts, Colin; Morton, Phoebe; Doody, Donnacha; Environmental Protection
In river catchments used as drinking water sources, high pesticide concentrations in abstracted waters require an expensive treatment step prior to supply. The acid herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA) is particularly problematic as it is highly mobile in the soil-water environment following application. Here, an agri-environmental scheme (AES) was introduced to a large-scale catchment (384 km2) to potentially reduce the burden of pesticides in the water treatment process. The main measure offered was contractor application of glyphosate by weed wiping as a substitute for boom spraying of MCPA, supported by educational and advisory activities. A combined innovation applied in the assessment was, i) a full before-after-control-impact (BACI) framework over four peak application seasons (April to October 2018 to 2021) where a neighbouring catchment (386 km2) did not have an AES and, ii) an enhanced monitoring approach where river discharge and MCPA concentrations were measured synchronously in each catchment. During peak application periods the sample resolution was every 7 h, and daily during quiescent winter periods. This sampling approach enabled flow- and time-weighted concentrations to be established, and a detailed record of export loads. These loads were up to 0.242 kg km−2 yr−1, and over an order of magnitude higher than previously reported in the literature. Despite this, and accounting for inter-annual and seasonal variations in river discharges, the AES catchment indicated a reduction in both flow- and time-weighted MCPA concentration of up to 21% and 24%, respectively, compared to the control catchment. No pollution swapping was detected. Nevertheless, the percentage of MCPA occurrences above a 0.1 μg L−1 threshold did not reduce and so the need for treatment was not fully resolved. Although the work highlights the advantages of catchment management approaches for pollution reduction in source water catchments, it also indicates that maximising participation will be essential for future AES.
Open Access
Reducing the time-dependent climate impact of intensive agriculture with strategically positioned short rotation coppice willow
(Elsevier, 2023-07-27) Livingstone, David; Smyth, Beatrice M.; Cassidy, Rachel; Murray, Simon T.; Lyons, Gary; Foley, Aoife M.; Johnston, Christopher; Environmental Protection
In this study the implementation of a short rotation coppice willow system, planted as a riparian buffer in an intensive agricultural setting, to intercept and reduce nutrient losses, was investigated. The aim of the work was to assess how such a system could reduce the overall climate impact of an intensive agricultural setting. A life cycle assessment was carried out for a combined Irish dairy farm and willow buffer system considering the impact category, climate impact. The climate impact was considering using the time-dependent climate impact metric, with the results given in terms of the impact on the global surface temperature. The results were compared to an Irish dairy farm in which no willow was planted. Such a system has not previously been investigated in this way and this was the first time-dependent climate impact assessment of a willow plantation planted on pastureland. Geographic information systems software, was used to map areas particularly susceptible to agricultural run-off and suitable for willow planting, using the land bank of the Agri-Food and Biosciences Institute research farm in Hillsborough, Northern Ireland, for the case study. The harvested willow was assumed to be combusted in a combined heat and power plant. By implementing the willow system the time-dependent climate impact of an Irish dairy farm could be reduced by 8% with only 3.7% of the land used for willow cultivation over a 101-year study period. The results also found an immediate reduction in climate impact following the implementation of the willow system. Total GHG emissions were reduced by 131 Mg CO2eq ha−1 over the study timeframe. The results can be more broadly applied to other agricultural sectors, such as arable farming where the climate impact savings of the willow system could be even higher.
Open Access
Soil phosphorus, hydrological risk and water quality carrying capacities in agricultural catchments
(Elsevier, 2024-03-15) Scott, Alison; Cassidy, Rachel; Arnscheidt, Joerg; Jordan, Phil; Environmental Protection
To support profitable agricultural production, nutrients, including phosphorus (P) are applied to soils. However, to avoid over-application and mobilisation of excess P, in-soil concentrations must be maintained at the agronomic optimum (crop requirement) through soil test P (STP) data. Areas above optimum STP (e.g., Olsen P) status have been linked to elevated instream soluble reactive P (SRP) concentrations. For example, when this status is combined with hydrologically sensitive areas (HSAs), excess P can be mobilised and transported directly to surface waters. Catchment carrying capacities for high STP are a possible management strategy to reduce these pressures. The aim of this study was to investigate the transferability of catchment carrying capacity approaches using primary and secondary datasets. Field by field STP status and LiDAR derived HSAs (2 m grid resolution) were compared with instream SRP concentrations using combinations of least squares regressions. The high range of STP catchment carrying capacities (15 % − 44 %, depending on the regression used) was influenced by the variation of instream SRP concentration thresholds (48 – 71 µg L-1) that are determined using altitude and alkalinity factors. However, a single SRP threshold of 35 µg L-1 reduced the catchment STP carrying capacity to a smaller range (10 % − 16 %), with a mean of 13 %. The analysis showed that instream particulate P concentrations were also related to above optimum STP but to a lesser degree and that all HSAs were vulnerable to P loss when soils were above optimum STP. Targeted management strategies should follow a “treatment-train” approach starting with reducing the catchment or farm area above agronomic optimum STP to a carrying capacity (proposed here as 13 %), followed by interception measures located at HSA breakthrough and delivery points to reduce both instream SRP concentration and load.
Open Access
The spatial and temporal dynamics of sediment phosphorus attenuation and release in impacted stream catchments
(Elsevier, 2023-09-24) Li, S.; Arnscheidt, J.; Cassidy, Rachel; Douglas, R.W.; McGrogan, H.J.; Jordan, P.; Environmental Protection
Sediments can attenuate phosphorus (P) from overlying water and reduce trophic status in zero and first order ditches and streams. These features can be considered as intermediate mitigation features between P mobilised from land, and onward delivery to river systems, if the risk of chemical P release from sediments is minimal. However, risk assessments are rarely based on temporal scale dynamics and especially at fine scale in both sediment and water column environments. In this study, in eutrophic stream catchments, bed sediments were tested fortnightly and spatially over one year for EPC0 (to derive phosphate exchange potential—PEP) and for P across a spectrum from labile to recalcitrant fractions. At the same time stream discharge and P concentrations were measured synchronously at high frequency and resolved to 1-hour intervals and indicated high water quality pressures at all flow rates. PEP indicated spatial and temporal changes most likely caused by periods of source disconnection/reconnection and sediment mobilisation during storm events, moving from periods of high attenuation potential to near saturation. Despite these spatial and temporal changes, PEP did not indicate much potential for chemical P release from the sediments (distributing mostly below or close to zero). However, this may be a misleading risk assessment by itself as physical P release, especially of the labile bicarbonate-dithionite (B-D) P fraction of sediments, was a more dominant process mobilised during storm events reducing by up to 84 % during a succession of summer storm events. The total P and total reactive P loads monitored leaving the catchments were coincident with these changes. The specific downstream trophic effects of this episodic P release will need to be assessed in terms of its bioavailability, in combination with other more noted diffuse and point P source processes.