Nutrient water quality of the Wye catchment, UK: exploring patterns and fluxes using the Environment Agency data archives H. P. Jarvie, C. Neal, P. J. A. Withers, A. Robinson, N. Salter To cite this version: H. P. Jarvie, C. Neal, P. J. A. Withers, A. Robinson, N. Salter. Nutrient water quality of the Wye catchment, UK: exploring patterns and fluxes using the Environment Agency data archives. Hydrology and Earth System Sciences Discussions, European Geosciences Union, 2003, 7 (5), pp.722-743. hal- 00304924 HAL Id: hal-00304924 https://hal.archives-ouvertes.fr/hal-00304924 Submitted on 1 Jan 2003 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. HydrologyHelen P. Jarvie, and ColinEarth Neal,System Paul Sciences, J.A. Withers, 7(5), Andrew 722743 Robinson (2003) and © Nicola EGU Salter Nutrient water quality of the Wye catchment, UK: exploring patterns and fluxes using the Environment Agency data archives Helen P. Jarvie1, Colin Neal1, Paul J.A. Withers2, Andrew Robinson3 and Nicola Salter3 1Centre for Ecology and Hydrology, Wallingford, Oxfordshire, OX10 8BB, UK 2ADAS, Sunnyhill House, 1 Thistlebarrow Road, Salisbury, Hampshire SP1 3RU, UK 3Environment Agency Wales, Rivers House, St Mellons Business Park, St Mellons, Cardiff, CF3 0EY, UK Email for corresponding author: [email protected] Abstract Water quality data, collected by the Environment Agency in England and Wales over 10 years (1991 2000) were used to examine the spatial distribution of nutrient pollution risk and for assessing broad-scale spatial and temporal variability in nutrient fluxes across the Wye catchment. Nutrient water quality across the upper and middle Wye catchment, and along the main River Wye, is generally very good. However, the main areas of concern lie in the small tributaries in the south and east of the catchment, which have lower dilution capacity and high agricultural and effluent inputs, and where mean Total Reactive Phosphorus (TRP) in some cases exceed 1mg-P l-1. Indeed, mass load calculations have demonstrated that the lowland south and east portion of the catchment contributes more than 85% of the whole-catchment TRP and more than 78% of nitrate (NO ) loads. Ratios of NO :Ca concentrations were used to fingerprint different water-types across the catchment, linked to 3 3 weathering and agricultural activity. The Wye catchment has been subject to two major sets of perturbations during the study period: (i) climatic fluctuations, with a drought during 1995-6, followed by a subsequent drought-break in 1997/8, and extreme high river flows in the autumn/winter of 2000/2001, and (ii) introduction of tertiary P-treatment at major sewage treatment works in the catchment. The implications of these perturbations for the nutrient water quality of the Wye catchment are discussed. Recommendations are made for more targeted monitoring to directly assess diffuse source nutrient contributions. Keywords: nutrients, phosphate, phosphorus, nitrate, nitrogen, river, Wye, PSYCHIC, flux Introduction candidate Special Area of Conservation (cSAC) under the This paper summarises and analyses a wealth of water European Unions Habitats Directive (Council of European quality data collected by the Environment Agency (EA), to Communities, 1992). Much of the lower Wye Valley is an assess large-scale nutrient functioning across the Wye Area of Outstanding Natural Beauty. The Wye supports a catchment, a large (4136 km2) and highly heterogeneous diverse fish fauna, many species of which are cited as catchment on the border between Wales and England (Fig. features in the cSAC designation. Of these, perhaps the most 1). This work forms a basis for more detailed and focused important in terms of conservation and socio-economic water quality monitoring and modelling on the Wye as part status, is the atlantic salmon (Salmo salar). The Wye is well of the Phosphorus and Sediment Yield CHaracterization In known as a historically important salmon fishery although Catchments (PSYCHIC) research programme. The Wye catches have declined in recent years and the proportion of catchment spans a wide range of riverine environments from much sought-after large spring fish has also reduced. base-poor upland streams to silty lowland rivers, with Most of the water quality research within the Wye diverse geology, land use and hydrological conditions. The catchment to date has focused on the uppermost headwaters River Wye is a river of high ecological status: there are 246 draining the Plynlimon Hills. These headwaters have been Sites of Special Scientific Interest in the Wye catchment a major centre for experimental catchment research since and the River Wye and some of its tributaries now form a the early 1970s (Neal and McCulloch, 1997). During the 722 Nutrient water quality of the Wye catchment, UK: exploring patterns and fluxes using the Environment Agency data archives R . W y e Rhayader R R . n Lu E o g l th g a . I n R Leominster Llandrindod Wells w o R rr A . R L u n Hay-on-Wye g o g e rf R I m . R Wy o r e F Llandwrtyd Wells R . Builth Wells W R y Yazor Brook e R Hereford . D R How Caple Brook i o . f r W n R e y y . e L H . o Wriggle Brook R n n d u R Rudhill Brook . M o Worm Brook R n Gamber Brook . n N Tr o ot w hy W E Garren Brook Monmouth S 10 0 10 20 Kilometers Fig. 1a. Map of the Wye catchment showing rivers, brooks and major settlements Wye at Cefyn Brwyn Wye at Ddol Farm ## Arrow at Titley Mill Lugg at Byton Elan at Elan Village Lugg at Butts Bridge ## # # ## Frome at Bishops Frome # # # # Ithon at Disserth # # # # ## # # # # Frome at Yarkill # # # ## Irfon at Cilmery # ## # Lugg at Lugwardine # Wye at Erwood # ## Wye at Belmont Llynfi at Three Cocks Yazor Brook at Three Elms Rudhill Brook at Sandford Bridge # Monnow at Grossmont Wye at Redbrook Fig. 1b. Map of the Wye catchment showing Environment Agency flow gauging stations late 1970s and early 1980s, mass-balance and timeseries nutrient water quality of the Wye. The study utilises the modelling of water quality was also undertaken across the extensive water quality data collected by the EA, which wider Wye catchment (Oborne, 1981, Oborne et al., 1980), allows better spatial resolution of nutrient pollution risk (for together with assessment of nutrient sources within two of more than 1200 river and effluent monitoring sites) and the lowland subcatchments, the Frome and Trothy (Houston temporal coverage in chemical concentrations (over a 10- and Brooker, 1981). Edwards and Brooker (1982) examined year period), than previously available. This complements longitudinal and temporal variations in water quality along the more localised site-specific studies described above. The the River Wye and the Elan Valley reservoirs. work focuses on the diverse water chemistry of the River This study takes the understanding of the Wye catchment Wye and its tributaries and describes the spatial and temporal further, by providing the first basin-scale assessment of the variability in nutrient fluxes. This is needed to understand 723 Helen P. Jarvie, Colin Neal, Paul J.A. Withers, Andrew Robinson and Nicola Salter Scottish River Dee; Langan et al., 1997; Smart et al., 1998), in relation to environmental management and research priorities at the regional and national level. Study area The River Wye rises on the Plynlimon mountains (741 A.O.D.) and flows approximately 215 km in a broadly south- east direction to the Severn Estuary. The geology of the catchment comprises rocks of Palaeozoic to Mesozoic age, which are overlain by glacial and alluvial drift (Fig. 2). The upland north-west of the catchment is composed of outcrops of Ordovician and Silurian sandstones, shales, grits and mudstones. The lowland catchment, south east of Hay-on- Wye is underlain by Old Red Sandstone, comprising readily- weathered marls of the Herefordshire lowlands and the more resistant sandstones of the Black Mountains. There is a Fig. 1c Map of the upper, middle and lower Wye catchment marked contrast in precipitation across the catchment, with a mean annual rainfall of 2450 mm (19511995) at Cefyn Brwyn in the upland north-west, compared with 717mm the nutrient functioning in one of the UKs most important (19711995) at Yarkhill in the lowland east of the catchment rural and agricultural rivers of high conservation value and (Institute of Hydrology, 1998). The high precipitation inputs to examine the rural/agricultural water quality endmember to the upland catchment and low groundwater inputs mean types for comparison with UK urban and industrially that the river regimes in the west tend to be flashier, whereas, impacted river systems (e.g. the Humber Rivers; Jarvie et in the eastern lowlands, there is more substantial al., 1998, 2002a) and relatively clean rural rivers (e.g. the groundwater supply, resulting in the lowland tributaries Carboniferous Limestone Lower Old Red Sandstone (Marl) Lower Old Red Sandstone (sandstone) Silurian : Ludlow beds Silurian : Wenlock beds Silurian: Tarranon and Llandovery beds Upper Ordovician Fig. 2. Geology map of the Wye catchment 724 Nutrient water quality of the Wye catchment, UK: exploring patterns and fluxes using the Environment Agency data archives exhibiting less variable flow regimes. The Elan Valley to lowland catchment, linked to the change from the reservoir system reduces downstream flood levels in the Ordovician and Silurian strata to the Old Red Sandstone Rivers Elan and Wye, but is also used to compensate low series.