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Sources and Impacts of Past, Current and Future Contamination of Soil

Appendix 5 : Contaminant re-mobilisation and transportation

S C Rose and R Lamb

JUNE 2007 (AMENDED FEBRUARY 2008)

Stephen Rose, JBA Consulting - Engineers & Scientists. South Barn, Broughton Hall, Skipton, North Yorkshire, BD23 3AE .

Telephone:+44(0)1756 799919 Fax: +44(0)1756 799449 Email : [email protected]

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5.1 Flooding

Flooding is an important process in the natural environment and cannot be entirely prevented. Water and its movement through the hydrological cycle is probably the major mechanism for contaminant transfer in the environment. In water, contaminants moved from source areas can either be transported in solution or adsorbed to particulate matter prior to deposition in sink areas. In addition, particles of ore material (e.g. galena [PbS] or sphalerite [ZnS]) mobilised by mining activities may be transported as suspended or bed sediment. Floodwaters have the capacity to generate the movement of large volumes of contaminants from both point and diffuse sources and their subsequent deposition in sink areas. The extent, depth, duration, frequency and timing of flood events could all, in some way, affect soil quality and function over time. Soil quality and function are also affected, in part, by the movement of water and any associated contaminants over or through the soil in response to rainfall, surface runoff and even shallow groundwater movement. Within England and Wales the floodplains are occupied by a number of land uses, including productive agricultural systems and built-up areas (residential, business and industrial). Approximately 10,000km2 (or 8% of the total area) of land in England is at risk from river flooding, including tidal rivers and estuaries (DTLR, 2000). In addition, approximately 2,500km2 of land (1.5% of the total area) is at risk of direct flooding by the sea. About 1.3M ha of agricultural land (12% of the total agricultural land area and 61% of the Grade 1 agricultural land area), worth about £7 billion, is at risk from flooding. In many places the floodplain has been disconnected from the river through the implementation of land drainage and flood defence schemes. This process has been taking place for many centuries. Flood defence structures (e.g. embankments, walls) are only designed to a specific standard of protection and will be overtopped in more extreme events. This review adopts the source-pathway-receptor approach (DETR, 2000) to ask the questions: what generates the flood and associated contaminant transport (sources), what route(s) do the flood and associated contaminants take (pathways) and what is the impact on soil function (receptor)? This generic approach has been applied by Defra and other government departments/agencies to environmental protection and flood risk management assessments (OST, 2004; EA, 2002a). The review also contains a list (Annex 5.1) indicating the locations of the UK studies that were considered.

5.1.1 Types of flooding There are five main types of flooding: • Fluvial flooding occurs when a river breaks or overtops its banks, inundating the surrounding area. The key factors that determine the occurrence of river flooding are intensity and duration of rainfall and initial catchment wetness condition. • Localised surface or ‘muddy’ flooding is generated via surface runoff and the channelling of flows through the landscape. Muddy flooding emerges from ephemeral, not permanent channels (inc. rills and gullies) on agricultural land. This type of flooding does not always contribute to river flooding if the surface runoff fails to reach a main watercourse. • Coastal/tidal flooding can occur during exceptionally high tides or during storm events when low pressure systems result in storm surges reaching our coastlines and funnelling water up our estuaries. Wind action causes increased wave heights which also contribute to coastal flooding. • Urban flooding: drainage networks underlying urban areas have been developed to drain surface runoff and foul water. These can be old, and have insufficient capacity if they have not been upgraded to reflect increased development. During more intense storm events urban drainage networks can be overwhelmed and surcharge, causing flooding. Urban flooding can also result from the blockage of culverts and drains by debris. • Groundwater flooding is mainly a chalk catchment phenomenon. If exceptional rainfall occurs in winter after the aquifer is already fully recharged then the additional water cannot be stored underground. It will emerge from hillslopes through springs and spring- fed headwater streams and can overwhelm drainage systems.

2 5.1.2 Impact on soil function Flooding, in the absence of any associated contamination, can directly or indirectly affect all soil functions. The three main soil functions that are directly affected by incidence of flooding alone are: i) biomass, food and fibre production; ii) environmental interactions and iii) support of ecosystems, habitats and biodiversity.

Biomass, food and fibre production. All growing vegetation requires water to grow and sustain itself. However, many plant species grown for production (especially arable and horticultural crops) can be adversely affected by periods of flooding. The timing of the flooding within the growing cycle of the plant in question and the duration of the flooding (and associated soil saturation) are the two main flooding factors that will determine just how adversely the plant will be affected. Many plants can withstand periodic inundation with a short duration. However, if the inundation occurs during, for instance, the critical germination period then the plant could be killed due to the lack of aeration in the root zoot. Also, root crops will rot in the ground if the soil surrounding them is flooded or saturated for prolonged periods. Flooding can also deliver contaminated water to fluvial and coastal/estuarine floodplains, thereby directly affecting the potential growth, yield and quality of crops.

Environmental interactions. Soils exist at the interface of the atmosphere, biosphere, geosphere and hydrosphere. Soils are vital to the natural buffering and filtering of numerous contaminants in the environment from natural and man-made sources. Many physical, chemical and biological processes are reliant on the availability of water for their effectiveness. Too much water, in the form of flooding and soil saturation, can be detrimental to a number of these interactions and processes and can lead to a state of imbalance and the potential transfer of contamination to another environmental compartment. Soils also regulate the flux of water from the point at which it reaches the soil surface (from rainfall or localised surface runoff) to the point it enters a watercourse, open water or groundwater body, or the point that it re-enters the atmosphere.

Ecosystems, habitats and biodiversity. Soil and underlying geological characteristics, hydrological and topographical conditions and climate, together with the considerable influence of mankind, have principally determined what terrestrial ecosystems are present on British landscapes. In the floodplain environment many terrestrial ecosystems (not including any cultivated areas) will have developed and continue to be sustained by a certain frequency and duration of flooding. For example, the quality of alluvial soils is dependant on flooding for the regeneration of soils, its structure and its nutrient content. Any prolonged change to the flood frequency and flood duration characteristics of an area, due to natural or human- induced causes, will potentially lead to changes in the functioning of ecosystems, habitat distribution and biodiversity.

The impact of floodwaters containing contaminants will have many additional detrimental impacts on the soil functions detailed above.

5.1.3 Small scale processes

Flood generation. The response of streams and rivers in a catchment to rainfall and snowmelt has been attributed to variable hillslope processes and pathways. Spatial variation in river response results from differences in topography, vegetation, soil and geology. Temporal variation in river response also results from the above, together with the timing and intensity of rainfall and the effect of antecedent moisture conditions in the catchment. Overland flow (or surface runoff) is the principle component for the generation of floods. Hewlett and Hibbert (1967) put forward the concept of ‘saturation overland flow’, whereby all rainfall infiltrates the soil surface before it moves downslope. Where the watertable is close to the soil surface, particularly in those areas next to rivers and streams, the watertable rises to meet the soil surface and subsequently all rainfall falling on these saturated areas runs off across the surface very quickly. As the storm or snowmelt continues, the ‘contributing area’ expands and then contracts after the rainfall or snowmelt stops. This phenomenon is known as the ‘variable source concept’ and is applicable to typical British conditions. These variable source areas can also be found on hillslopes where flow convergence occurs. The

3 implication of the different runoff generation mechanisms is that not all of a given hillslope or catchment area necessarily contributes to the contaminant load in any particular flood event.

Sediment. Sediment is an essential, integral and dynamic component of river catchments (SedNet, 2004). In natural and agricultural catchments sediment originates from the weathering and erosion of minerals, organic matter and soils in upstream areas and floodplains, together with the erosion of river banks and beds. Fine and medium-sized particles, i.e. <63 microns (μm), are the most important for contaminant transport (SedNet, 2004). These include all clay minerals, organic matter and the common fine-grained minerals (e.g. quartz, feldspar, carbonates). Coatings of iron and manganese (hydro-) oxides are common on these particles. The physical and chemical properties of these particles (e.g. large specific surface area and high ion exchange capacity) enable them to act as efficient scavengers of contaminants discharged into the river and catchment systems.

Sediment entrainment and transport. The fundamental processes by which sediment is lifted and moved downstream or downslope are similar for both the soil surface and within river/stream channels. As the velocity of flowing water increases, so does its ability to initiate sediment entrainment and movement (through rolling, sliding or saltation), although this process is not necessarily linear (Embleton and Thornes, 1979). If the velocity continues to increase the flow becomes more turbulent and sediment may become suspended in the flow. Cohesive clay particles, particularly if they are wet, are far more difficult to entrain by flowing water than light sandy material. Since these fine particles preferentially trap contaminants, it may therefore be appropriate in models or budget calculations to consider these different size fractions separately. On ground surfaces rain splash can loosen soil surface particles and initiate sediment entrainment and transport, especially if the ground surface is sloping. Sediments are transported in three broad phases (Defra, 2003): • Dissolved load : Ionic solutes dissolved in the water. • Suspended load : Load dominated by silt and clay, though sand may be included at peak flows. Sediment is maintained in the water column by turbulence. This material is important for floodplain construction and infilling regions of low velocity within the channel. • Bedload : Dominated by sediments coarser than fine sands and transported along the river/stream bed or soil surface. Sediment transport varies with discharge over time and in space within a river system. The natural cycle of the development and destruction of a variety of bedforms within channels also influences sediment transport dynamics over time and space.

5.1.4 Macro scale processes

Catchment sediment yield. All sediment in a river system is derived from catchment erosion that is localised in extent or more widespread. Catchment erosion is active in most landscapes, but can be greatly enhanced by human activities on the ground surface which disturb the natural landforms and vegetation. Urbanisation, for example, and its associated soil disturbance can increase erosion, but the sediment supply may be reduced below natural levels once the urbanisation is complete. Catchment erosion types vary spatially between upland, middle course and lowland regions. Table A5.1 lists the typical sediment sources in three areas of the catchment. Only a fraction of the sediment detached by erosion will reach a river in any period. This quantity is known as the sediment yield. This sediment yield (often expressed in terms of mass flux per unit catchment area) is a function of topography, geology, hydroclimatology, soil types, land cover and land management practices. The relationship between catchment erosion and sediment yield depends on two main factors: the efficacy of surface processes responsible for carrying eroded material from this point of origin to the channel; and, second, the distance over which the sediment must be carried by these processes. Sediment yield is highest in the upland headwaters where steep slopes, thin soils, and a harsh climate provide widespread opportunities for erosion processes to dominant and provide a source of coarse sediment (Defra, 2003). Overstocking and forestry operations (e.g. ploughing, ditching, clear felling) in these more erosion sensitive areas have also been

4 shown to generate higher sediment yields in headwater catchments (McHugh, 2000; Moffat, 1988; Leeks, 1992).

Table A5.1. Typical catchment sediment sources (taken from Defra, 2003) Upper course Middle course Lower course Rock fall Valley side slope Overland flow Scree slope Terrace slope Tributaries Debris flow Soil creep Cultivated farmland Landslide Floodplain erosion Wind blown soils Freeze-thaw Tributary stream Construction sites Sheet flow Cultivated farmland Urban runoff Rills and gullies Field drains and ditches Gravel workings Overgrazed, burnt or rabbit Urban runoff Marine sediments (estuaries) infested areas Ditches (forest and roads) Ditches (forest and road) Quarries Mining and gravel extraction

In the middle reaches of a river basin, the channel has less interaction with the valley side slopes and the catchment sediment supply mainly consists of mixed-sized sediment re- eroded from older floodplain and colluvium valley fills, together with coarse sediment derived from steep tributary streams. Sediment yield in the middle reaches of a river basin are lower than in the headwater reaches as erosion rates and delivery are reduced. However, human activities, such as intensive livestock and arable farming, and mineral extraction have led to elevated sediment yield in these areas. In the lower course of a river, relief is low, soils are generally deep and the channel is located within a much wider valley, which limits opportunities for direct inputs of eroded material to the river. The sediment yield is fine-grained and would naturally be limited. However, human occupation of lowland catchments for thousands of years, together with the resulting farming, urban and industrial development has led to high levels of fine-grained sediment being carried by extensive drainage networks to the main channel.

Floodwater contaminant transport. Within river systems, flooding and floodwaters can lead to the movement and deposition of sediment and/or contaminants by a number of processes: • Delivery of contaminated overland flow water and/or sediment from an upstream source site to a downstream deposition site (without any conveyance within a watercourse) • Delivery of contaminated water and/or sediment from an upstream source site to a watercourse which then floods ‘out-of-bank’ further downstream depositing sediment and/or contaminants onto the floodplain • Resuspension of contaminated material in a watercourse (bed and banks) and deposition onto the floodplain further downstream if the watercourse floods ‘out-of bank’ depositing sediment and/or contaminants • Delivery of contaminated water/soil from one part of the floodplain to another during inundation events. Within any particular flood event one or more of the above processes can take place and the rates of these fluxes will vary with respect to space and time.

5.1.5 Time dependency

Sediment associated contamination. Over the centuries human-induced changes in land use and land management have increased the rate of many erosion processes. Sediment transport in rivers generally takes place in pulses. In smaller catchments (<500km2) 50-90% of the annual sediment flux is transported during periods of days to weeks (SedNet, 2004). This effect does occur in large catchments but is far less pronounced. Natural sedimentation processes take place in the channel, on the floodplain and in lakes/reservoirs as the flow rates decreases. Natural sedimentation is restricted in the presence of flood embankments. In most rivers the majority of the remaining sediment will be deposited in the estuary and coastal zone. Forested catchments contribute far more sediment to rivers than moorland areas in the British uplands, particularly as a result of certain felling operations created under mid 20th

5 century ditching and planting regimes (Old et al., 2003a). Studies have shown 70% of the sediment load in forested and moorland areas being carried in less than 5% of the time, indicating the importance of extreme events for sediment transport in upland headwaters areas of catchments. Urban watercourses typically have ‘flashy’ hydrological regimes and poor water quality, due to the extent of impermeable surfaces, piped/culverted drainage systems and the presence of combined sewers overflows. Old et al. (2003b) undertook some detailed flow and turbidity measurements in the highly urbanised Bradford catchment in West Yorkshire. During one 15 minute period of an intense convectional rainfall event in June 2001, downstream discharge increased by two orders of magnitude and suspended sediment concentration increased by three orders of magnitude. This large flux of fine sediment (potentially containing significant amounts of urban area derived contaminants) is likely to have a considerable environmental impact downstream of Bradford on the River Aire. River floodplains act primarily as deposition areas for sediment transported by floodwaters. Recent studies have shown that a significant proportion of the suspended sediment flux transported through a river system may be deposited on the floodplain during out-of-bank flood events. Walling et al. (1999) estimated that approximately 40% of the total suspended sediment load delivered to the main channel of the Yorkshire Ouse (to Skelton) and the Tweed (to Norham) was deposited on the adjacent floodplain. An additional study on sediment transport in eastern UK rivers draining to the North Sea over the period 1993-1999 (Neal et al., 2000) showed that 20-40% of the total suspended sediment flux within individual flood events was lost to the floodplain. There has also been an increasing awareness that suspended sediment plays an important role in the transport of nutrients and contaminants through fluvial systems (Foster and Charlesworth, 1996 Walling et al., 1996). Horowitz (1991) estimated that the concentration of many contaminants bound to suspended solids can be at least an order of magnitude higher than their concentration in the dissolved phase. The floodplain is therefore an important storage zone for a wide range of sediment associated nutrients and contaminants as well as major non-point sources of these substances where lateral channel migration and floodplain reworking take place, primarily during flood conditions (Macklin, 1996). Owens et al. (2001) suggests that in many UK river basins, most of the sediment deposited on floodplains is likely to be stored for periods of between 10 and 1,000 years or longer so the potential contamination can last a long time. This contamination can have adverse effects on water quality and aquatic habitats and the agricultural use of the floodplain soils receiving contaminated sediment. The spatial distribution of the deposited sediment (and any associated contaminants) across the floodplain can be quite variable and influenced by a number of factors (Macklin, 1996; Brewer and Taylor, 1997), including: fluvial characteristics (e.g. mobile braided and single-thread meandering rivers, stable low gradient rivers); flood characteristics (e.g. depth, extent, flow rate, sediment deposition rate); and, floodplain characteristics (e.g. topography, surface roughness, presence of embankments, drainage routes). When the total suspended solids (TSS) concentrations exceed 100mg/L, more than 90% of the conveyed load of most toxic metals (e.g. cadmium, copper, chromium, mercury, lead, zinc), polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs) are present in river clay-silt sediments compared to the dissolved fraction (SedNet, 2004). At TSS concentrations <10mg/L, the dissolved fraction of these contaminants may equal or exceed the sediment-associated fraction. In European rivers, the average TSS levels commonly range from 5mg/L to 100mg/L. In turbid estuaries, TSS levels naturally reach 600mg/L or more. During floods TSS levels are commonly multiplied by one of two orders of magnitude, which indicates that most metals, PCBs and PAHs and organic carbon are transported in association with the suspended particulate material. Channel bed and bank deposits also represent important sinks for nutrients and contaminants. Reworking and re-mobilisation of these sediments during flood events can provide a source of contamination to downstream areas for many years after direct inputs of contamination to the river have ceased (Bradley and Cox, 1990). Flooding and floodwaters can have a direct impact on soil quality at any particular location via a number of processes, including particle entrainment, erosion, transportation (solution or sediment-associated) and deposition.

6 Contamination associated with historical metal mining activities. Britain has a very long history of non-ferrous metal mining. In England the main locations for vein ore mining were in Cornwall, Devon, Mendips, West Shropshire, Southern Pennines, Northern Pennines and the Lake District. In Wales the main vein ore areas were Central Wales and North Wales (e.g. Harlech, Llanrwst, Parys and Halkyn-Minera). Metal mining was important during Roman times and then again from the 15th to the beginning of the 20th Centuries (Lewin and Macklin, 1987). The peak period for metal mining took place in the 19th Century. The extraction and processing of the ore included a significant discharge of waste material into the local rivers. In addition, the presence of waste tips and tailings dumps have continued to provide a source of metals long after the mining activities ceased. Metal pollutants that enter the fluvial system, in solution or adsorbed to sediment, follow the same environmental pathways as any other ion or sediment-associated element. Although metals may enter rivers as simple ions, they react with and or are complexed by dissolved organic material, suspended or fine-grained bottom sediments. As a result particulate metals dominate river metal fluxes, unless the river is heavily polluted. Downstream dispersal rates and patterns of sediment-associated metals reflect: (i) hydraulic sorting according to differential particle density and size; (ii) chemical dispersal through solution or biological uptake; (iii) sediment-mixing processes with addition of uncontaminated material from tributaries or bank erosion resulting in dilution of heavy metals; and, (iv) floodplain deposition and storage (Lewin and Mackin, 1987). Metals that are not abstracted into the fluvial storage are either retained in estuaries or pass out to the ocean. Bradley and Cox (1990) investigated the cycling of metals in the River Derwent catchment upstream of Matlock, which has a long history of lead mining. They estimated that the loadings of metals to the floodplain surface were 1800 mg lead, 500 mg zinc, 50 mg copper and 10 mg cadmium per m2 per year respectively. The average sedimentation rate in the floodplain was in the range 0.8-4.6mm per year. Hudson-Edwards et al. (1999) reported on 2000 years of sediment-associated heavy metal storage in the Yorkshire Ouse catchment. The Yorkshire Ouse catchment has been affected by over 2000 years of lead and zinc mining in the Yorkshire Dales and 250-300 years of industrial and urban development around Leeds and Bradford. Heavy metal storage in the Yorkshire Ouse floodplain has been greatest since c. 1750. Sediments on the floodplain dating from the post-1750 period often contained levels of heavy metals (especially for lead and zinc) that exceeded recommended trigger and guideline values for contaminated land or agricultural land. The Aire at Beal (including drainage from the Leeds conurbation) was particularly contaminated with all the heavy metals measured. Metal mining and industrial activity may not be the only causes of the high levels of heavy metal storage in the Yorkshire Ouse floodplain. The rate of sedimentation on the floodplain (c. 0.24-0.85cm/year) has been at its highest in the period since c. 1750, which will have been affected by significant population growth, agricultural expansion and climatic influences that triggered the erosion and transport of fine grained sediments and associated heavy metals. The heavy metals stored in these floodplain soils therefore represent sources of future contaminants to rivers, estuaries and coastal zones. Geomorphological, hydrological and biogeochemical conditions within the floodplain will govern the degree and extent of heavy metal remobilisation that will occur in the future. In the upper Clyde catchment in Scotland, there has been a 700 year history of metal mining, especially for lead (Rowan and Franks, 2002). Sediment-associated concentrations of Pb were found to exceed 150,000 mg/kg in the unstable stream and floodplain deposits in the mining areas. Active channel bank erosion of 30-50 mm/year is now remobilising the contaminants in floodplain storage and transporting them downstream. At a distance of 40km downstream of the main mining areas, Pb concentrations in excess of 1,500 mg/kg have been measured in the sediments on the floodplain, indicating a significant level of contamination. Dennis et al. (2003) investigated the consequences of the October-November 2000 floods on heavy metal dispersal in the River Swale catchment, North Yorkshire. Both lead and zinc were mined and processed in the upper Swale catchment in large quantities in the 19th Century. Concentrations of zinc, lead and cadmium in samples collected from channel edge and floodplain sediments after the flood were found to exceed the guideline values given in the MAFF Code of Good Agricultural Practice for the Protection of Soil (MAFF, 1998a). Elevated metals levels were observed 5-10km downstream of the inputs from intensively mined tributaries. This indicates that the remobilisation of contaminated material

7 during major floods can have a serious impact on ecological and agricultural interests downstream of these old mining areas. Table A5.2 provides a summary of heavy metal concentrations measured in floodplain sediments in a number of northern England catchments. Concentrations well in excess of the most recent minimum MAFF and Environment Agency guideline levels for soils (MAFF, 1998a; EA, 2002a; EA, 2002b), and the UK Inter-Departmental Committee on the Redevelopment of Contaminated Land (ICRCL, 1987) threshold levels for contaminated land, have been measured. The latest Environment Agency Soil Guideline Values (EA, 2002a; EA 2002b) are a screening tool for use in the assessment of land affected by contamination. They can be used to assess the risks posed to human health from exposure to soil contamination in relation to land-use. They represent “intervention values”, indicating that soil concentrations above this level might present an unacceptable risk to the health of site-users and that further investigation and/or remediation is required.

Table A5.2. Concentrations of heavy metals in floodplain sediments for Northern England catchments Catchment Pb (mg/kg) Zn (mg/kg) Cu (mg/kg) Cd (mg/kg) Swale 240-4,500 142-3,000 10-37 n/a Swale-Ure 192-932 198-472 18-30 n/a Nidd 283-1,100 102-284 12-25 n/a Wharfe 196-1,690 188-803 33-55 n/a Nent 2,770-13,000 791-38,200 17-228 2-160 West Allen 2,750-9,500 600-8,000 20-85 n/a Hamps 31-309 92-3,207 11-5,318 0-7 Manifold 16-1,108 109-6,391 12-1,562 0-22 Aire 12-442 43-670 19-500 1-14 Tyne 268-2,860 478-5,300 14-64 0-14 Tees 522-6,880 404-1,920 19-77 1-6

MAFF guideline (min) 300 200 80 3 ICRCL threshold (min) 500 300 130 3 EA Soil Guideline Value 450 n/a n/a 1 (min) Sources: Hudson-Edwards et al. (1996, 1997 and 1999), Macklin et al. (1997), MAFF (1998a), ICRCL (1987), EA (2002b)

The results indicate the magnitude of these historic contaminant sources, which will also be present in a number of other metal mining and metal processing regions of England and Wales. The cessation of carboniferous coal mining and ironstone mining, together with their associated dewatering operations, in the second half of the 20th Century has produced a number of environmental impacts (Younger, 2002; Younger, 2000). The main carboniferous coalfields in England and Wales are Durham and Northumberland, North Wales and Lancashire, South Wales, Yorkshire and East Midlands. Minewater discharge from abandoned workings can produce both surface flooding and water contamination (acid or alkaline). Surface flooding can result in the loss of valuable agricultural land (and pollution of the soil) and damage to properties and businesses. Iron ochre from ferruginous discharges associated with ironstone mines has been known to block culverts in built-up areas creating localised flooding (Younger, 2000). Around 400km of watercourse in the UK is currently degraded by abandoned coal-mine discharges, and a further 200km is similarly contaminated by abandoned metal-mine discharges (Younger, 2002). Contaminant concentrations (especially Fe, Mn and SO4) are usually highest shortly after the mine has flooded to the surface, and the quality then gradually improves over time. However, unacceptably high levels of pollution may continue for hundreds of years until the pollutant-source minerals are exhausted.

Contamination associated with Industrial activities. Walling et al. (2003) undertook a comparison of the river channel and floodplain storage of sediment-associated heavy metal (Cr, Cu, Pb, Zn), total P and PCB contamination in the industrialised Rivers Aire and Calder,

8 and the agricultural River Swale in Yorkshire. In the Aire and Calder the contaminant content of the fine sediment generally increased in a downstream direction reflecting the presence of the main urban and industrial areas in the middle and lower parts of the catchment. In contrast, the concentrations of most of the contaminants in the Swale remained fairly constant along its length reflecting the relatively unpolluted nature of the river. However, the Pb and Zn concentrations did decrease downstream in the Swale reflecting a legacy of mining activities in the headwaters. Comparison of the estimates of the total mass of sediment- associated contaminants within the channels of the Aire and Swale, with estimates of the annual contaminant fluxes at the catchment outlets, indicate that channel storage is equivalent to <3% of the annual flux at the catchment outlet. In contrast, estimates for the contamination deposition flux to the floodplains emphasised the importance of the floodplain as a conveyance loss (Table A5.3). In the Aire the floodplain deposition flux was equivalent to between ca. 2% (PCBs) and 36% (Pb) of the catchment outlet flux. In the Swale negligible PCBs were found in the floodplain deposition flux, whereas the floodplain deposition flux for the other contaminants ranged from 18% (total P) to 95% (Pb).

Table A5.3. Mean annual total deposition flux of sediment-associated contaminants to the Swale and Aire floodplains (1997-1999). Contaminant Swale Aire Cr (kg/year) 359 250 Cu (kg/year) 904 376 Pb (kg/year) 27821 1302 Zn (kg/year) 20018 2428 Total P (kg/year) 11128 11479 Total PCBs (kg/year) 0 0.161

Carton et al. (2000) undertook a detailed study of the Cr content of both suspended sediment and floodplain sediment in the River Aire, Yorkshire, which includes the major Leeds-Bradford conurbation. Average Cr concentrations associated with suspended sediment and floodplain sediment ranged from c. 50-100mg/kg in the headwaters to c. 300mg/kg at the tidal limit. The background level of Cr in the <63μm fraction of soils within the catchment ranged from 6-96 mg/kg, indicating that the Cr levels in the river sediment were not naturally derived. A maximum suspended sediment Cr concentration of 627mg/kg was recorded in the River Calder, a tributary of the Aire and this reflected the nature of the industry in the Calder catchment. These high levels of Cr have long-term environmental implications as they may be stored within the river system for many years before being re- worked at a later date during periods of erosion and re-working. The Peak District in the southern Pennines contains large areas of blanket peat. These areas are situated between the cities of Manchester and Sheffield, which were in the heartland of the Industrial Revolution in the 19th century, producing vast amounts of contaminated emissions. Very high concentrations of industrially-derived, atmospherically transported particulates, including heavy metals, are now stored in the upper peat layer of the Peak District, making them some of the most contaminated blanket peats in the world (Rothwell, 2005). Lead concentrations in excess of 1000mg/kg have been measured in the upper peat layers in these areas. Extensive erosion of the peat catchments in the Peak District, involving low pH soil solution processes, is now releasing heavy metals and other contaminants (both in dissolved and sediment–associated forms) into the fluvial environment. Larger floods, particularly after periods of warm, dry weather, lead to flushes of high heavy metals contamination in the fluvial systems downstream of the blanket peats. A number of the watercourses draining these peatlands eventually discharge into drinking water supply reservoirs of the region and the sediments in these reservoirs are known to be contaminated with heavy metals (J Rothwell, pers. comm.). It could be concluded that floodplain soils downstream of these eroding peatlands will also be contaminated by heavy metals, however very little investigation has been made of actual contamination levels in these particular sink areas. CSL (2000) undertook an investigation of PCDD/Fs (dioxins) and PCBs in floodplain and non-floodplain grassland to determine whether milk from livestock grazing these areas suffered from contamination by dioxins and PCBs taken up by the cows. Samples were taken from farms on the Trent floodplain downstream of urban and industrial areas (e.g.

9 Nottingham) and the Doe Lea/Rother/Don system downstream from Sheffield. These were compared to samples taken from the Dee floodplain upstream of Chester, where little urban and industrial development was present. The Dee samples, as expected, had the lowest mean PCB concentrations in the milk and the Doe Lea/Rother/Don had the highest. Milk samples from cows grazing flood prone locations contained significantly higher PCB concentrations than samples from non-floodplain grazing sites, indicating that floodwater was contributing PCBs to the floodplain soils from urban and industrial sources upstream. The results suggested that if flooding does contribute to the contamination of pastures by PCBs, then the impact is more likely to be accumulative over time than instantaneous. As with other substances, this indicates that it may be necessary to model contaminant pathways and receptors over time in order to understand fully the implications for soil function.

Contamination associated with urban activities. Poor water quality in urban areas is often caused by the presence of contaminated sediments. Old et al. (2004) monitored the small (58km2), steep and urbanised Bradford Beck catchment in West Yorkshire between 1999 and 2001 for rainfall, flow and turbidity (calibrated to suspended sediment concentration). Water quality in Bradford Beck was poor due to urban pollution sources (including automotive tyre and brake wear and possible unauthorised industrial discharges), contaminated land drainage and intermittent CSOs (67 in the catchment). Samples of fine bed material and suspended sediment were collected (including during storm events) and analysed for As, Cd, Cr, Cu, Pb, Ni and Zn. Channel bed sediments in Bradford Beck were found to be contaminated with Cd, Cr, Cu, Pb, Ni and Zn with the highest concentrations in the most heavily urbanised areas. The data suggested that Al, As, Cd, Cu and Zn accumulated in fine bed sediments between high flow events, which were then flushed out during large flow events. Fluxes of suspended sediment-associated metals (As, Cu, Pb, Zn, Al and Fe) during one extreme high flow summer event were higher than those typically recorded during high flow conditions at a downstream gauging station on the River Aire at Beal (1,932km2). Even though Bradford Beck only represents 3% of the total catchment of the River Aire at Beal it does contribute significant amounts of metals to the river system which could, during flood events, contaminate soils on the floodplain areas further downstream.

Eutrophication of floodplains. The deposition of nutrient-rich sediments on the floodplain has been exploited by agricultural societies for many centuries. The mass of sediment periodically deposited on floodplains can be considerable and is dependent on the sediment load of the river at any point in time and the historical land use and land management within the catchment (Burt and Haycock, 1996). Johnston (1991) found sediment deposition rates within some international floodplain wetlands to be of the order of 0.6g/m2/year to 100kg/m2/year. Sediment accumulation will be dependent on the connectivity between the floodplain and the main channel, the water velocity over the floodplain and the floodplain roughness. Walling et al. (1997) and Withers et al. (1999) have shown that the sediment- associated load of total-P can account for 25-93% of the annual total-P load of British rivers. The deposition of a substantial amount of suspended sediment onto the floodplain during flood events would result in a substantial reduction in the net phosphorus flux downstream. In contrast, Walling et al. (1997) also found that the sediment-associated transport of total-N only represented 3-8% of the annual total-N load. The nitrogen flux is therefore dominated by solution-associated transport processes. Walling et al. (2000) investigated the deposition of sediment-associated phosphorus to the floodplains of 20 rivers in southern England and Wales. Sediment cores taken from the floodplains indicated that the average rates of total-P accumulation since 1963 ranged from 1.3-11.6 g/m2/year. Estimates of the increase in total-P content of suspended sediment deposited on the floodplains over the period 1950-1992 ranged from 10-170%. The highest mean total-P concentrations of the post-1963 sediment were associated with catchments characterised by intensive agriculture (with fertiliser applications) and high population densities (with effluent discharges). The lowest values as expected were associated with upland pasture with limited fertiliser inputs and low population densities. Monitoring of a 55km stretch of the River Swale in Yorkshire by Bowes and House (2001) indicated that during out-of-bank bank flooding events, there was a large retention (58% of total input) of particulate phosphorus within the system, due to the mass deposition of phosphorus-rich

10 sediment onto the floodplain. Soluble phosphorus was also retained within the system by sequestration from the water column by the high concentration of suspended solids. Floodplains can become a significant sink for sediment-associated phosphorus as plant uptake is limited. However, the changing redox status of floodplain soils during flood events and fluctuating watertable levels indicate that floodplains can become a source of phosphorus under certain conditions (Burt and Haycock, 1994). The duration of the flood also affects the phosphorus cycling. In general, however, it seems that the implications of mobilisation of P by floods may be greater for the downstream aquatic environment (in terms of eutrophication) rather than on agricultural soil function per se. The fate of nitrogen (N) in flood events is still not well known. Brunet et al. (1994) studied the lateral deposition of fine sediments and changes in floodplain water quality during large flood events on the River Adour in south west France between 1991-1994. The storage of particulate organic forms of N, the adsorption of ammonium and the denitrification of nitrate in shallow standing water meant that only 2% of the total N budget for a catchment was taken up by vegetation, the remainder being flushed away. Soluble forms of N will only be retained in stretches where the flow is low enough to permit plant or algal uptake or denitrification.

Pathogenic and radionuclide transfer in floodwaters. Floodwaters do have the potential to transport pathogenic and radionuclide material adsorbed to organic matter and clay particles in the environment. However, the dilution effect of large volumes of floodwater, together with the relative short-lived nature of pathogens in soils, suggests that widespread soil contamination by pathogens transported in floodwater is unlikely. Most soil contamination associated with radionuclides (apart from accidents, such as Chernobyl in 1986) tends to be very localised to the sources and contamination levels tend to be extremely low. Floodwaters are therefore unlikely to transport much radionuclide material. However, coastal and estuarine flooding could potentially transfer radionuclide contamination derived from coastal sources onto land surfaces.

5.1.6 Coastal flooding Considerable areas of the UK coastline are at risk from coastal flooding impacting natural, semi-natural and built environments. Parts of coastal England and Wales are also important for agriculture. These areas also face considerable challenges in terms of flood risk management. The coast is vulnerable to rises in sea level, storm surges and saline intrusion as well as coastal ‘squeeze’ as coastal habitats (including some productive farmland) are squeezed up against sea coastal defences. When seawater floods soils in coastal areas the residues of sodium and chloride that it brings will restrict vegetative growth (for non-salt tolerant plants/crops) and damage the soil structure (MAFF, 1998a). If the flooding does not remain for a long period, then the excess chloride will usually be flushed out by fresh rainwater within 1-3 years, depending on the drainage properties of the soil. However, if the additional sodium taken up by the clay particles in the soil is not immediately replaced by calcium or magnesium, the clay particles will disperse and the soil will become difficult to cultivate. Soils with a high silt or clay content are most at risk of this structural deterioration. Peaty and coarse sandy soils are less prone to damage. The current Defra and Environment Agency policy for promoting sustainable flood risk management schemes wherever appropriate rather than continuing with a huge investment in engineered hard flood defences has meant that, in places, ‘managed retreat’ or ‘managed realignment’ of the coastline (including estuaries) has been undertaken, or is planned for the future. This will lead to increased coastal/tidal flooding in some areas, particularly along the eastern and southern coasts of England.

5.1.7 Fluvial flood risk management The intervention of humans in the natural flow and flooding cycle of rivers and watercourses has taken place over many centuries. Considerable lengths of rivers within floodplains and built-up areas have undergone channel modifications (e.g. straightening, widening, deepening) and embanking (including walls) constructed to protect the land and built environment from flooding and to aid agricultural and urban development. In addition, water level control structures (e.g. weirs, sluices, barrages) have been constructed across many watercourses for a variety of reasons, including water resource management and flood risk management. These alterations to the natural channel have affected the nature of a

11 number of the physical processes that take place in channels, through changes to water velocity, stream power and sediment erosion/transportation dynamics. The interaction of the channel with the out-of-bank area or floodplain has also been greatly influenced by the channel alterations (particularly long lengths of river embankments). The transfer of fluvial flood water and any sediment or contaminant from the channel to the floodplain, together with the drainage of floodwater/runoff and contamination from the floodplain back into the channel is greatly influenced by the presence of water control structures. Burt and Haycock (1996), in their examination of the role of floodplains as transitional zones between the catchment and river channel, concluded that the management of floodplains and the engineering of flood embankments to control flood risk must be considered as having a profound effect on the ability of floodplains to regulate storm water nitrogen loads and phosphorus transfers. Work undertaken in the Swale catchment (Carter, 1998) reported an increase in channel incision in the floodplain which, in part, had been caused by the cessation of metal mining in the upstream headwaters and the construction of flood embankments along the river channel. The decrease in sediment delivery from upstream areas, together with the increased stream power caused by more water being confined inside an embanked channel has led to the reworking of metal contaminated sediments (in channel bed and banks) which have been deposited on the floodplain downstream. Macklin (1996) has identified the potential contamination of foodstuffs bound for the UK human food chain (via crops and livestock meat) from contaminated floodplains and the potential socio-economic impact this has brought to the local farmers. Carter (1998) reported one case where this had happened, with consequent economic losses to the farmer. The Defra and Environment Agency policy to explore more sustainable flood risk management options now and into the future should consider these potential channel and floodplain interactions, in terms of water and sediment dynamics and any associated soil contamination issues. This should be extended to include water level control regimes on vulnerable floodplains which will affect the soil moisture status through the year, the soil redox status, soil pH and hence the soils ability to adsorb and/or degrade contaminants, or transfer them to other environmental compartments.

5.1.8 Environmental change Longfield and Macklin (1999) investigated the historical variations in the flood frequency and magnitude in a stage record on the Ouse at York covering a 119 years. Fluxes of sediment and metal contaminants were also assessed over this period. The late 19th century was characterised by low flood frequency and magnitude, however the intensive nature of the metal mining and on-site processing that was taking place at the time would have caused a high flux of sediment-associated contaminants. Flood frequency and magnitude declined during the period 1904-1943 and metal mining ceased, resulting in a decline in sediment-associated contaminant fluxes. Between 1944 and 1968 increased upland drainage, agricultural intensification and high flood frequency and magnitude all contributed to considerably elevated sediment and metal fluxes. In contrast, the period 1969- 1977 had very low fluxes during a low flood frequency and magnitude period. In the last 20 years there have been a series of extreme flood events in the Ouse catchment, causing the remobilisation of metal-contaminated sediments, and high contaminant fluxes. The work has suggested that any future increase in the frequency and vigour of rainfall events (e.g. through climate change) will generate more extreme floods and consequently produce increased fluxes of sediment-associated contaminants. Neal et al. (2000), reporting conclusions from the Land Ocean Interaction Study (LOIS) on eastern UK rivers draining to the North Sea, state that the mobilisation of sediment from upper catchment areas, floodplain sources, pathways and sinks may all change with the likely increasing frequency of high-flow events. Leenaers and Schouten (1989) investigated the metal budget of the Geul catchment in the Netherlands, which has a legacy of metal mining and its associated pollution. They report that changes in land use and land management in the catchment during the last 30 years has increased the size of floods with short recurrence intervals (≤ 2 years) by 40-50%, resulting in more frequent small-scale inundations. In addition, the metal-contaminated stream banks are now being eroded at higher rates, causing high metal levels in suspended sediments which are then deposited on downstream floodplains. This supports earlier work by Walling (1979) and Bosch and Hewlett (1982) who also observed changes in flood regime as a result of land use change in the catchment.

12 Over the last 50 years, significant changes in rural land use and management have taken place as a result of EC and UK agricultural policy (Defra, 2004a). Typically this has led to more intensive farming, land use change, land drainage, increased stocking rates and the removal of field infrastructure (e.g. hedgerows). At a local scale (up to small catchment size) this has changed the runoff generation processes, with the increase occurrence of muddy floods, which has also had considerable additional impacts on soil erosion, diffuse pollution and groundwater recharge. However, there is still only limited evidence that local changes in runoff are transferred to the larger surface water network and floodplains downstream. Current and future policies on land management (including soil management) should give due consideration to other policies on pollution control or flood risk management to achieve the desired environmental, social and economic benefits whilst minimising any detrimental effects on soil quality and function. The recent UK Climate Impacts Programme Report on climate change scenarios for the United Kingdom (UKCIP, 2002) provides the most recent definitive summary of the predicted changes to UK climatic variables in the 2020s, 2050s and 2080s. A number of these variables will directly affect the generation of floods (both fluvial and tidal), flood duration and flood magnitude in the future. This also means that some soil functions and characteristics, together the processes of sediment and contaminant entrainment, transportation and deposition will be affected. The loss of soil organic matter, as a consequence of elevated temperatures and the subsequent increased oxidation, could lead to a deterioration in the structural stability of topsoil, making it more prone to erosion. The loss of soil organic matter would also reduce the natural capacity of the soil to adsorb and/or degrade many contaminants, therefore, potentially making them available for movement into another environmental compartment. Clayey soils are likely to be prone to greater cracking during longer, drier, hotter summers thereby providing a rapid route for water and any associated contamination to bypass the soil profile during intense summer and early autumn storms and reach drainage systems quickly, thereby potentially contributing to the generation of floods. The predicted magnitude of sea level rise, together with the increased incidence of storm surges will put substantial areas of the British coast at a higher risk of coastal flooding. Half of Britain’s Grade 1 agricultural land is below the 5 metre contour and land in those areas affected by increased flooding will become unproductive as a result of the water and saline intrusion that will occur. The intensely agricultural Fens area in East Anglia lies below sea level and is already artificially drained. This area is particularly vulnerable to sea level rise and potential loss of fertile arable and horticultural land due to salt contamination and waterlogging. Predicted climate change scenarios for the UK, involving higher summer temperatures, wetter and stormier winters, may result in an increased flux of both sediment-associated and dissolved heavy metals from eroding peatland catchments in the southern Pennines, which would continue to contaminate the watercourses, floodplains and drinking water reservoirs downstream of these areas (Rothwell, 2005).

5.1.9 GIS Spatial analysis The extent of a nominal ‘extreme’ flood in England and Wales has been mapped for the Environment Agency Flood Map (EA, 2005a; Waller et al., 2003). The Flood Map provides the outline for a 1,000 year return period (0.1% annual probability) fluvial and tidal flood (Flood Zone 2), which can be used to indicate the ‘maximum’ extent of the fluvial and coastal/tidal floodplain (note that this analysis assumes a situation where there are no flood defences in place). It therefore shows the maximum area of possible interaction between the fluvial and coastal/tidal floodwater and the land surface on the floodplain. It should be noted that the Flood Zone outlines do not include localised or ‘muddy’ flooding, urban flooding or groundwater flooding. By overlaying the Flood Zone 2 (FZ2) outline on top of other spatial datasets, such as land cover, agricultural land classification and erosion vulnerability, it is possible to provide areal estimates of what land, and its current function, might be impacted by fluvial and coastal floodwater and hence might act as a source or sink for contaminants transported in the floodwaters. The national datasets made available to the project for spatial analysis within a Geographic Information System (GIS) framework were: • Environment Agency Flood Map (Flood Zone 2) (EA, 2005a)

13 • CEH Land Cover Map 2000 (CEH, 2000) • MAFF Provisional Agricultural Land Classification Map (ALC) (MAFF, 1977) • Cranfield University Potential Soil Vulnerability Map (EA, 2002c) The analysis has been confined to the North East region of England as the computer processing of the complete national datasets for England and Wales requires greater resources than available in this review project. The complexity of the ALC dataset also precluded its inclusion in this analysis. The North East region includes the counties of Northumberland, Tyne and Wear, Durham, North Yorkshire, Teeside and South Yorkshire. This comprises the river basins of the Tyne, Tees, Yorkshire Derwent, Yorkshire Ouse, Don and the rivers of the Holderness peninsula. The North East region is a good example for this project as it provides a wide variety of land covers, soil types, topography, and intensity of agricultural production, erosion vulnerability and mining/industrial/urban legacy. The total land area of the North East region is around 23,000km2. The total FZ2 area within North East region represents about 14% of the total land area in North East region.

Land cover. The CEH Land Cover Map 2000 (LCM2000) for England and Wales, derived from remotely sensed imagery, was provided under licence by Defra in its most detailed form with 72 individual land cover classes (CEH, 2000). These 72 classes were rationalised into 10 major land cover classes, namely: arable/horticultural, improved grassland, semi-natural grassland, moorland/heathland, bog/fen/marsh/swamp, inland bare ground, built-up, inland water, forest/woodland, and coastal/estuarine. The raster (gridded) data are based on 25m x 25m grid squares, though the minimum area mapped as a single polygon is 0.5ha (0.005km2 or 8 grid squares). As discussed in earlier sections, arable land, particularly on slopes during periods of limited vegetative cover, together with disturbed ex-mining/quarrying areas are more vulnerable to soil erosion and hence the potential transportation of sediment-associated contaminants. Degraded moorland on erodible peaty soils in headwater areas might also deliver contaminants to the floodplains. The North East region contains all these land cover types and historical mining legacy, together with significant centres of development originating from England’s Industrial Revolution. Typically, the wider floodplains in the middle and lower reaches of river systems tend to be the main areas where floodwaters deposit contaminants extensively across the land surface, whatever the land cover. In these areas more pollution sensitive wildlife habitats, often found more commonly in land covers such as bog/fen/marsh swamp and semi-natural grassland could be negatively impacted by these floodwaters, especially if episodes of contaminant input continued over time. The analysis of the various land covers within FZ2 are given in Table A5.4. The arable/horticultural and improved grassland classes cover an area of 1,824km2, or about 58% of the total area within FZ2. This is a significant area which could potentially be impacted by the consequences of extreme fluvial and coastal/tidal flooding and any associated soil contamination effects including, ultimately, human health. In addition, the potentially pollution sensitive conservation land cover classes of semi-natural grassland and fen/marsh/swamp/bog represent around 13% of the FZ2 area. Flooding of these areas with contaminated water originating from upstream could be detrimental to a number of protected habitats and species. Around 12% of the FZ2 area is covered by built-up land, providing both sources (e.g. contaminated industrial sites, landfill sites) and sinks for contaminants.

Table A5.4 North east region – areal extent of land cover classes within Flood Zone 2. Land cover class Area (km2) % FZ2 area Arable/horticulture 1,358 43.5 Improved grassland 466 14.9 Forest, woodland 257 8.2 Semi-natural grassland and bracken 381 12.2 Rough vegetation (heathland and moorland) 22 0.7 Fen, marsh, swamp, bog 21 0.7 Inland bare ground 22 0.7 Built-up 377 12.1 Inland water 117 3.8 Tidal/coastal 101 3.2 TOTAL 3,122 100.0

14

Figure A5.1 provides an indication of the range of land covers represented in the lower Derwent and Ouse confluence within the FZ2 outline. The significant areas of arable/horticultural, improved grassland, semi-natural grassland and built-up land within these wide floodplains as potential sinks or sources of soil contamination are very evident.

Soil erosion vulnerability. The national dataset on potential soil erosion vulnerability from the Environment Agency R&D project P2-209: ‘Prediction of sediment delivery to watercourses from land – Phase II’ (EA, 2002c) was provided under licence from the Environment Agency. The dataset provided an identification of the risks of erosion from soils under arable, grassland and upland land covers using soil and slope characteristics. The 1km x 1km gridded maps generated from this project were not validated, though the results from the 1 in 10 year return period erosion event broadly confirmed the findings from other researchers. The methodology used in this project is currently being revised and improved within an on-going Environment Agency R&D project.

Figure A5.1. Lower Derwent/Ouse (Yorkshire) – land cover classes within the FZ2 outline.

The erosion vulnerability dataset for the 1 in 10 year return period erosion event was analysed in a similar way to the land cover dataset to determine areal extents within the FZ2 outline. Six erosion vulnerability classes were derived from the P2-209 dataset, ranging from no erosion vulnerability to very high erosion vulnerability. A further class covering the built-up areas that were not classified was also included. Results are given in Table A5.5.

Table A5.5 North east region – areal extent of erosion vulnerability classes within Flood Zone 2. Erosion vulnerability class Rate (m3/ha/year) Area (km2) % FZ2 area Very high 1-5 0.0 0.0 High 0.1-0.999 14.3 0.6 Moderate 0.034-0.099 278.6 11.5 Low 0.013-0.0339 508.5 21.0 Very low 0.0001-0.0129 521.0 21.6 None 0 936.8 38.8 Built-up - 156.2 6.5 TOTAL - 2,415.4 100.0

15

The discrepancy in the total area value between the erosion vulnerability analysis and the land cover analysis can be explained by the inherent differences in the two datasets, both in terms of their spatial resolution and the spatial extent. Also, the land cover dataset includes the whole inter-tidal area along the coast and within all the estuaries, inland water areas and some other land covers, which were not included in the erosion vulnerability dataset. About 293km2 or 12% of the north east region within FZ2 would be classified as being of moderate to very high erosion vulnerability, which could potentially, under suitable conditions, provide sources of contaminant transport from fluvial flooding. In contrast, approximately 81% of the north east region within FZ2 has a no erosion to low erosion vulnerability classification. Further combined analysis of the FZ2 dataset, LCM2000 dataset and erosion vulnerability dataset would permit the identification and quantification of those areas most prone to soil erosion, which are also under land covers which may provide sources of soil contamination to downstream floodplain areas. Other, more detailed spatial datasets on the presence of specific contaminants known to be transported by floodwaters could also be included in this type of analysis, if appropriate.

5.1.10 Modelling contaminant pathways and fate The spatial analysis of land cover types and Agricultural Land Classification within the Environment Agency’s Flood Zones provides a simple quantification of the potential areas of land that may be considered as sources or receptors for soil contaminants transported by flood water. However, there is essentially no representation of contaminant pathways within this analysis. This is because the pathways from source to receptor involve processes that vary both in time and from place to place. Whilst the static GIS analysis gives us some idea of a plausible maximum extent of source or receptor areas, it cannot represent the evolution of contaminant inputs to the soil over time, and, in particular, the remobilisation of sediment- associated substances. Generally, the particles that are stored in floodplains have longer residence times than channel sediment because of being less accessible to erosion. Many environmental contaminants also break down over time through radioactive decay, chemical reactions or bioprocessing. The long-term fate of contaminants is therefore controlled by the different timescales of degradation and residence time in the channel and floodplain. Residence time therefore becomes a factor in understanding the impact of the contamination on soil function. The decay processes may occur at different rates for different substances (for example relatively short time scales for most pathogens but longer timescales for radionuclides). Mobilisation, transport and deposition can also be viewed over different time scales, although for the purpose of understanding the long term impact on soil function it is probably appropriate to view these processes in terms of discrete ‘events’.

Deterministic modelling. One approach to represent the dynamic nature of contaminant transport is to model deterministically the important controlling processes, namely the hydraulics of river and floodplain flooding, the entrainment and deposition of sediment, the attachment and fate of sediment-associated contaminants and the movement of soluble contaminants, possibly including infiltration into the soil water during flooding. Even when compartmentalised in this way, each of these processes would require a moderately complex mathematical model, with coefficients, boundary and initial conditions that have to be specified for any simulation. Different choices are possible, depending on the resolution and reliability of the available data to characterise these various parts of a model. Although all the elements of a complete deterministic model are ‘available’ (in the sense that there are concepts, equations and computer codes that can be used) there have been few integrated models that link the various elements in one complete system. Most hydraulic modelling of rivers is carried out using one dimensional (1D) computer codes, which average the variations in hydraulic properties such as velocity both laterally and with depth. The main purpose of such models is usually to determine the timing, level or extent of flood water as an aid to flood risk management. There are numerous such hydraulic models in existence, although for practical purposes three proprietary models have been adopted by the Environment Agency for flood risk modelling; these are HEC-RAS from the US Army Corps of Engineers, and two models sold by commercial providers, called ISIS

16 (Wallingford Software Ltd.) and MIKE11 (DHI Ltd.). In all cases, the fundamental mathematical and physical principles of the models are similar. Most 1D hydraulic modelling systems include optional sediment and water quality modules. However, the necessary assumption of depth and laterally averaged flows is not consistent with field knowledge of the considerable spatial variability in contaminant fluxes, especially the load associated with sediments (Graf, 1990, Walling et al., 2003). A more detailed and complex approach is to build contaminant transport into a two- or three dimensional (2D or 3D) river and floodplain hydraulic model. This approach has recently been demonstrated in the UK by Stewart et al. (1998), who based their model on the 2D depth-averaged hydraulic model TELEMAC-2D (a commercial fluid dynamics code produced in France). The hydraulic model acts as a ‘platform’, providing simulated values of local velocities and water depth. Data requirements are a detailed and accurate topographic grid (for example a LiDAR survey), characterisation of surface roughness, and initial and boundary conditions, in terms of water levels and velocities or flow rates. The 2D model is then used to drive the advection-diffusion model SUBIEF (Moulin and Gailhard, 1996) for contaminants in solution and bound to particles in suspension. The combined model requires specification of contaminant concentrations at the inflow boundary and initial concentrations within the model domain. It is also necessary to specify a dispersion coefficient and four parameters relating to erosion and deposition of particles. Rather than assuming a zero flux boundary at the ground, a 1D vertical soil infiltration model is integrated into the hydraulic/contaminant model. This allows pollutants to enter the soil water during a flood event. The 1D soil infiltration model is the well-known Richards equation, which allows the soil conductivity to vary as a function of soil moisture content. The soil infiltration component requires specification of hydraulic conductivity, initial soil moisture content and saturated soil moisture content. The entire scheme operates in one direction only, i.e. it permits infiltration but does not allow soil moisture to re-enter the floodplain flow. The type of 2D hydrodynamic model described by Stewart et al. (1998) solves a set of equations known as the shallow water or Saint Venant equations. Although the computer codes now available are robust and well proven, the solution remains computationally demanding and requires detailed specification of parameters and boundary conditions. Experience shows that this type of model, whilst very effective over specific river reaches, is not suitable for large scale application to entire river basins or regions. Currently, river reaches of ~50km are a pragmatic upper limit on the scale of application (although the actual limit in any specific case will depend on the model grid mesh resolution and hence the complexity of the physical system, and of course on available computing hardware). Ultimately, the computational constraints may be overcome, but unless there is detailed data available to characterise the model’s boundary conditions and parameters, then the simulations are, in effect, hypothetical. There is no doubt that such simulations may help to show the influence of topography and floodplain hydraulics on contaminant transport, particularly in locations where there are sufficient background data to support the model. The TELEMAC-2D model had been applied to a 1km reach of the River Culm and a 40km reach on the River Severn (Bates et al., 1995) and Stewart et al. (1998) combined these models with SUBIEF to simulate hypothetical pollution events. A generic, conservative contaminant substance was assumed and loads were specified in terms of assumed rates of dissolved and sediment-bound fluxes. At both study sites, the modelled contaminant plume was found to leave the main river channel and occupy the floodplain during simulations of moderate-sized flood events. The modelled soil infiltration of contaminated water was not found to be significant, with no more than 3% of the flood water entering the soil, however this still accounted for up to 15% of the pollutant mass. Removing the soil infiltration model had no noticeable effect on the flood hydraulics. In the 1km reach on the Culm, 18% of contaminated suspended sediment was deposited on the floodplain. This value seems plausible; it is towards the lower end of figures reported in the literature from chemical mass balance studies for suspended sediment accumulation on floodplains. On the Culm, Walling et al. (1986) had measured values between 8% and 54% for flood events. The 2D modelling suggested that the contaminated sediment deposition is very unevenly distributed across the floodplain, with half of the deposited sediment load settling in only 10% of the floodplain area, in localised sinks that were a function of topography and local hydraulic effects. A 2D hydrodynamic model can be expected to provide an acceptably accurate simulation of the hydraulics of flood flows provided the quality and resolution of the boundary

17 conditions (including the underlying topographic grid, surface roughness and flow boundaries) are sufficient. There are also simpler, more approximate 2D models that solve a sub set of the complete shallow water equations. These models, which can be referred to as 2D diffusion wave schemes, sacrifice some accuracy and representation of hydraulic processes (for example by neglecting turbulence). The main motivation for the approach is to generate robust, approximate results based on simple parameterisation suitable for automated application over thousands of kilometres of river and floodplain. One such model, JFLOW, has been used to generate flood extents for the entire river network of England and Wales in work commissioned by the Environment Agency (Bradbrook et al., 2005). The national application of JFLOW was for floodplain flows only (that is, it did not include river channels within the 2D model domain) driven by statistical estimates of a hypothetical flood of specified probability. The model does, however, generate simulated depths and velocity vectors across the floodplain (Figure A5.2). These values therefore exist for all areas lying within the Environment Agency’s Extreme Flood Outline. The data have the potential to be used in predicting contaminant mobilisation and deposition.

Figure A5.2. Simulated depth averaged velocity vectors produced using a 2D diffusion wave model.

Probabilistic modelling. The 2D hydraulic modelling approach promises to resolve important features of the physics of flood flows and contaminant transport at a level of detail relevant for modelling specific flood events at the reach scale. However, computationally simpler and less data-hungry methods are better suited to assess the mobilisation and transport of contaminants at a larger scale, and the evolution of the contaminant load over longer periods of time. Here, it is the suspended rather than dissolved phase that is most significant, because of the longer residence times. A general and flexible framework for modelling the fate of contaminants is the probabilistic model known as the discrete time Markov chain. Kelsey et al. (1987) proposed using this type of approach for contaminant modelling and it has been developed further by Malmon et al. (2002, 2003, 2005). This approach focuses on the fate of sediment-associated substances (although it has also been applied in modelling pathogen fate, Yeghiazarian et al., 2004). The discrete Markov chain is a stochastic model that represents a random variable (in this case a sediment particle) that can be in any of several states (in this case conceptual sediment stores) and can move between states at each step (in this case a time step). In the case of sediment-associated contaminants, it is convenient and reasonable to interpret the step as being one year. The movement between states is governed by a matrix of transition probabilities and the probability of moving from one state to another does not depend on which states the chain was in before the current step. To apply the theory to model the contaminants within a river and floodplain system, the river has to be conceptualised as a set of discrete sediment reservoirs or stores. Each of these conceptual stores is a state within the Markov chain, and together they form the state space of the model.

18 It is assumed that all particles within a given state are equally likely to experience mobilisation, transport or deposition over a time scale of the order of 10 years or so. The conceptual basis for the model is therefore strongest if the states are defined in such a way as to meet this assumption. This is achieved by defining states as geomorphological units that have relatively homogenous long term erosion and deposition characteristics. In the simplest case, the floodplain and channel bed can be distinguished, and there may be separate states identified for reaches of the river where the geomorphology changes, for example in terms of slope or channel shape. The state space can also be designed to reflect particle size sorting. Consider a simple example in which a river is split into two reaches, each of which has a separate channel and floodplain unit (Figure A5.3) Together, these sediment stores comprise four elements of the state space. In addition, there is a fifth state, known as an absorbing state, which represents the loss of contaminated sediment from the downstream boundary of the model.

Figure A5.3. Schematic of the discrete Markov chain state space for a simple river and floodplain.

State 2 (channel 2 in 1st reach) 3 1 4 State 1 (floodplain in 1st reach)

‘absorbing 1st reach 2nd reach state’

The transition probability matrix for this system is:

⎡ P1→1 P1→2 P1→3 P1→4 P1→x ⎤ ⎢P P P P P ⎥ ⎢ 2→1 2→2 2→3 2→4 2→x ⎥

P = ⎢ 0 0 P3→3 P3→4 P3→x ⎥ ⎢ ⎥ ⎢ 0 0 P4→3 P4→4 P4→x ⎥ ⎣⎢ 0 0 0 0 1 ⎦⎥

where Pi→j is the probability of a particle moving from state i to state j in a single step. The notation x is used to represent the absorbing state. It can be seen that the matrix allows for the possibility that a particle originally residing in state 1, may move to any of the states 2, 3 and 4 in a single step, or may be transported out of the system. It is permitted for a particle to move between channel and floodplain in one reach, or to remain in the same state. However, there is by definition a zero chance of a particle moving from states 3 or 4 into the upstream reach (state 1 or 2). Likewise, there is a zero chance of moving from the absorbing state back into the ‘active’ part of the system whilst there is a probability of one that a particle in the absorbing state will remain there. If it is assumed that the system is in long term steady state or geomorphological equilibrium (that is, the masses of sediment in the various states do not change systematically over time) then straightforward matrix calculations give the probability of particles moving from any one state to any other state over a given number of steps, and the distribution of transit times for the same transport. Because the state space has been partitioned into homogenous units with stable, long term masses, the flux of particles between states at any time can also be computed. The computations can be extended to model the average residence time of particles in each state.

19 A further extension allows the model for particle transit times to be combined with a suitable decay function to describe the net contaminant fluxes through the system. Malmon et al. (2002) showed how this analysis predicts the fate of 137Cs at the Los Alamos National Laboratory in New Mexico, USA, with an initial flushing of contaminated sediment from the channel, followed by a much longer, gradual increase in the flux of contamination from floodplain sediments. The approach has not been proven in use in the UK context; however it could be suitable for broad scale application because of its generality. The critical issues are delineation of the state space and estimation of the transition probabilities. The following section discusses briefly how this might be done in the UK.

State space identification and sediment budget requirements. Under the programme of work for the development of Catchment Flood Management Plans (CFMPs) in England and Wales, a suitable geomorphological analysis may already be in place for some rivers. This process has also been automated (EA, 2005b) to split river reaches according to their geomorphological sensitivity, as inferred from parameters derived by GIS analysis, including sinuosity, drift geology and numbers of tributaries. Delineation carried out in this way can identify river reaches defined so as to be homogenous in terms of their sensitivity to erosion or deposition, as required. The automated GIS analysis can be used to derive the dimensions of each geomorphic unit. Malmon et al. (2003) describe how these probabilities can be computed from estimates of the sediment budget for a particular system. The calculations are based on the conditional probabilities of mobilisation in a given state, transport to another state, deposition in the new state, deposition in the same state or transport out of the system. The approach therefore characterises the contaminant pathway through erosion, remobilisation, transport and deposition. The transition probabilities could be determined, in principle, from detailed field data or modelling (such as in the 2D hydraulic modelling described above). However, for a generalised, broad scale analysis, the requirement is for estimates of the channel and floodplain mass (T), the long-term average annual sediment flux (T/yr), the channel erosion/deposition rate (T/yr) and the floodplain erosion/deposition rate (T/yr). Some possible broad scale approaches are suggested below. Note that uncertainties in the estimates should ideally also be considered for any future application.

Channel and Floodplain Mass. Tools developed for the geomorphological assessment of a CFMP also have the potential to be used to determine the channel and floodplain masses. The extent of the Environment Agency’s Flood Map can be used to estimate the floodplain areas of each storage unit while the channel mass can be estimated by the use of Ordnance Survey 1:10,000 scale maps or vector digital data, where available. The density of the sediment unit can be based on average dry bulk density of channel sediment values for the UK (~1.1 to 1.3 g/cm3). The depth of active sediment available for erosion could be estimated either from experimental data (which is limited) or as an effective scour depth, based on an empirical formula (see below).

Annual Sediment Flux. Integration of the flow-duration curve and a sediment-discharge rating can be used to estimate the annual sediment flux (Biedenharn et al., 2000). This method is widely used in the literature. Both flow duration-curve (which defines the proportion of time for which a given flow is experienced) and the sediment-discharge rating curve are process-based and can be derived either from measured values or using models that allow for prediction of future flow regime and sediment yield. Also, the sediment rating curve can be defined to reflect specific components of the sediment runoff process, for example it can be calculated specifically for the particle size fraction of most interest. The flow duration curve can be derived at a gauged site from the flow record. For ungauged locations, generalised methods are available to estimate the flow duration curve, such as the model available to the Environment Agency in the Low Flows 2000 software produced by CEH Wallingford (Young et al., 2003, Holmes et al., 2002). Although there are not any continuous monitoring suspended or bed load sites in the UK (except for specific individual studies), it is possible to estimate the bed load for a river by using a suitable bed- load transport equation. The sediment rating curve is based on hydraulic conditions and the bed gradation. A range of sediment transport functions can be used for this analysis; see Williams et al. (2002).

20 The data required for the sediment transport functions are typically parameters such as particle size, specific gravity, velocity, depth, slope, width and water temperature. For some parameters, it will be sufficient to assume average values for rivers in the UK, for others the values can be derived from topographic data and either hydraulic models (where available) or a general flow equation such as Manning’s equation. These calculations can be programmed into GIS-based software or carried out using existing software packages.

Channel Erosion/Deposition Rate. The channel erosion and deposition rate can be measured or derived from a hydraulic aggradation/degradation model, where available. More generally, where detailed analysis and/or hydraulic models are not available, empirical formulae can be used to predict channel erosion/deposition rates as a form of ‘general scour’. Sear et al. (2003) note that this is quite predictable given knowledge of channel geometry and flow rate. For a broad scale assessment, the GIS tools applied for CFMP analysis can be used to determine channel dimensions and sinuosity, from which a typical scour depth for the annual average flood flow can be calculated (e.g. Thorne and Abt, 1993). The annual rate of scour and fill can then be approximated by multiplying the typical scour depth by the width and length of the reach and by an average bulk density of channel sediment.

Floodplain Erosion/Deposition Rate. Floodplain rates of sedimentation/erosion have been measured and modelled using a wide range of techniques including the use of natural and anthropogenic radionuclides, alluvial stratigraphy, dendrochronology, sediment reconstruction and numerical modelling. There have been many studies carried out on British rivers, and it is expected that a literature review can provide sufficient information on floodplain sedimentation rates; this report includes a short bibliography listing some of the available literature.

5.2 Other mechanisms

Contaminants present in the soil can be remobilised and/or transported by a number of other natural and artificially induced mechanisms.

Wind. Any sediment-associated contaminants present in the surface layer of soils will be susceptible to remobilisaton and transportation by the wind if the sediment is light enough to be moved. Unvegetated non-cohesive material (e.g. light sandy soils) and organic matter (e.g. peaty soils), especially when dry, are particularly vulnerable to wind blow. Light sandy soils in the East Midlands, Vale of York and the peaty soils of the East Anglian Fens are most prone to wind erosion (MAFF, 1998b).

Agricultural cultivations. The mechanical turning, mixing or inversion of soil during cultivation activities (especially ploughing) can provide opportunities for contaminants to be mobilised and transported to other environmental compartments. Cultivations tend to break down soil clogs and can encourage soil surface drying, which will affect physical, chemical and biological processes taking place in the soil.

Soil disturbance in the built environment. The disturbance of contaminated soil during urban construction and development work can also provide opportunities for contaminants to be mobilised and transported to other environmental compartments. The stripping and storage of topsoil during construction and development is often associated with the degradation of topsoil structure and topsoil stability, which when replaced on the land (often on subsoil compacted by construction vehicles and machinery) make them more vulnerable to erosion. However, soils known to be contaminated by certain elements or compounds can be treated by appropriate physical, chemical or biological methods prior to re-use to reduce the risk of contaminant re-mobilisation.

5.3. Legislative context

There is no legislation that directly links flooding and floodwaters to soil contamination. There is, however, some legislation and guidance that indirectly links these aspects.

21 Water Framework Directive (WFD) (2000/60/EC). The purpose of this Directive is to establish a framework for the protection of inland surface waters, transitional waters, coastal waters and groundwater which: • prevents further deterioration and protects and enhances the status of aquatic ecosystems and, with regard to their water needs, terrestrial ecosystems and wetlands directly depending on the aquatic ecosystems; • promotes sustainable water use based on a long-term protection of available water resources; • aims at enhanced protection and improvement of the aquatic environment, inter alia, through specific measures for the progressive reduction of discharges, emissions and losses of priority substances and the cessation or phasing-out of discharges, emissions and losses of the priority hazardous substances; • ensures the progressive reduction of pollution of groundwater and prevents its further pollution, and • contributes to mitigating the effects of floods and droughts River basin management plans (RBMPs) will be developed (by stakeholder groups, led by the Environment Agency) with defined environmental objectives. The RBMPs will also contain programmes of measures to limit and reduce the contamination of all water bodies and groundwater and their associated ecosystems. Land management and pollution control measures (for point and diffuse pollution sources) will be key elements of RBMPs, which are clearly linked to soil contamination issues. Within catchments, large scale land management changes will undoubtedly have an effect on flood generation processes and the flood magnitude. In the context of the ecological drive of the Directive as a whole, mitigating the effects of floods would be best served by restoring the natural function of floodplains, with their ability to hold up floodwaters and reduce the flood pulse. Reconnection of the more natural floodplain would clearly affect the future flood extent and, as a consequence, sediment erosion, transportation and deposition processes. WFD was transposed into English and Welsh law by Statutory Instrument 2003 No. 3242 - The Water Environment (Water Framework Directive) (England and Wales) Regulations 2003. Statutory Instruments (SIs) are a form of legislation which allow the provisions of an Act of Parliament (or European Directive) to be subsequently brought into force or altered without Parliament having to pass a new Act.

PPS25 – Development and Flood Risk. Planning Policy Statement 25 (PPS25) This was published in 2006 by the Department for Communities and Local Government (DCLG) and replaced Planning Policy Guidance 25 – Development and Flood Risk (DTLR, 2000). This policy statement, aimed at the built environment, explains how flood risk should be considered at all stages of the planning and development process in order to reduce future damage to property and loss of life. It should also help to ensure that floodplains are used for their natural purposes, continue to function effectively and are protected from inappropriate development. Local planning authorities need to consider the issues raised by flooding on the wider river catchment and the need to work with natural processes in planning future development. This is consistent with the requirements of the Water Framework Directive in respect of RBMPs. They should also consider how a changing climate is expected to affect the risk of flooding over the lifetime of developments. Point 1 in PPS25 covers, in part, environmental aspects of flooding: “Flooding from rivers and coastal waters is a natural process that plays an important role in shaping the natural environment. However, flooding threatens lives and causes substantial damage to property. The effects of weather events can be increased in severity both as a consequence of previous decisions about the location design, design and nature of settlement and land use, and as a potential consequence of future climate change. Although flooding cannot be wholly prevented, its impacts can be avoided and redcued through good planning and management.” Potential damage from flooding is both uncertain and unpredictable but it can be significant and, in the case of soil loss, potentially irreversible. Because of this the Government considers that the objectives of sustainable development require that action through the planning system to manage development and flood risk should be based on the precautionary principle. In particular, Planning Policy Statement 1: Delivering Sustaimable Development (ODPM, 2005) sets out the Government’s objectives for the planning system, and how planning should facilitate and promote sustainable patterns of development, avoiding

22 flood risk and accommodating the impacts of climate change. The Rio Declaration in 1992 defined the precautionary principle: “where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost- effective measures to prevent environmental degradation”. The planning process should therefore take due regard for soil loss issues. This is particularly the case on so-called “brownfield” development sites where soil contamination and its re-mobilisation could be a potential hazard (depending on the historical use of the land). . TAN (Wales) 15 – Development and Flood Risk. This Technical Advice Note (TAN) is the Welsh equivalent of PPG25, published in 2004 by the Welsh Assembly Government. TAN15 makes the following statement with regard to surface runoff from new development - “All types of land use change will impact on the natural hydrological cycle in one way or another and flooding is not confined to floodplains, as heavy rain falling on waterlogged ground can cause localised flooding almost anywhere. In all flood risk zones, development should not increase the risk of flooding elsewhere. Runoff from developments in these areas can, if not properly controlled, result in flooding at other locations and significantly alter the frequency and extent of floods further down the catchment.”

Common Agricultural Policy (CAP) Reform. The new Single Payment under the Common Agricultural Policy (CAP) reform will be linked to cross compliance conditions to come into effect in 2005. These include standards of Good Agricultural and Environmental Conditions (GAEC) in relation to protection of soils. The Government have decided on an evolutionary approach which will in due course move to the production by each farmer of risk-based soil management plans. Defra expect the plans to have the potential to result in local benefits for the control of water run-off from soils.

Environmental Stewardship. The Government launched the new Environmental Stewardship scheme in March 2005. This new agri-environment payment scheme aims to secure widespread environmental benefits through the adoption of appropriate land management options. Entry Level Stewardship (ELS) is a ‘whole farm scheme’ open to all farmers and land managers. If ELS is taken up across large areas of the countryside it will help to improve water quality and reduce soil erosion, which will also assist in meeting of the WFD requirements. Flood management has been included as a secondary objective in the new Higher Level Stewardship (HLS). Under the HLS scheme, options that contribute to flood management will be adopted where they contribute to one or more of the primary objectives of the scheme which relate to wildlife conservation, resource protection, landscape, the historic environment and public access. For example, options designed to create new grassland habitat on existing arable land will reduce soil erosion and may well reduce runoff by improving water retention.

The Soil Code. The Code of Good Agricultural Practice for the Protection of Soil (MAFF, 1998) includes a section on the causes and consequences of soil erosion (including contamination and flooding), how to avoid it and what remedial measures can be put in place to reduce the effect. The Soil Code also has a section on the impact of sodium and chloride contamination by seawater flooding, including remedial measures in the event of this contamination occurring.

5.4. Actions and research needs

A number of actions and research needs have been identified, viz: • Undertake GIS-based modelling, supported by selected field sampling for validation purposes, of river channels and floodplains known or suspected to be contaminated by heavy metals derived from mining operations, metal processing industries or industrial emissions. Undertake risk-based assessment to determine what remediation, if any, is needed and what is the most appropriate future land use in these floodplain areas to minimise impacts on human, animal or plant health. A key issue however will be deciding on an appropriate metal concentration that corresponds to an unacceptable risk, and hence potential remediation.

23 • Undertake a quantitative national assessment of floodplain soils and sediments for target contaminants known to affect human, animal or plant health (e.g. organic pollutants, radionuclides and biological contaminants) and how they interact with floodwaters. The selection of the target contaminants should be based on i) persistence; ii) bioaccumulation/adsorption; iii) toxicity; iv) relevance to river basins/floodplains; and v) flux levels. • Assess the potential effects of the current more sustainable flood risk management policy options, including managed retreat of fluvial and coastal flood defences and reconnection of fluvial floodplains, on future contaminant dynamics and associated impacts on soil function. • The recommended national assessment of floodplain soils can be carried out using GIS analysis in the first place. This will focus attention on regions of river systems where modelling is needed to understand the long term fate of contaminants and their impact on soil function. There should be an initial review of the role of 2D and probabilistic modelling. The latter is likely to be suitable for large-scale application. • In view of the possible implications of coastal realignment and sustainable river flood management, there should be due consideration of policy links between soil function and flood and coastal erosion risk management. This reiterates the general recommendation on soil-friendly policy frameworks in Action Point 6 from the First Soil Action Plan for England: 2004-2006 (Defra, 2004b).

24 References (Contaminant re-mobilisation and transportation)

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25 DTLR 2000. Planning Policy Guidance 25 – Development and Flood Risk. Department of Transport, Local Government and the Regions. EA 2005a. Environment Agency Flood Map (www.environment-agency.gov.uk/floodmap). EA 2005b. River Ribble Catchment Flood Management Plan: Scoping Report Report to the Environment Agency by JBA Consulting and Entec Ltd., September 2005, 149 pp. EA 2002a. Assessment of risks to human health from land contamination: an overview of the development of soil guideline values and related research. Environment Agency Report CLR7, ISBN 1 857 05732 5. EA 2002b. Contaminated Land Exposure Assessment (CLEA). Soil Guideline Values (SGC) for lead and cadmium. Environment Agency, Bristol. EA 2002c. Prediction of sediment delivery to watercourse from land. R&D Project Technical Report No. P2-209, ISBN 1-84432-078-2. Environment Agency, Bristol. Embleton C and Thornes J (Eds.) 1979. Process in geomorphology. Edward Arnold (Publishers) Ltd, London. Foster IDL and Charlesworth SM 1996. Heavy metals in the hydrological cycle: Trends and explanation. Hydrological Processes 10(2), 227-261. Hewlett JD and Hibbert AR 1967. Factors affecting the response of small watersheds to precipitation in humid areas. In: Forest Hydrology, eds. WE Sopper and WH Lull. Proc Int. Symp. On Forest Hydrol., Penn State Univ., University Park, Pennsylvannia, 275-290. Graf WL 1990. Fluvial dynamics of Thorium-230 in the Church Rock event, Puerco River New Mexico. Annals of the Association American Geographers, 80, 327-342. Holmes MGR, Young AR, Gustard A and Grew RA (2002). A region of influence approach to predicting flow duration curves within ungauged catchments. Hydrology and Earth System Sciences, 6, 721-731. Horowitz AJ 1991. A primer on sediment trace-element chemistry. Second Edn. Lewis, Michigan, USA. Hudson-Edwards KA, Macklin MG and Taylor MP 1999. 2000 years of sediment-borne heavy metal storage in the Yorkshire Ouse basin, NE England, UK. Hydrological Processes 13(7), 1087-1102. Hudson-Edwards K, Macklin M and Taylor M 1997. Historic metal mining inputs into the Tees river catchment. The Science of the Total Environment 194/195, 437-445. Hudson-Edwards KA, Macklin MG, Curtis CD and Vaughan DJ 1996. Processes of formation and distribution of Pb-, Zn-, Cd- and Cu-bearing minerals in the Tyne basin, Northeast England: implications for metal-contaminated river systems. Environment Science & Technology 30(1), 72-80. ICRCL 1987. Inter-departmental Committee on the Redevelopment of Contaminated Land. Guidance Note 59/83 (2nd Edition), Department of the Environment, London. Johnston CA 1991. Sediment land nutrient retention by freshwater wetlands: effects on surface water quality. Critical Reviews in Environmental Control 21, 491-565. Kelsey HM, Lamberson R and Madej MA 1987. Stochastic model for the long term transport of stored sediment in a river channel. Water Resources Research, 23, 1738-1750. Leeks GJL 1992. Impact of plantation forestry on sediment transport processes. In: Dynamics of Gravel-bed Rivers, eds. P Billi, RD Hey, CR Thorne and P Tacconi. Wiley, Chichester, 651-668. Leenaers H and Schouten CJ 1989. Soil erosion and floodplain soil pollution: Related problems in the geographical context of a river basin. International Association of Hydrological Systems (IAHS) Publ. no. 184, 75-83. Lewin J and Macklin MG 1987. Metal mining and floodplain sedimentation in Britain. International Geomorphology, 1986 Part 1. John Wiley & Sons Ltd, Chichester, West Sussex, 1009-1027. Longfield SA and Macklin MG 1999. The influence of recent environmental change on flooding and sediment fluxes in the Yorkshire Ouse basin. Hydrological Processes 13(7), 1051-1066. Macklin MG, Hudson-Edwards KA and Dawson EJ 1997. The significance of pollution from historic metal mining in the Pennine orefields on river sediment contaminant fluxes to the North Sea. The Science of the Total Environment 194/195, 391-397. Macklin MG 1996. Fluxes and storage of sediment-associated heavy metals in floodplain systems: assessment and river basin management issues at a time of rapid environmental change. Chap 13 in: Floodplain Processes, eds. MG Anderson, DE Walling and PD Bates). John Wiley & Sons Ltd, Chichester, West Sussex, 441-460.

26 MAFF 1998a. Code of Good Agricultural Practice for the Protection of Soil. Ministry of Agriculture, Fisheries and Food, London. MAFF 1998b. Controlling soil erosion – an advisory booklet for management of agricultural land. Ministry of Agriculture, Fisheries and Food, London. MAFF 1977. Agricultural land classification of England and Wales. ADAS, Ministry of Agriculture, Fisheries and Food, London. Malmon DV, Reneau SL, Dunne T, Katzman D and Drakos PG 2005. Influence of sediment storage on downstream delivery of contaminated sediment. Water Resources Research, 41, W05008, doi:10.1029/2004WR003288. Malmon DV, Dunne T, and Reneau SL 2003. Stochastic Theory of Particle Trajectories through Alluvial Valley Floors. The Journal of Geology (Chicago), 111, 525–542. Malmon DV, Dunne T and Reneau SL 2002. Predicting the fate of sediment and pollutants in river floodplains. Environmental Science & Technology, 36(9), 2026-2032. McHugh M 2000. Extent, causes and rates of upland soil erosion in England and Wales. Soil Survey and Land Research Centre, Cranfield University. Moffat AJ 1988. Forestry and soil erosion in Britain – a review. Soil Use and Management 4(2), 41-44. Moulin C and Gailhard J 1996. SUBIEF V3P1 Users Manual, Report HE-43/95/074/B. Laboratoire National d’Hydraulique, Direction des Etudes et Reseacrches, Electricite de France, Paris. Neal C, House WA, Leeks GJL, Whitton BA, Williams RJ 2000. Conclusions from the special issue of Science of the Total Environment concerning ‘The water quality of UK rivers entering the North Sea’. The Science of the Total Environment 251/252, 557-573. ODPM (2005). Planning Policy Statement 1 – Delivering Sustainable Development. Office of the Deputy Prome Minister (now Department for Communities and Local Government), London. Old GH, Leeks GJL, Packman JC, Stokes N, Williams ND, Smith BPG, Hewitt EJ and Lewis S 2004. Dynamics of sediment-associated metals in a highly urbanised catchment: Bradford, West Yorkshire. Water and Environmental Management Journal 18(1), 11-16. Old GH, Smith BPG, Stegemann JA, Leeks GJL and Naden PS 2003a. Fine sediment dynamics in UK catchments. Proc Workshop on: Impact bioavailability and assessment of pollutants in sediments and dredged materials under extreme hydrological conditions. SedNet, Berlin, Germany. From: www.SedNet.org. Old GH, Leeks GJL, Packman JC, Smith BPG, Lewis S, Hewitt EJ, Holmes M and Young A 2003b. The impact of a convectional summer rainfall event on river flow and fine sediment transport in a highly urbanised catchment: Bradford, West Yorkshire. The Science of the Total Environment 314-316, 495-512. OST 2004. Foresight: Future Flooding. Scientific Summary: Volume I Future risks and their drivers. eds. Evans E, Ashley R, Hall J, Penning-Rowsell E, Saul A, Sayers P, Thorne C and Watkinson A. Office of Science and Technology, London. Owens PN, Walling DE, Carton J, Meharg AA, Wright J and Leeks GJL 2001. Downstream changes in the transport and storage of sediment-associated contaminants (P, Cr and PCBs) in agricultural and industrialised drainage basins. The Science of the Total Environment 266, 177-186. Rothwell JJ, Robinson SG, Evans MG, Yang J and Allott THE 2005. Heavy metal release by peat erosion in the Peak District, southern Pennines, UK. Hydrological Processes, 19, 2973-2989. Rowan JS and Franks SW 2002. Heavy metal mining and flood plain response in the upper Clyde basin, Scotland. In: The structure, function and management implications of fluvial sedimentary systems. International Association of Hydrological Systems (IAHS) Publ. no. 276, 143-150. Sear DA, Newson MD and Thorne CR 2003. Guidebook of Applied Fluvial Geomorphology, Defra/ Environment Agency Flood and Coastal Defence R&D Programme, R&D Technical Report FD1914, Defra, London. SedNet 2004. Contaminated sediments in European rivers basins. From: www.SedNet.org. Stewart MD, Bates PD, Price DA and Burt TP 1998. Modelling the spatial variability in floodplain soil contamination during flood events to improve chemical mass balance estimates. Hydrological Processes, 12, 1233-1255.

27 Thorne CR and Abt SR 1993. Velocity and scour prediction in river bends. Report submitted to United States Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Mississippi, Contract Report HL-93-1. UKCIP 2002. Climate change scenarios for the United Kingdom. The UKCIP02 Briefing Report. UK Climate Impacts Programme. Waller S, Bradbrook K, Huband M, Mitchell G and Gilkes P 2003. The extreme flood outline. In: Proc of the 38th Defra Flood and Coastal Management Conference, Keele University, 16-18 July 2003, 07.2.1-07.2.11. Walling DE., Owens PN, Carter J, Leeks GJL, Lewis S, Meharg AA and Wright J 2003. Storage of sediment-associated nutrients and contaminants in river channel and floodplain systems. Applied Geochemistry 18, 195-220. Walling DE, He Q and Blake WH 2000. River flood plains as phosphorus sinks. In: The role of erosion and sediment transport in nutrient and contaminant transfer. International Association of Hydrological Sciences (IAHS) Publ. no. 263, 211-218. Walling DE, Owens PN and Leeks GJL 1999. Rates of contemporary sedimentation and sediment storage on the floodplains of the main channel systems of the Yorkshire Ouse and River Tweed, UK. Hydrological Processes 13(7), 993-1009. Walling DE, Webb BW and Russell MA 1997. Sediment-associated nutrient transport in UK rivers. In: Freshwater contamination. Proc Rabat Symposium, International Association of Hydrological Sciences, IAHS Publ. No 243, 69-81. Walling DE, He Q and Nicholas AP 1996. Floodplains as suspended sediment sinks. Chap 12 in: Floodplain Processes, eds. MG Anderson, DE Walling and PD Bates). John Wiley & Sons Ltd, Chichester, West Sussex. 399-440. Walling DE, Bradley SB and Lambert CP 1986. Conveyance losses of suspended sediment within a flood plain system. In: Drainage Basin Sediment Delivery, ed. Hadley RF, IAHS Publication 159, p 119-131. Walling DE 1979. The hydrological impact of building activity – a study near Exeter. In: Man’s Impact on the Hydrological Cycle in the UK, ed. GE Hollis. Geo Books, Norwich, 135-151. Williams AT, Ronald RC, Dinah NC 2002. SAM Hydraulic Design Package for Channels. Engineer Research and Development Center. US Army Corps of Engineers. Withers PJA, Dils RM and Hodgkinson RA 1999. Transfer of phosphorus from small agricultural basins with variable soil types and land use. In: Impact of land use change on nutrient loads from diffuse sources. Ed. L Heathwaite. Proc Birmingham Symposium International Association of Hydrological Sciences, IAHS Publ. No 257, 41-50. Yeghiazarian LL, Kalita P, Kuhlenschmidt MS, McLaughlin SJ and Montemagno CD 2004. Field Calibration and Verification of a Pathogen Transport Model. Water Environment Research Foundation Report 00-WSM-3, IWA Publishing, London, ISBN: 1-84339-711-0. Young AR, Grew R and Holmes MGR 2003. LF2000: A national water resources assessment and decision support tool. Water Science and Technology, 48(10), 119-126. Younger PL 2002. Coalfield closure and the water environment in Europe. Transactions of the Institution of Mining and Metallurgy (Section A: Mining Technology) 111(3), 201-209. Younger PL 2000. Mine water pollution in the long-abandoned Cleveland ironstone field, north-east England. Proc of the 7th National Hydrology Symposium, British Hydrological Society, Newcastle. 2.51-2.60.

28 Annex 5.1 Locations of UK studies relating references considered in contaminant re- mobilisation review

Year Author(s) River/site Contaminants

Phosphorus, dissolved 2001 Bowes and House Swale silicon 1990 Bradley & Cox Derwent (Derbyshire) Heavy metals

1997 Brewer and Taylor Severn Heavy metals

1996 Burt and Haycock UK, Europe review Nitrate, Phosphorus

1998 Carter River Swale Heavy metals Carter, Owens, Walling, & Sediment associated 2003 River Calder and Aire Leeks contaminants Carton, Walling, Owens and 2000 River Aire Chromium Leeks Sediment associated 1997 Coulthard, Kirkby & Macklin Cam Gill Beck contaminants 2000 CSL Dee, Trent, Doe/Lea/Rother PCDD/Fs, PCBs

2005 DEFRA Review of water erosion in UK N/A Dennis, Macklin, Coulthard & 2003 River Swale Heavy metals Brewer Department of the Review of Erosion, Deposition, and 1995 N/A Environment Flooding in UK Discussion of soils at risk from erosion 1990 Evans N/a in England & Wales 1997 Hudson-Edwards Tees Heavy metals

2003 Hudson-Edwards General review Heavy metals Hudson-Edwards, Macklin & 1997 Pennines Heavy metals Dawson Hudson-Edwards, Macklin & 1999 Ouse Heavy metals Taylor Hudson-Edwards, Macklin, 1996 Tyne Heavy metals Curtis, & Vaughan 1992 Leeks Wales Sediment Derwent (Yorkshire), Rheidol, Nent, 1987 Lewin & Macklin Rea Brook, E & W Allen, Severn, Heavy metals Twymyn, Yeo, Axe 1992 Mackin UK wide Heavy metals

1996 Macklin Tyne, Nent, Derwent (Derbyshire) Heavy metals Marks, Soloman, Johnson, 1997 Watson, Royle, Richardson & Areas in UK at risk from soil erosion Sediment Goodlass 2000 McHugh Upland England and Wales Sediment Sediment 1988 Moffat Forestry and soil erosion in UK

General review of soil erosion in 1985 Morgan Sediment England Wales

29 Year Author(s) River/site Contaminants

Neal, House, Leeks, Whitton, Sediment associated 2000 UK rivers entering the North Sea & Williams contaminants Old, Leeks, Packman, Smith, Sediment associated 2003 Lewis, Hewitt, Holmes, & Bradford Beck contaminants Young Owens, Walling, Carton, 2001 River Aire and River Calder Heavy metals Meharg, Wright, Leeks Reynolds, Emmett, Thompson, Loveland, Jarvis, Sediment associated 2002 Welsh rivers Haygarth, Thomas, Owen, contaminants Roberts, Marsden 2005 Rothwell, Allott & Evans Upper North Grain (Derbyshire) Heavy metals Rothwell, Robinson, Evans, 2005 Upper North Grain (Derbyshire) Heavy metals Yang, Allott 2002 Rowan & Franks Upper Clyde Basin Heavy metals

1996 Skinner & Chambers General review of water soil erosion Sediment Sediment associated 1998 Stewart, Bates, Price & Burt General Review contaminants Surridge, Heathwaite, & 2005 Strumpshaw Fen (Norfolk) Phosphorus Baird 2002 Walling & Owens River Swale Heavy metals Sediment associated 2001 Walling, Russell & Webb Severn, Avon, Exe contaminants 2000 Walling, He & Blake UK Phosphorus Sediment associated 2000 Walling & Woodward Avon, Stour, Exe, Culm, Avon contaminants 2000 Walling, Blake River Stour Phosphorus

1997 Walling, Webb & Russell UK Nitrate, phosphorus

1996 Walling, He & Nicholas Culm, Stour (Dorset) Sediment Sediment associated 1999 Welsh Environment Agency General Review contaminants 2000 Younger Cleveland Acidity, metals Younger, Amezaga, Nuttall & Durham, Northumberland, Yorkshire, 2001 Acidity, metals Doyle Nottinghamshire coalfields

30 Short bibliography on sediment transport measurement and modelling

Blake WH, Walling DE and He Q 1999. Fallout beryllium-7 as a tracer in soil erosion investigations. Applied Radiation and Isotopes 51, 599-605. Carton J, Walling DE, Owens PN and Leeks GJL. 2000. Spatial and temporal variability of the chromium content of suspended and flood-plain sediment in the River Aire, Yorkshire, UK. In: The Role of Erosion and Sediment Transport in Nutrient and Contaminant Transfer. IAHS Publication No. 263, 219-226. Chapman AS, Foster IDL, Lees JA, Hodgkinson RJ & Jackson RH 2003. Phosphorus transfer from field to river via land drains in England and Wales. A risk assessment using field and national data bases. Soil Use and Management. 19, 347-355. Foster IDL, Mighall TM, Wotton C, Owens PN and Walling DE 2000. Evidence for medieval soil erosion in the South Hams region of Devon, UK. The Holocene 10, 261-271. Foster IDL, Lees JA, Jones AR, Chapman AS, Turner SE & Hodgkinson RA 2002. The possible role of agricultural land drains in sediment delivery to a small reservoir, Worcestershire UK; a multiparameter tracing study. In: The Structure, Function and Management Implications of Fluvial Sedimentary Systems, eds. Dyer FJ, Thoms MC & Olley JM. IAHS Symposium, Alice Springs, Sept. 2002. IAHS Publication, 276, 433-442. Foster IDL, Chapman AS, Hodgkinson RM, Jones AR, Lees JA, Turner SE & Scott M 2003. Changing suspended sediment and particulate loads and pathways in underdrained lowland agricultural catchments, Herefordshire and Worcestershire, UK. Hydrobiologia, 494, 119-126. Foster IDL & Lees JA (in press). Evidence for past erosional events from lake sediments. In: Climate Change and Soil Erosion, eds. Boardman J and Favis-Mortlock D. Imperial College Press, London. Golosov V, Walling DE and Panin A 2000. Post-fallout redistribution of Chernobyl-derived caesium-137 in small catchments within the Lokna River basin, Russia. In: The Role of Erosion and Sediment Transport in Nutrient and Contaminant Transfer. IAHS Publication No.263, 49-57. Golosov VN, Walling DE, Kvasnikova EV, Stukin ED, Nikolaev AN and Panin AV 2000. Application of a field-portable scintillation detector for studying the distribution of 137Cs inventories in a small basin in central Russia. Journal of Environmental Radioactivity 48, 79-94. Gruszowski KE, Foster IDL, Lees JA & Charlesworth SM 2003. Sediment sources and transport pathways in a rural catchment, Herefordshire, UK. Hydrological Processes, 17 (13) 2665-2681. He Q, and Walling DE 2000. Calibration of a field-portable gamma detector to obtain in situ measurements of the 137Cs inventories of cultivated soils and floodplain sediments. Applied Radiation and Isotopes 52, 865-872. Jenns N, Heppel K, Burt TP, Wwalden J & Foster IDL 2002. Investigating contemporary and historical sediment inputs to Slapton Higher Ley: an analysis of the robustness of source ascription methods when applied to lake sediment data. Hydrological Processes. 16 (17), 3467-3486. Okunishi K, Walling DE and Saito T 2000. A hydrological model of the mobilisation of fine suspended sediment from slopes. Transactions Japanese Geomorphological Union 21-3, 243-60. Owens PN, Walling, DE and Leeks GJL 1999. Use of floodplain sediment cores to investigate recent historical changes in overbank sedimentation rates and sediment sources in the catchment of the River Ouse, Yorkshire, UK. Catena 36, 21-47. Owens PN, Walling DE and Leeks GJL 1999. Deposition and storage of fine-grained sediment within the main channel system of the River Tweed, Scotland. Earth Surface Processes and Landforms 24, 1061-1076. Phillips J M and Walling DE 1999. The particle size characteristics of fine-grained channel deposits in the River Exe Basin, Devon, UK. (1999) Hydrological Processes 13, 1-19. Phillips JM, Webb BW, Walling DE and Leeks GJL 1999. Estimating the suspended sediment loads of rivers in the Lois study area using infrequent samples. Hydrological Processes 13, 1035-1050. Quine TA, Walling DE, Chakela QK, Mandaringana OT and Zhang X 1999. Rates and patterns of tillage and water erosion on terraces and contour strips: evidence from caesium-137. Catena 36, 115-142.

31 Russell MA, Walling DE and Hodgkinson RA 2000. Appraisal of a simple sampling device for collecting time-integrated fluvial suspended sediment samples. In: The Role of Erosion and Sediment Transport in Nutrient and Contaminant Transfer. IAHS Publication No.263, 119-127. Siggers GB, Bates PD, Anderson MG, Walling DE and He Q 1999. A preliminary investigation of the integration of modelled floodplain hydraulics with estimates of overbank floodplain sedimentation derived from Pb-210 and Cs-137 measurements. Earth Surface Processes and Landforms 24, 211-231. Walling DE, Golosov VN, Kvasnikova EV and Vandecasteele C 2000. Radioecological aspects of soil pollution in small catchments. Eurasian Soil Science 33, 776-784. Walling DE, Owens PN, Waterfall BD, Leeks GJL and Wass PD 2000. Particle size characteristics of fluvial suspended sediment in the Humber and Tweed catchments, UK. The Science of the Total Environment 251/252, 205-222. Walling DE, He Q and Blake WH 2000. River floodplains as phosphorus sinks. In: The Role of Erosion and Sediment Transport in Nutrient and Contaminant Transfer. IAHS Publication No. 263, 211-218. Walling DE, Owens PN and Leeks GJL 1999. Fingerprinting suspended sediment sources in the catchment of the River Ouse, Yorkshire, UK. Hydrological Processes 13, 955-975. Walling DE, Owens PN and Leeks GJL 1999. Rates of contemporary overbank sedimentation and sediment storage on the floodplains of the main channel systems of the Yorkshire Ouse and the River Tweed, UK. Hydrological Processes 13, 993-1009. Walling DE, Owens PN, Foster IDL, & Lees JA 2003. Changes in the sediment dynamics of the Ouse and Tweed basins in the UK, over the last 100-150 years. Hydrological Processes.17, 3245-3269. Walling DE 1999. Using fallout radionuclides in investigations of contemporary overbank sedimentation on the floodplains of British rivers. In: Floodplains: Interdisciplinary Approaches, eds. SB Marriott and J Alexander. Geological Society, London, Special Publication no. 163. 41-59. Walling DE and Amos CM 1999. Source, storage and mobilisation of fine sediment in a chalk stream system. Hydrological Processes 13, 323-40. Walling DE and Fang D 1999. Longer-term variability of sediment transport to the oceans. In Hydrological and Geochemical Processes in Large Scale River Basins. Proceedings of the Manaus Symposium, November, 1999 (CD). Brasilia, HIBAM. Walling DE and He Q 1999. Changing rates of overbank sedimentation on the floodplains of British rivers over the past 100 years. In: Fluvial Processes and Environmental Change, eds. AG Brown and TA Quine. Wiley, Chichester. 207-222. Walling DE and He Q 1999. Improved models for estimating soil erosion rates from 137Cs measurements. Journal of Environmental Quality 28, 611-622. Walling DE and He Q 1999. Using fallout lead-210 measurements to estimate soil erosion on cultivated land, Soil Science Society of America Journal 63, 1404-1412. Wallling DE, He Q and Blake W 2000. Use of 7Be and 137Cs measurements to document short- and medium-term rates of water-induced soil erosion on agricultural land. Water Resources Research 35 (12), 3865-3874. Webb BW, Phillips JM and Walling DE 2000. A new approach to deriving ‘best-estimate’ chemical fluxes from rivers draining the LOIS study area. The Science of the Total Environment 251/252, 45-54. Zavoianu I, Walling DE and Serban P (eds.) 1999. Vegetation, Land Use and Erosion Processes, Institute of Geography, Romanian Academy, Bucharest. Zhang X, Walling DE and He Q 1999. Simplified mass balance models for assessing soil erosion rates on cultivated land using caesium-137 measurements. Hydrological Sciences Journal 44, 33-45

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