JBA Consulting http://www.jbaconsulting.co.uk 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] 1 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