Groundwater is always on the move and interacts with soil, surface waters and sediments
Basically, soil, groundwater, surface water and sediments cannot be separated and should be dealt with as parts of one system. In risk based management the interaction pathways (determining the emission pathways between contaminant sources and water systems to be protected) need to be taken into account. Two major pathways can be considered:
- Subsurface interactions (Soil - groundwater – surface water interactions):
This concerns groundwater quality/quantity and the functioning of the soil as filter/buffer/transform/storage medium. In megasite management, the understanding and the use of soil and acquifers and their natural filter and clean-up processes is very important. Technologies as enhanced and natural attenuation and heavy metal immobilisation are based on these processes. These technologies are crucial parts of management scenarios for practical any megasite. In the risk based approach, groundwater systems can be sources, pathways or receptors. Groundwater pressure areas (receptors) need to be dealt with in relation to the Groundwater Daughter Directive. - Surface interactions (Soil/floodplains – sediments – surface water interactions):
Surface water system affect soil (for instance river systems affect floodplain soils through inundation) and vice versa. Suspended solids are the main quality determining medium for surface water, sediments, and floodplain soils. Particulate matter transport from land towards surface waters by precipitation, surface run-off or wind erosion, and deposition of vaporised contaminants into water systems can affect surface water and sediment quality. Sediments and flood-plain soils mirror long term trends in lake and river basin water quality. Land-use, large scale historical contamination (i.e. megasites), diffuse pollution, surface run-off and wind erosion, are key factors in surface water quality at the scale of catchments as well as river basins. These processes, and possible intervention measures are taken into account in the risk based management approach for megasitesas developed in the IMS. The Water Framework Directive requires the set up of River Basin Management plans to tackle these types of problems. The welcome IMS helps to establish megasite management plans as part of River Basin Management Plans.
Both subsurface and surface interactions are taken into account in the risk based approach developed in the IMS. This starts with the conceptual model building and is subsequently further developed in the risk assessment and building/evaluation of scenarios. In the IMS approach, most attention is given to subsurface interactions and emission pathways through groundwater water systems. Therefore some additional background information is given on this subject below.
Basic description of a groundwater system
Groundwater systems are characterized by the flow of the water. The flow of the water can be characterized by its physical dimensions like position of infiltration, length of the pathway, groundwater velocity, position of seepage, etc. These dimensions are dependent on the soil characteristic (e.g. porosity). Traditionally geohydrologists have focussed on groundwater quantity, because groundwater basically was free of pathogens and contaminants, due to the filtering capacity of the soil. When groundwater quality became an issue, knowledge about the fate and transport of micro-contaminants was added to the quantitative knowledge of the water system.
Classification of groundwater systems
Though the basic model is very simple, natural diversity of soils make the systems very complex. With a closer look one could discern several ‘systems’ on different scales, overlapping, interdependent and changing in time and space. This complexity makes it impossible to understand most groundwater systems, without quantitative computer models. The models are validated by geohydrological data, like water pressures, flows, etc.
Because there is a lot of experience in modelling groundwater systems, it is possible to recognize patterns in the groundwater systems and to classify them. Groundwater systems could be classified based on:
- Geological scale (local systems, subregional systems, regional systems)
- Human interference (natural systems, manmade systems)
- Dominant surface water (Rhine system, Ruhr-system, etc.)
- Functionality (drinking water area, salty systems, contaminated systems)
- Territorial dimensions (city systems, nature reserve systems, industrial area systems)
Groundwater systems in the IMS
Knowledge of the groundwater system of a megasite, and its interaction with surrounding groundwater and surface water systems, is the core of the IMS. Due to the characteristics of a megasite (many sources of contamination, polluted groundwater, effects over a long time-scale and over a large geographical scale) it is not possible to assess risks and to define remedial actions without detailed knowledge of the groundwater system.
Knowledge of the groundwater system is built up in tiers:
- The first tier is to collect perceptual knowledge of the surface aspects of the system (geography, soil type, surface water, groundwater extractions, etc) and to get a rough idea of the system. This kind of knowledge is needed to feed the story that starts the IMS.
- The second tier is to collect existing data and use it to make a conceptual model of the groundwater system and its functioning.
- The third step is to go into detail, collect specific additional data and make a computer model of the system and its behaviour over time.
- The fourth step is to translate the specialist model information into management information that could be used in the IMS.
