Attenuation of groundwater pollution by bank filtration
Introduction
This Special Issue has its origins in a two-day workshop on the Attenuation of Groundwater Pollution by Bank Filtration held at the Dresden University of Technology, Germany, in June 2000 (website www.bankfiltration.org). The workshop was funded by the European Science Foundation under its Groundwater Pollution (GPoll) Programme and attracted 23 participants from 12 countries within Europe and the United States. The workshop was organised around five sessions (determination of infiltration zones and velocities, the river bed as an attenuation barrier for groundwater pollution, fate of synthetic organic compounds during bank filtration, elimination of microbial pathogens during bank filtration, and bank filtration as an effective pre-treatment for water production) and representative papers under these session themes are presented in this Special Issue.
Additional relevant information is currently available from the extended abstracts in the proceedings of the International Riverbank Filtration Conference held in November 1999 in Louisville, Kentucky, United States and from a NATO Advanced Research Workshop on Riverbank Filtration on Understanding Contaminant Biogeochemistry and Pathogen Removal held in September 2001 in Tihany, Hungary. Proceedings from an International Riverbank Filtration Conference held in November 2000 in Düsseldorf, Germany (Jülich and Schubert, 2000) include full papers and give an overview of research in the field of bank filtration in Germany and The Netherlands. These recent workshops and conferences on bank filtration underline the importance of bank filtration as a valuable water resource. Future publications include the full text of papers from the NATO Workshop (Ray, 2002) and a forthcoming compendium of papers on bank filtration (Ray and Linsky, 2002).
Complementary special issues that consider hydrological, ecological and modelling aspects of the inverse situation of the role of groundwater discharge in maintaining river flows are presented by Sear and Armitage, 1999, Hiscock et al., 2002, Younger, 1995.
In many countries of the world, alluvial aquifers hydraulically connected to a water course are preferred sites for drinking water production given the relative ease of shallow groundwater exploitation, the generally high production capacity and the proximity to demand areas (Doussan et al., 1997). Although proximity to a river can ensure significantly higher recharge and pumping rates, water quality problems may be encountered during exploitation of river bank well-fields (Bertin and Bourg, 1994). Even with these problems, groundwater derived from infiltrating river water provides 50% of potable supplies in the Slovak Republic, 45% in Hungary, 16% in Germany and 5% in The Netherlands. In Germany, the City of Berlin depends 75% on bank filtration while Düsseldorf, situated on the Rhine, has been using river bank filtration since 1870, with bank filtration the most important source for public water supply in this densely populated and industrialised region (Schubert, 2002).
In the United States, the water supply industry has adopted the broadly defined regulatory concept of ‘groundwater under the direct influence’ (GWUDI) of surface water (variably defined and implemented in response to local conditions by each State, Tribe or other regulatory agent). Groundwater sources in this category are considered at risk of being contaminated with surface water-borne pathogens (specifically disinfection-resistant pathogenic protozoa such as Cryptosporidium). Under the draft Proposed Long Term 2 Enhanced Surface Water Treatment Rule, the US Environmental Protection Agency (EPA) is considering awarding additional Cryptosporidium treatment credit for systems using bank filtration that meets certain design criteria (which could include defined separation distances between river and well) (US Environmental Protection Agency, 2001). Future, increased exploitation of water from alluvial aquifers along river banks is expected in the US given the rise in demand for drinking water, the ease of abstraction and the positive effects of bank filtration on the quality of the infiltrating surface water (Solley et al., 1998).
Section snippets
Hydraulic controls on bank filtration
River bank filtration can occur under natural conditions or be induced by lowering the groundwater table below the surface water level by abstraction from adjacent boreholes. Fig. 1 shows the typical flow conditions associated with different types of bank filtration schemes. For the quantitative and qualitative management of bank filtration systems, the catchment zones, infiltration zones, mixing proportions in the pumped raw water, flow paths and flow velocities of the bank filtrate need to be
Attenuation of contaminants by bank filtration
Fig. 2 shows schematically the attenuation processes that are known from various bank filtration sites. Compared with surface water abstraction, bank filtration with its effective natural attenuation processes has the following advantages: elimination of suspended solids, particles, biodegradable compounds, bacteria, viruses and parasites; part elimination of adsorbable compounds; and the equilibration of temperature changes and concentrations of dissolved constituents in the bank filtrate.
Modelling of bank filtration processes
The mathematical description of contaminant behaviour during bank filtration is often simulated using mass transport modelling based on groundwater flow models. An example of the type of insight that can be gained is presented by Kim and Corapcioglu (2002) who developed a kinetic model to simulate contaminant transport during bank filtration in the presence of dissolved organic matter and bacteria. The model equations are solved numerically with a fully implicit finite difference method and the
Conclusions and further research directions
Over a century of experience exists in the operation and maintenance of bank filtration schemes in Europe but current understanding of bank filtration schemes is primarily based on empirical knowledge. There are no common guidelines on conditions necessary for the optimisation and protection of bank filtration schemes. Although knowledge exists for site-specific problems of water supply related to different hydrogeological and hydrochemical settings, there is a lack of comparative data
Acknowledgements
The authors are grateful to the European Science Foundation for financial assistance under its GPoll Programme in supporting the international workshop held at the Dresden University of Technology (TUD) in June 2000. Dagmar Schoenheinz and staff at the Institute of Water Chemistry at TUD are thanked for their expert organisation of the workshop sessions. We are greatly indebted to the many referees who provided helpful comments on earlier drafts of the papers contained in this Special Issue and
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