RBF_Cover (for eps) - National Water Research Institute

194
During the field investigation program, hydrogeological, hydraulic, hydrological, chemical,
physico-chemical, biological, and microbiological data were collected and evaluated. The results of
the water-quality studies revealed characteristic quality changes in the filter media: the chemical
oxygen demand concentration gradually decreased from the river towards abstraction sites,
indicating the decomposition of organic pollutants. Hydro-biological studies indicated that the
Danube River had considerable fecal coliform pollution, but that bacteria removal during RBF was
very efficient. Coliphages used as indicators of virus pollution were also measured. The results of
these tests verified that coliphages were not able to pass through filter media.
The retardation of specific organic micropollutants is very different depending on their biodegradability,
sorption affinity, etc. The mobile, persistent organic micropollutants (e.g., atrazine,
simazine, trichloroethene, and benzene) have low removal efficiency (less than 30 percent); on
the other hand, some pesticides (e g., terbutrin, 2,4-D, and carbaryl) and petroleum hydrocarbons
are removed substantially. The retardation or remobilization of some heavy metals is influenced
by redox conditions.
Adsorption experiments in the laboratory provided information on the partition coefficients of
various micropollutants between spiked Danube water and the material of filtration medium taken
from RBF sites (Laszlo and Literathy, 2002). Two riverbed materials were used: one represented the
sandy alluvium having low organic material; the other was taken from a sedimentation zone of the
riverbed where river water infiltrates through sediment having high organic material content and fine
particle size. The partition coefficient varied widely for the different organic micropollutants and
heavy metals, depending mainly on the character of the pollutants. The type of bed material was also
important: the partition coefficients were higher in silty riverbed material than in the sandy matrix
for all pollutants.
Numerical transport modeling — based on measured hydraulic, hydrogeological, and physicochemical
parameters — concluded that relatively short-duration accidental pollution has limited
risk for RBF schemes along the Danube River due to a wide range of travel times from the river to
abstraction wells. Higher risk would be long-lasting, continuous pollution of the Danube.
Another concern is the impacts of river training and gravel dredging on the quality of riverbankfiltered
water (Laszlo et al., 1990). Training structures (groynes, parallel dykes) and dredging
operations affect the hydraulic conditions in the river that are conducive to silting in areas with
reduced flow velocities. Adverse hydrochemical changes occur in the silted filter layer, especially
the dissolution of iron and manganese, and higher concentrations of ammonium are observable.
Bed degradation owing to dredging causes changes in the inflow ratio to the abstraction wells,
increasing the proportion of polluted groundwater from the background areas in the wells.
Conclusions
RBF has been an effective method for drinking-water supply in Hungary for a long time.
The sustainability of RBF abstraction schemes depends on several factors, including the quality of
river water, characteristics of the filtration media, retention time within the aquifer, and quality
of the groundwater adjacent to the extraction site.