The impact of urban groundwater upon surface water - eTheses ...

GROUNDWATER FLUX
attenuation may be particularly important for the chlorinated solvents and heavy metals which
display concentrations in the discharging groundwater which are lower than other data such as
abstraction well sampling and estimates of recharge loading suggest. Further investigation is
required to determine if the most significant natural attenuation occurs within the source zone,
the aquifer or the riverbed.
Agreement between the different methods of mass flux estimation varied between
determinands but values were generally within one order of magnitude of each other. For
example, annual mass flux calculated from baseflow analyses and riverbed piezometer data
for Ca, Cl, TCE and Zn were 1,300,000, 1,270,000, 95 and 719 kgy -1 respectively. This
compares with 1,450,000 (Ca), 1,170,000 (Cl) and 183 (TCE) kgy -1 calculated from the
combined surface water sampling and discharge measurements. The mass flux of Zn
estimated from the regular Environment Agency surface water sampling ranged between 897
and 1,762 kgy -1 . The calculations are based on limited data sets and further work is required
to assess the requirements for sample density. Some of the contaminants (e.g. F, TCA) may
receive a significant contribution to the total mass flux from a single plume while other
contamination (e.g. NO3) is more diffuse. Very detailed investigations are required for
individual plumes to obtain an accurate estimate of mass flux. However, based on limited
calculations, none of the plumes by itself will raise surface water concentrations above the
EQS limits and the principal impact of the plume will be on the ecology within the specific
discharge zone where locally high concentrations may exist prior to dilution by surface water.
All the methods of groundwater flux estimation should be used in conjunction, as none in
isolation is sufficient to assess the impact of the groundwater on the surface water quality in
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