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

WATER QUALITY INTERACTIONS
within 500 m of the river during the current survey did not show significantly higher
concentrations (41, 46, 104 mgl -1 ) than the other 11 wells sampled across the aquifer but the
data set is too limited to draw any conclusions from this.
The longitudinal profile (Figure 7.6c) shows background levels of chloride increasing from 20
to 80 mgl -1 across the aquifer perhaps associated with changing land use from parkland to
industrial (Figure 4.2). A high concentration plume of fairly limited lateral extent was
detected in the riverbank (1,568 mgl -1 ) and riverbed piezometers (1,873 mgl -1 ) of one profile
but was not detected in profiles 250m upstream or downstream. Surface water sampling
across the plume at 100 m intervals detected a rise in surface water chloride concentrations
from 160 to 169 mgl -1 in 100 m on 11/5/01, and from 205 to 207 mgl -1 on 28/2/01. However,
these changes do lie within the range of analytical error and the surface water concentration
profile is difficult to interpret because of variations over time in the chemical flux coming
downstream. Repeat surface water sampling on 26/7/00 at profile 8 showed a rise in chloride
concentrations from 150 to 200 mgl -1 in 6 hours. The variation in STW discharges also
explains the sharp change in the chloride concentrations (223 to 153 mgl -1 ) observed in
surface water sampling between the 19 th and 20 th of July 2000. The effect of dilution from the
waters of the River Rea, which does not contain any STW discharges, is visible at the
downstream end of the aquifer on the 19/7/00. Consecutive samples were taken within 15
minutes on either side of the confluence and a fall in chloride concentration from 183 to 161
mgl -1 was observed.
The high chloride concentrations in the river (median 162 mgl -1 ) compared with those
generally found in the groundwater (median 59 mgl -1 ) provide a useful tracer to establish the
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