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

WATER QUALITY INTERACTIONS
depth of surface water penetration into the riverbed. Data from the multilevel riverbed
piezometers generally show an increase in chloride concentrations over the upper 20cm to 60
cm of the riverbed which is interpreted as an upward flow of groundwater mixing with surface
water (Figure 7.8c). The degree of mixing was calculated from the ratio of the concentrations
of chloride in the surface water and the groundwater at depth. Profile 2 has >95% surface
water at 35 cm depth while in some other profiles no mixing with surface water is apparent at
depths of just 20 cm . Temporal variation in the extent of the mixing zone is also apparent,
with a decrease from 40 cm (24/8/00) to 20 cm (8/8/01) in the western multilevel piezometer
at Profile 8. The extent of the hyporheic zone is dependent on local groundwater heads and
discharge rates and river stage, gradient and bedforms. The deepest penetration of surface
water occurs at Profiles 2 and 7 which are narrow, engineered sections of channel, with river
depths >70cm and low predicted groundwater discharge based on Darcy flow calculations.
Conversely, Profiles 5 and 8 have wide channels (~12m) with DWF river depths of ~30 cm
and high predicted groundwater discharge. Profiles 2 and 8 are within the river meander
modelled in Section 7.4.2, (Figure 7.1), with modelled discharge rates of 0.2 and 3.7 m 3 d -1 per
metre length of channel respectively. A proportion of the total volume of surface water
flowing along the study reach will pass through the hyporheic zone at some point. This
provides an important environment in which microbially mediated reactions, sorption and
precipitation may take place that could considerably modify the surface water quality. The
residence time of the surface water within the hyporheic zone is unknown but in Profile 2 it is
long enough for an anoxic environment to develop in which nitrate from both groundwater
and surface water sources is being reduced.
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