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

WATER QUALITY INTERACTIONS
in the upstream flux. Magnesium and potassium (Figure 7.11c) display a similar increasing
trend across the aquifer.
Good evidence of the imported origin of the surface water is seen by the contrast with the
groundwater for the Na, Ca, Cl and SO4 data (Figure 7.12a and b). A relationship is apparent
in all the types of urban groundwater sampled between the concentrations of sodium and
calcium in the approximate milli-equivalent ratio 1:4 (Figure 7.12a). This may be due to the
fact that contamination by both NaCl and CaSO4 often occur together at a site and therefore
higher levels of contamination have a proportionate increase in both Na and Ca
concentrations. The ratio of chloride to sulphate shows a similar relationship (Figure 7.12b)
but with a higher degree of scatter between the points. The closer grouping of data points for
the calcium/sodium relationship may indicate that additional controls such as ion-exchange
are taking place. The Na:Ca ratios show a fairly distinct divide between samples of
groundwater and surface water with little overlap, implying a limited extent to the
groundwater/surface water mixing zone. A greater overlap is observed between groundwater
and surface water concentrations for chloride and sulphate. This may be coincidental, or it
may imply that surface water influent to the groundwater system has undergone a decrease of
the Na/Ca ratio via ionic exchange, with the relatively conservative Cl/SO4 ratio remaining
unchanged. However, the large heterogeneity in the system, and in the sample types and
depths, introduces a high degree of uncertainty and makes it difficult to draw any valid
conclusions on this.
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