Page 216 - Hydrogeology Principles and Practice

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Groundwater quality and contaminant hydrogeology 199

it gives the impression of an allowed level of pollu- has also been a long debate as to the relative health
tion. An alternative approach is to state that ground- benefits of drinking hard water. In fact, no health-
water should not be polluted at all. In the EU, this based guideline value is proposed for hardness since
precautionary approach to protecting the chemical it is considered that the available data on the inverse
status of groundwater is adopted and comprises a relationship between the hardness of drinking water
prohibition on direct discharges to groundwater and, and cardiovascular disease (CVD) are inadequate to
to cover indirect discharges, a requirement to mon- permit the conclusion that the association is causal
itor groundwater bodies in order to detect changes (World Health Organization 2002). However, a con-
−1
in chemical composition and reverse any upward centration of 500 mg L is at the upper limit of aes-
trend in pollution. Under the Directive on the Protec- thetic acceptability. Further discussion follows in the
tion of Groundwater Against Pollution Caused by next section and Box 6.1.
Certain Dangerous Substances (80/68/EEC; Council
of European Communities 1980), the most toxic sub-
stances are listed under Lists I and II (Appendix 9). 6.2.1 Water hardness
List I substances, including organophosphorus com-
pounds, mercury and cadmium, should be prevented Water hardness is the traditional measure of the
from being discharged into groundwater. List II sub- capacity of water to react with soap and describes
stances, including metals, fluoride and nitrate, should the ability of water to bind soap to form lather, a
have discharges of these substances into groundwater chemical reaction detrimental to the washing process.
minimized. Only a few specific directives have been Hardness has little significance in terms of hydro-
established at European level for particular issues, in- chemical studies, but it is an important parameter
cluding the Directive on Diffuse Pollution by Nitrates for water users. Today, the technical significance of
(91/676/EEC; Council of European Communities water hardness is more concerned with the corrosive
1991). Taken together, it is envisaged that the above effects on water pipes that carry soft water.
measures, which are subsumed under the EU Water Despite the wide usage of the term, the property of
Framework Directive (see Section 1.8), should pre- hardness is difficult to define exactly. Water hardness
vent and control groundwater pollution and achieve is not caused by a single substance but by a variety of
good groundwater chemical status for the future. dissolved polyvalent metallic ions, predominantly Ca
Further to water quality objectives, water quality and Mg, although other ions, for example Al, Ba,
standards present the detailed rules that govern how Fe, Mn, Sr and Zn, also contribute. The source of the
the objectives should be met. To be workable, the metallic ions are typically sedimentary rocks, the most
standards must be relatively simple so that routine common being limestone (CaCO ) and dolomite
3
monitoring can detect water quality failures. A stand- (CaMg(CO ) ). In igneous rock, magnesium is typic-
3 2
ard may allow some variability, or derogation, in ally a constituent of the dark-coloured ferromagne-
terms of a given concentration being met for a certain sian minerals, including olivine, pyroxenes, amphiboles
percentage of samples but with no single sample and dark-coloured micas, and slow weathering of
allowed to exceed a maximum allowable concentra- these silicate minerals produces water hardness.
tion. A further consideration is where, in the cycle of Hardness is normally expressed as the total con-
2+ 2+
water abstraction, treatment and supply, to apply the centration of Ca and Mg ions in water in units of
−1
standard. Standards applied at the tap control the mg L as equivalent CaCO . For this purpose, hard-
3
standard of water used for human consumption and ness can be determined by substituting the concen-
2+ 2+ −1
are termed drinking water quality standards. A number tration of Ca and Mg , expressed in mg L , in the
of large organizations, including the EU, United States following equation:
EPA and WHO, have published drinking water quality
2+ 2+
standards and these are summarized in Appendix 9. Total hardness = 2.5(Ca ) + 4.1(Mg ) eq. 6.1
The hardness of groundwater can become a water
quality issue especially where it affects industrial Each concentration is multiplied by the ratio of the
and domestic uses where the water is heated. There formula weight of CaCO to the atomic weight of the
3