Page 74 - Soil and water contamination, 2nd edition

Environmental compartments 61

deep confined aquifers. Another is for artificially recharging groundwater near drinking water

pumping stations, either to counteract the decline of ecosystems or crop yields following the
fall in the water table and reduction of upward seepage , or to increase the aquifer s water
’
yield. Deep well injection for the storage of liquid wastes is usually done into deep aquifers
(up to several hundreds of metres deep) that are confined vertically by impermeable layers
and contain water that is saline or otherwise unpotable. However, leakage or dispersal of
these contaminants to neighbouring aquifers may result in unintentional contamination of
other groundwater bodies. The target contaminant groups for deep well injection are volatile
and semi-volatile organic compounds (VOCs and SVOCs ), oil, explosives, and pesticides . In
Russia and the USA, liquid radioactive waste s have also been disposed of in this manner. For
artificial groundwater recharge near groundwater pumping stations, surface water is usually
employed. The surface water is generally not pre-treated before injection and therefore
any pollutants it contains may contaminate the aquifer. Furthermore, the aquifer may
become contaminated due to chemical reactions between the recharge water and the native
groundwater and/or the aquifer material.

3.3.3 Physico-chemical conditions in groundwater
Daily and seasonal variations in temperature at the soil surface are dampened in the soil
profile . As a consequence, the groundwater temperatures at a given location 10 m below the
water table are relatively stable. Groundwater temperature varies with latitude, being warmer
near the equator and colder nearer the poles. In general, the average annual temperature of
groundwater is about two degrees (°C) higher than the mean annual air temperature and
increases by 1 to 5 °C (average about 2.5 °C) per 100 m depth. Larger increases may occur
near local volcanic or geothermal activity.
As noted above, oxygen diffuses much more slowly in the saturated zone than in the
unsaturated zone. As a consequence, the oxygen becomes depleted if it is consumed by
the decomposition of organic matter faster than it is replenished by diffusion. Although the
organic matter content in the saturated zone is usually much less than in soil (exceptions
are peats and mucks, more than 80 percent of which consist of organic matter), all oxygen
will be consumed after some time and so the redox potential decreases. Because groundwater
flow s, this process is reflected in a spatial separation of oxic (aerobic ) groundwater and anoxic
(anaerobic ) groundwater. Therefore, oxic groundwater is usually found in the upper layers
in infiltration areas, where the aquifer consists of coarse materials poor in organic matter.
Nevertheless, reducing conditions tend to prevail in groundwater. As we saw in Section 2.10,
the change in redox conditions in groundwater has an important effect on the speciation and
solubility of substances, particularly of heavy metals .
The process of weathering and dissolution of minerals, which occurs in the unsaturated
zone and is accelerated by the presence of acids in the soil solution, continues in the saturated
zone. As a consequence, the concentration of dissolved substances in groundwater increases
with time and thus in distance from the point of infiltration , until chemical equilibrium is
reached. Some minerals, such as carbonates (e.g. calcite ; CaCO ) and evaporites (e.g. halite;
3
NaCl) dissolve rapidly, significantly changing the mineral composition in the unsaturated
zone. The weathering of other minerals, such as silicates (e.g. feldspars and clay minerals ),
which proceeds much more slowly, usually has only a minor effect on water composition.
Thus, the amount of total dissolved substances in groundwater depends on rock type.
Chapter 5 will further discuss the effect of rock type on water composition.
Furthermore, the sediment composition has a great influence on the retention of
contaminants. Both fine-grained clay minerals and organic matter are able to sorb and hold
large quantities of cations and organic pollutants. The mechanisms responsible for this
sorption process will be further elucidated in Chapter 4.

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