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Module 1 — Understanding Groundwater
Permeability of Sediments
Groundwater is stored in the small spaces between particles that make up the sediment
and rock formations. These pore spaces are interconnected and groundwater moves slowly
through them. Permeability is a measure of the ease with which groundwater travels
through the pore spaces. Groundwater moves very slowly through sediments with low
permeability, such as clay. This allows more time for minerals to dissolve. In contrast,
sediments with high permeability, such as sand, allow groundwater to move more quickly.
There is less time for minerals to dissolve and thus the groundwater usually contains lower
levels of dissolved minerals.
There is also a difference in dissolved solids between groundwater in recharge zones
and water in discharge zones. Recharge zones are uplands areas where precipitation readily
enters the ground through permeable, sandier sediments. Generally, water in recharge zones
has a low level of mineralization. Discharge areas are low areas where groundwater flow
eventually makes its way back to (or near) the ground surface. Groundwater found in such
areas can be extremely high in minerals such as sodium, sulfates and chlorides. Examples
are saline seeps, sloughs and lakes.
Chemical Makeup of Sediments
Another factor affecting groundwater quality is the chemical makeup of minerals. Some
chemicals are more soluble than others, making them more likely to become dissolved in the
water. For example, groundwater in contact with sediments containing large concentrations
of sodium, sulfate and chloride will become mineralized at a faster rate than if other
chemicals were present.
Climatic Variations
Climatic variations such as annual rainfall and evaporation rates also play an important
role in groundwater quality. In semi-arid regions discharging groundwater often evaporates
as it approaches the surface. The minerals from the water are deposited in the soil, creating
a salt buildup. Precipitation infiltrating through the soil can redisslove the salts, carrying
them back into the groundwater. For example, in east central and southern Alberta where
annual precipitation is from 25-40 cm (10-16 in.) and the evaporation rate is high, TDS are
about 2500 parts per million (ppm). In areas with higher precipitation and lower evaporation
rates, precipitation that reaches groundwater is less mineralized. For example, in western
Alberta where annual precipitation is more than 45 cm (18 in.) groundwater in surficial
deposits contains less than 800 ppm of TDS.
A basic understanding of the factors that
affect groundwater quality can help you
make decisions on well depths and the best
water quality for a given application.