Groundwater and Surface Water
In order to understand drinking water contamination,
it is necessary to first understand from where our drinking
water comes. For most urban residents, relying upon municipal water systems, drinking water comes from two major sources: groundwater and surface water. These two sources of drinking water will be referenced throughout this guide to water contamination.
Groundwater refers to any subsurface water that occurs beneath the water table in soil and other geologic forms (Rail, 2000). Scientists estimate that groundwater makes up 95% of all freshwater available for drinking. Groundwater is a significant source of water for many municipal water systems in the United States. Rural residents, withdrawing their water from wells, also rely upon groundwater.
Surface water refers to water occurring in lakes, rivers, streams, or other fresh water sources used for drinking water supplies. While most drinking water in the United States is withdrawn from groundwater sources, surface water remains a significant water resource.
Each source of water has a unique set of contaminants; groundwater
stores pesticide chemicals and nitrate while surface water contains
most bacteria and other microorganisms. Because of the interconnectedness
of groundwater and surface water, these contaminants may be
shared between the two sources. Neither water source can ever
be entirely free from water contaminants. Due to the cycle of
water (hydrology), the two sources of drinking water feed each
other, sharing contaminants.
Groundwater is generally stored in aqueducts, underground layers
of porous rocks that are saturated with water. These aqueducts
receive water as soil becomes saturated with precipitation or
through stream and river runoff. As the aqueducts exceed their
capacity for water storage, they will bleed water back into
streams or rivers. The aqueducts maintain a natural balance
of water, alternately receiving or giving water as their saturation
levels oscillate. Throughout this process, water constantly
moves between surface and groundwater sources, sharing contaminants.