Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 77

groundwater is a primary concern facing the growing human population on Earth and
especially those within arid and semiarid watersheds. Although the use of groundwa-
ter dates back thousands of years, it has only begun to be understood in the last couple
of centuries—and major gaps in our knowledge still persist today. Scientists now under-
stand the role of groundwater in the hydrologic cycle but still lack detailed information
about groundwater in urban areas. This information includes groundwater's geographic
distribution and migration pathways, anthropogenic influence, contaminant behavior and
transport, and its connection with and influence on surface water.
At the watershed scale, more knowledge is required to understand the geology of
groundwater and nongroundwater-bearing strata as well as its quality, flow, residence
time, age, and the anthropogenic contributions and impacts to this resource. In addition,
we need a deeper understanding of the fundamental hydrologic and hydrogeologic pro-
cesses where groundwater plays a critical role, such as infiltration, surface runoff, and the
recharge of surface waters.
The many reasons underlying the lack of understanding concerning groundwater can
be organized into a few broad categories:
1. Hydrogeological
• An absence of readily available groundwater in a particular area. There is a
tendency to study only things we can see.
• An abundance of groundwater in a particular area. In this case, groundwater
may be taken for granted and accepted without critical inquiry.
• An in-depth knowledge and understanding of geological and hydrological
principles is required. These skills are limited to a relatively small segment of
the population.
2. Geographical
• Other readily available and exploitable sources of water such as a large river or
lake are nearby. As a result, groundwater is ignored.
3. Financial
• Groundwater is expensive to investigate, study, and develop as a water source.
Money woes can limit curiosity.
Groundwater has only been studied outside of academic circles when something nega-
tive occurs. This includes events where contaminated groundwater is consumed from
an impacted aquifer, or when the water supply runs out or is significantly reduced. This
is particularly disturbing because approximately 50% of the population in United States
obtains its drinking water from groundwater, and more than 40% of irrigation water
comes from groundwater.
The importance of groundwater cannot be overstated, because groundwater accounts
for over 95% of all the freshwater on the planet—excluding the hard to reach water stored
in ice caps and glaciers. The estimated total world reserve of groundwater is estimated to
be approximately 10,528,827 km 3 (2.5 million mi 3 ) (Table 3.1), which is about 100 times more
than all the freshwater in lakes, streams, rivers, and swamps on Earth. To envision this
amount of water is difficult, but suppose all of the groundwater in the world was spread
out on the surface of the United States. The water depth would be close to 0.8 km (0.5 mi).
Let us go global. If all the groundwater was placed on the land surface of the Earth, it
would be 46 m (150 ft) deep.