Groundwater, when pumped up from underneath the planet’s surface, is usually cheaper, easier to access and is less vulnerable to being polluted than water found on the earth’s surface. Because of this, it is normally used in public water supply chains. Groundwater is the largest single source of potable water storage in the USA. Underground aquifers possess much more water compared to the capacity of reservoirs and lakes, and this includes the Great Lakes. Some USA municipalities rely only on groundwater.
Professional hydrologists judge the estimated volume of underground water by taking water levels in locally sunk wells and by looking closely at the geologic record after well-drilling to help determine the breadth and depth of water-containing sediments, as well as rocks. Before a commitment is made to a full-sized well, hydrologists may require the drilling of a test well, or multiple test wells. They study the depths where water is found and inspect samples of soil, rocks and the water for lab analyses. They often perform a wide range of geophysical tests on the finished well, maintaining an accurate log of what they find and the test results. Hydrologists can then determine pumping efficiency rate by watching the degree to which water levels drop in the test well and others close by. Pumping too fast may cause it to dry up or interfere with other wells nearby. Near the coast, too much pumping can allow for saltwater intrusion. By analyzing the data, hydrologists can closely estimate the maximum, minimum and optimum output of the well.