Groundwater

Groundwater is the fresh water located within the subsurface pore spaces of soil and rocks. It also includes water flowing through fractures in rock formations beneath the water table. The water table is the specific depth where soil pores and rock voids become completely saturated with water. This hidden resource is vital for life on Earth. It makes up about 30 percent of all readily available fresh water in the world. In fact, groundwater accounts for approximately 99 percent of the world's liquid fresh water.

To understand how groundwater works, we can view it as a system of inputs, outputs, and storage. The primary natural input is infiltration, where water from precipitation, streams, or rivers soaks into the ground. This water must then percolate downward to reach the groundwater. Once underground, the water can be stored for long periods. This storage makes groundwater a long-term reservoir in the natural water cycle. Its residence times can range from just a few days to many millennia. The natural outputs occur when groundwater discharges at the surface through springs and seeps or flows into the oceans.

Water is often stored in units called aquifers. An aquifer is a unit of rock or an unconsolidated deposit that can yield a usable quantity of water. These formations vary based on the local geology. Generally, the most productive aquifers are found in sedimentary geologic formations. These include unconsolidated alluvial materials that have accumulated in river valleys. In contrast, weathered or fractured crystalline rocks often yield much smaller quantities of water. Scientists study the distribution and movement of this water through a field called hydrogeology, or groundwater hydrology.

Hydrogeologists use various methods to study these underground systems. They can estimate the volume of an aquifer by measuring water levels in local wells. They also examine geologic records from drilling to find the depth and thickness of water-bearing rocks. Before building production wells, experts often drill test wells. These tests help collect soil and water samples for laboratory analysis. Pumping tests are also performed to determine how water flows through the specific aquifer. This data is essential before making large investments in water extraction.

Humanity relies heavily on this resource for many different purposes. Groundwater provides at least 50 percent of the global population's drinking water. It also accounts for about 40 percent of the world's irrigation water and one-third of water used for industrial purposes. In the United States, groundwater is the largest source of usable water storage. California is the state that withdraws the largest amount of groundwater annually.

However, global groundwater depletion is estimated to be between 100 and 300 cubic kilometers per year. This loss is mainly caused by the expansion of irrigated agriculture in drylands.

One of the most amazing discoveries in hydrogeology involves the age of groundwater. In the Great Artesian Basin in Australia, which covers almost 2 million square kilometers, water can be extremely old. By analyzing trace elements, scientists found that some water in these aquifers is more than 1 million years old. This water moves incredibly slowly. In this basin, groundwater travels at an average rate of about 1 metre per year. As the water flows westward from its recharge source at the Eastern Divide, it increases in age.

While groundwater is a massive resource, human use can cause serious environmental problems. Pollution is a major concern because contaminated groundwater is less visible than surface pollution. It is also much more difficult to clean up than pollution in rivers. Common sources of pollution include industrial chemicals, landfills, and excessive fertilizers or pesticides from agriculture. Other sources include mining wastewater, leaking underground oil tanks, and septic systems. Additionally, coastal areas face the risk of saltwater intrusion, where salt water enters fresh groundwater supplies.

Unsustainable extraction can also lead to physical changes in the Earth. When too much water is removed, it can cause land subsidence, which is the sinking of the ground. This has led to sinking cities like Bangkok. In California's Central Valley, multiple meters of elevation have been lost.

Even the Earth's axial tilt has shifted by 31 inches because of human groundwater pumping. These issues are further complicated by climate change and its effects on the global water cycle.