3.1 The water resource – amounts and kinds of water

Properties of water

Although water is common in many parts of the world, it is unusual among liquids. Unlike many substances, water doesn’t become denser and denser as it gets colder. Solid metal sinks in liquid metal. Solid paraffin sinks in liquid paraffin. But solid water floats on liquid water, and for this reason, when ponds and lakes freeze over in winter, most of the contents remain liquid, continuing to support aquatic ecosystems and their fish, invertebrates, and aquatic plants.

Water is called the universal solvent because so many substances dissolve in water. As a result of this property, blood and plant sap can carry circulate through plant and animal bodies, delivering gases and nutrients and carrying away wastes. Water can support aquatic ecosystems in the same way.

Lastly among water’s interesting and useful properties is its slow response to changing air temperature. It takes considerable energy to heat water, and once heated, it cools slowly. This property of thermal inertia, a result of water’s high specific heat capacity , is the reason that coastal areas near oceans and large lakes often have rather mild climates relative to areas at the same latitude that are farther from water. Because water cools slower than the surrounding air in the winter and warms slower in the summer, nearby land experiences less extreme temperature swings. Moist soil also warms and cools more slowly than dry soil, due to its water content.

Kinds of water

Of the various kinds of water, humans rely most heavily on surface freshwater; it is, arguably, the most heavily managed natural resource on the planet. Yet, this comprises only a tiny fraction of all the water on Earth (Fig 1).

Liquid freshwater in groundwater is about 30% of 2.5% = 0.7% of total global water. Liquid, surface freshwater in rivers, lakes, and wetlands is about 24% of 1.2% of 2.5% = 0.007%. Less than 1% of the world’s water is easily used for drinking water. Further, a 2021  United Nations report estimated that about 40% of monitored freshwater bodies did not meet the standard for good ambient water quality due to water pollution. Many freshwater bodies are not monitored.

Figure 1 divides water resources between saltwater and freshwater, and then further divides freshwater into a variety of liquid and frozen forms. Liquid freshwater resources can be surface water or groundwater (Fig 2). Surface water occurs above the ground, in lakes, rivers, and wetlands. Groundwater, as the name suggests, is water in the ground. But it may be water immediately under the land’s surface or it may be water under rock layers far below the Earth’s surface.

An unconfined aquifer is a water layer just below the land’s surface, in the soil or sand or gravel (Fig 2). The top of the unconfined aquifer is called the water table. If you ever dug a hole near a lake shore or ocean shore, to reach water, you reached the water table. Where the land dips below the water table, surface water occurs in the forms of lakes, rivers, and wetlands. Surface water, rainfall or melting snowfall can add water to the unconfined aquifer (called recharge) quickly because the water merely needs to percolate through the soil to reach the unconfined aquifer. If water is plentiful, the unconfined aquifer can contribute water to surface-water bodies, as in the figure above, where blue lines point to the stream; if water is less plentiful, then the water table will be lower and flow direction will likely be from surface water into the unconfined aquifer.

Confined aquifers are separated from the land’s surface by one or more confining layers that significantly slow percolation of water (an aquitard – for example, a clay layer) or prevents it entirely (an aquiclude – for example, a rock layer) (Fig 2). Confined aquifers can occur in several layers, separated by confining layers. Just as the unconfined aquifer isn’t pure water but is soil or sand or gravel saturated with water, confined aquifers are layers of porous rock or sand or gravel saturated with water. Precipitation recharges confined aquifers over long periods, ranging from years to millennia.

A well that taps into an unconfined aquifer and removes water creates a local depression in the water table (Fig 2). The area and depth of the depression is a balance between the amount of water withdrawn and the amount of recharge from local precipitation. Withdrawal may thus be sustainable or unsustainable. If the depression is extensive or prolonged, the ground in the area may become compressed without water to support it, and the land may subside, creating a surface depression called a sinkhole.

A well that taps into a confined aquifer is tapping an essentially nonrenewable resource – human use occurs faster than recharge, so the confined aquifer is constantly depleted. As the confined aquifer is drained, the overlying confining layers lose the support of the water and may crack or subside, leading to subsidence at the surface, as well. In addition, collapse of deeper layers may lead to drainage of any unconfined aquifer as well as surface water in rivers or lakes, into the deeper layers.

In the upper diagram, the water table holds enough water to push against the ocean and maintain freshwater under the land surface. In the lower diagram, water withdrawal from the water table reduces the amount of pressure from the water table outward against the ocean; salt water is drawn towards the well.

Over pumping of aquifers in coastal regions can lead to saltwater intrusion (Fig 3), a process in which saline ocean water moves into the aquifer when freshwater is withdrawn, and potentially renders it unfit for use as a water supply. Saltwater intrusion has occurred in many coastal areas in the US and is particularly problematic in Florida. The problem of saltwater intrusion is exacerbated by rising sea levels due to global warming.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.