Groundwater moves slowly relative to surface water, so it is useful to consider the time it takes for water to travel through the groundwater portion of the hydrologic cycle (Figure 38). The time required for a water molecule at any point along a flow path in a groundwater flow system to reach another location along the flow path is called groundwater travel time. The time it takes for a water molecule at the water table in a recharge area to travel to where it exits to the surface (the stream in the case of Figure 38) is referred to as groundwater residence time because that is the time the water molecule “resides” in the groundwater portion of the hydrologic cycle. Labels on the curved arrows of Figure 38 show that groundwater flowing along longer, slower and deeper paths spends more time in the subsurface, so it has a longer residence time when it reaches the stream. As shown in Figure 38, water enters a groundwater system at many locations over a wide area, but generally the flow paths converge to discharge on a smaller area. Thus, groundwater discharging to a stream will include water with a wide range of residence times.
The nature of geologic material strongly influences the velocity of groundwater flow and thus influences the travel and residence time. Velocities in the three broad classifications of geologic materials (granular sediments, fractured rock, and cavernous karst) are generally much different from one another. Velocity varies widely even in similar types of geologic material because the velocity is a combination of the ease with which water can pass through the material as well as the driving force created by the water level difference. Given that caveat, at shallow depths typical velocities are on the order of 0.01 to 1 m/day in granular aquifers, 0.5 to 50 m/day in fractured rock, and 10 to 500 m/day in cavernous karst material. Again, considering the residence time of groundwater in the flow system shown in Figure 38, if the geologic material was sand, groundwater residence times may be on the order of years to decades, but if the geologic material was silt and/or clay, which is much less permeable than sand, the residence times would be much longer, such as decades to many millennia.
Residence time of water in the groundwater portion of the hydrologic cycle is much longer than in other compartments of the hydrologic cycle such as the surface water compartment or the atmosphere. The residence time of the water in the groundwater zone is more like that of water molecules that make up large glaciers than that of liquid surface waters. Residence time in any compartment of the hydrologic cycle is estimated as the volume of water in a compartment divided by the average flux of water into and out of that compartment. The exact values are uncertain, but their order of magnitude is easily estimated. For example, the average residence time of a molecule of water in the atmosphere is on the order of 10 days, while a molecule spends about 1 year in the vadose zone, roughly tens to hundreds of years in lakes, about several thousands of years in the ocean, and on the order of hundreds of 1000s of years in the ice caps (Figure 39). In contrast, residence times of water in the fresh water part of the groundwater water zone range from years from hundreds to thousands of years, while residence times of deep, saline water beneath the fresh water zone are much longer, on the order of millions of years. Most of this deep water does not exchange with the hydrologic cycle except during geological upheavals which occur at a frequency on the order of tens of millions of years. The depth to this deep saline water varies with locale depending on topography and hydraulic conductivity, but generally occurs between a depth of 200 to 1000 meters.
Water dissolves minerals from subsurface materials as it flows through the pores of those materials, so generally, the longer water resides in the groundwater system, the higher the concentration of dissolved minerals. Consequently, given the potentially broad range of residence time for water discharging to streams and the varying character of the subsurface materials that the water passes through, the discharging groundwater can have a wide range of chemical compositions.
Travel times and residence times are important with respect to the movement of contaminants in groundwater. If contaminants enter the recharge areas and are not readily assimilated by the geologic materials, nor broken down by subsurface microbes, it may take years or decades to arrive at the stream (thus threaten the stream within a human lifetime) or it may take centuries to millennia (thus only become a threat to the stream in the long‑term future).
Groundwater has a long‑term “memory”, because it takes a long time to fill and empty a large reservoir, thus water has a long residence time in the subsurface. In fact, some of our aquifers contain waters that were recharged during the wetter and cooler periods of Earth’s history through the glacial‑interglacial cycles over the past to tens‑of‑thousands to hundreds‑of‑thousands of years. Such groundwater is called fossil groundwater, owing to the fact that it is not being replenished similar to fossil fuels that we extract from the subsurface.
After the discussion of groundwater systems thus far, it is useful to consider examples of groundwater in various settings to help the reader become more familiar with groundwater around the Earth. To that end, the following section illustrates how groundwater occurs in some unique terrains including mountains, karst (i.e., locales with caves), and permafrost settings.