Groundwater is often characterized as a “renewable resource.” Yet data now accumulating indicate that much of the current development of groundwater is depleting the resource at rates that cannot be sustained – in many places groundwater is being “mined” at high rates – contradicting its renewability over human timeframes. This poses a challenge to groundwater scientists and managers – can the resource be developed in a sustainable manner, and if so, how can that goal be accomplished? This is the premier task for the groundwater community in the 21st Century.
“Groundwater sustainability” has been defined in various ways. Alley et al. (1999) define groundwater sustainability as “development and use of groundwater in a manner that can be maintained for an indefinite time without causing unacceptable environmental, economic, or social consequences.” However, they note that “the definition of ‘unacceptable consequences’ is largely subjective and may involve a large number of criteria.” It might, for example, include streamflow depletion, drying of springs or wetlands, loss of vegetation, and/or water-level declines in wells. Price (2002) argues that sustainability should be related to a specific time period over which it is to be evaluated.
The concept of safe yield of a groundwater system is often used by water managers to place limits on the total number of wells and/or total pumping from a given aquifer. This need arises because groundwater is a common pool resource in which extraordinarily high usage by one or more parties may be highly beneficial to those parties (and their self-interests) but harmful to the long-term viability of the resource (through excessive depletion) and to everyone else’s continued future use of the common resource. Overdevelopment of an aquifer is a classic example of “The Tragedy of the Commons” (Hardin, 1968).
Meinzer (1923) defined safe yield as “the rate at which water can be withdrawn from an aquifer for human use without depleting the supply to such an extent that withdrawal at this rate is no longer economically feasible” (Alley and Leake, 2004). Freeze and Cherry (1979) state, “Todd (1959) defines the safe yield of a groundwater basin as the amount of water that can be withdrawn from it annually without producing an undesired result. Any withdrawal in excess of safe yield is an overdraft.” Freeze and Cherry (1979) indicate that there is widespread dissatisfaction with the term among hydrologists. Alley and Leake (2004) indicate that the dissatisfaction arises in large part because the term is vague. Misinterpretation implies a fixed underground water supply. Groundwater supply depends on the particular locations of wells and is only fixed when the locations of wells are specified. A yield that is safe from one perspective, such as depletion of groundwater storage, might not be so safe from the standpoint of discharge areas of aquifers, such as lakes, springs, and wetlands (Alley and Leake, 2004). Pumping rates that are considered safe by well owners may yield streamflow depletion that is an undesired result for surface-water users. Freeze and Cherry (1979), among others, suggest that an optimization approach within a socioeconomic framework would be a better way to assess an optimal (rather than safe) level of development in a groundwater basin.
The desirability and value of sustainable development of groundwater, and management approaches to help achieve that, have been discussed by Gleeson et al. (2012). Sustainable groundwater development seeks to preserve the resource for use by future generations. But “sustainability” should be assessed in a larger perspective that includes impacts on surface-water flows, other environmental consequences (e.g., land subsidence and water-quality changes), as well as other linkages, such as socio-economics (Alley and Leake, 2004; Hiskock et al., 2002; Kendy, 2003; MacEwan et al., 2017; National Research Council, 2013; and Van der Gun and Lipponen, 2010).
In contrast, “groundwater mining” is the removal of water from storage in the aquifer that cannot be renewed (or replaced) within a human timeframe (Thomas, 1955; and Bredehoeft and Alley, 2014). By definition, such rates of groundwater development cannot be maintained indefinitely. However, the length of time that such mining can continue depends on the stock of groundwater in storage (i.e., the volume of recoverable groundwater in the aquifer) and the rate of withdrawal through wells. Clearly there are cases in which substantial rates of groundwater development and mining can continue for decades and even centuries. It is possible that the economic, social, and political benefits of the water provided by such groundwater mining may be very large, and that some mining is acceptable within a socioeconomic framework.
If sustainable development of a groundwater resource is not practically achievable, then the question might be whether to manage the resource in order to extend its life or just let development eventually lead to functional limitations on groundwater withdrawals. Should non-sustainable groundwater development (sometimes called “overdraft” or “overexploitation”) ever be considered acceptable? Should society weigh the shorter-term economic and societal benefits of a time-limited use of the groundwater resource in an aquifer against the “costs” and environmental effects of the development, and how would that be done? Price (2002) points out that many developments in human history were non-sustainable, but contributed substantially to human progress. Perhaps non-sustainable groundwater development (groundwater mining) is acceptable to society if it is done with a full knowledge and understanding that such groundwater use can only continue for a limited (but reliably estimated) time. Depletion of groundwater resources without an understanding of its existence, timing, and consequences should be considered unacceptable policy. Hydrogeologists can provide that predictive understanding, which offers a long-view scientific basis for policy makers and water managers to make sound and defensible policy decisions.