and water flow, presented the ground‐breaking mathematical solution that describes the transient behaviour of water levels in the vicinity of a pumping well.
Two additional major contributions in the advancement of physical hydrogeology were made by Hubbert and Jacob in their 1940 publications. Hubbert (1940) detailed work on the theory of natural groundwater flow in large sedimentary basins, while Jacob (1940) derived a general partial differential equation describing transient groundwater flow. Significantly, the equation described the elastic behaviour of porous rocks introduced by Meinzer over a decade earlier. Today, much of the training in groundwater flow theory and well hydraulics, and the use of computer programmes to solve hydrogeological problems, is based on the work of these early hydrogeologists during the first half of the twentieth century.
The development of the chemical aspects of hydrogeology stemmed from the need to provide good quality water for drinking and agricultural purposes. The objective description of the hydrochemical properties of groundwater was assisted by Piper (1944) and Stiff (1951) who presented graphical procedures for the interpretation of water analyses. Later, notable contributions were made by Chebotarev (1955), who described the natural chemical evolution of groundwater in the direction of groundwater flow, and Hem (1959), who provided extensive guidance on the study and interpretation of the chemical characteristics of natural waters. Later texts by Garrels and Christ (1965) and Stumm and Morgan (1981) provided thorough, theoretical treatments of aquatic chemistry.
By the end of the twentieth century, the previous separation of hydrogeology into physical and chemical fields of study had merged with the need to understand the fate of contaminants in the sub‐surface environment. Contaminants are advected and dispersed by groundwater movement and can simultaneously undergo chemical processes that act to reduce pollutant concentrations. More recently, the introduction of immiscible pollutants, such as petroleum products and organic solvents into aquifers, has led to intensive research and technical advances in the theoretical description, modelling and field investigation of multi‐phase systems. At the same time, environmental legislation has proliferated, and has acted as a driver in contaminant hydrogeology and in the protection of groundwater‐dependent ecosystems. Today, research efforts are directed towards understanding natural attenuation processes as part of a managed approach to restoring contaminated land and groundwater and also in developing approaches to manage groundwater resources in the face of global environmental change.
Hence, hydrogeology has now developed into a truly interdisciplinary subject, and students who aim to become hydrogeologists require a firm foundation in Earth sciences, physics, chemistry, biology, mathematics, statistics and computer science, together with an adequate understanding of environmental economics and law, and government policy. Indeed, the principles of hydrogeology can be extended to the exploration of water on other planetary systems. Finding water on other planets is of great interest to the scientific community, second only to, and as a prerequisite for, detecting evidence for extraterrestrial life. As an example, a discussion of the evidence for water on Mars is given in Box 1.2.
Box 1.2 Groundwater on Mars?
Significant amounts of global surface hydrogen as well as seasonally transient water and carbon dioxide ice at both the North and South Polar Regions of Mars have been detected and studied for several years. The presently observable cryosphere, with volumes of 1.2–1.7 × 106 km3 and 2–3 × 106 km3, respectively, at the north and south poles, contains an equivalent global layer of water (EGL), if melted, of a few tens of metres deep (Smith et al. 1999; Farrell et al. 2009). Surface conditions on Mars are currently cold and dry, with water ice unstable at the surface except near the poles. Geologically recent, glacier‐like landforms have been identified in the tropics and the mid‐latitudes of Mars and are thought to be the result of obliquity‐driven climate change (Forget et al. 2006). The relatively low volume of the EGL, coupled with widespread indications of chemical and geological landforms shaped by areas of recent groundwater seepage (Malin and Edgett 2000) and extensive palaeohydrological activity (Andrews‐Hanna et al. 2010; Michalski et al. 2013; Salese et al. 2019), has resulted in the search for other extant water resources, as well as evidence of how much water, hydrogen and oxygen was stripped from the Martian atmosphere about 4 Ga.
The most likely reservoir for extensive storage of water on Mars is groundwater and a global Martian aquifer was long been assumed to exist beneath the permafrost at a depth where crustal temperatures maintained by geothermal heating may support liquid water. Depending on latitude, this melting isotherm is tentatively estimated to be located between depths of 5–9 km and to be overlain by a layer of mixed soil and ice (Farrell et al. 2009; Harrison and Grimm 2009). It is thought that topographic and temperature gradients act to create a significant and prolonged difference in hydraulic head between the melt water‐fed, polar groundwater ‘mound’ and the equatorial aquifer and this is assumed to facilitate significant subsurface flow over geological timescales to establish a global equilibrium depth to the melting isotherm (Baker et al. 1991; Clifford 1993).
Martian groundwater research advanced greatly in the 1980s and early 1990s when the currently accepted ideas regarding subterranean dynamics and subsurface structure were hypothesized. Contemporary investigations are examining these assumptions using the imagery and data now collected by the extensive array of Martian orbiters, landers and rovers, notably NASA's Mars Odyssey satellite, launched in 2001, and the ESA Mars Express, in orbit since 2003. As Mars has a very thin atmosphere and no planetary magnetic field, solar cosmic rays reach the planet's surface unimpeded where they interact with nuclei in subsurface layers up to 2 m in depth, producing gamma rays and neutrons of differing kinetic energies that leak from the surface. Instruments on board the Mars Odyssey orbiter can detect this nuclear radiation and use it to calculate the spatial and vertical distribution of soil water and ice in the upper permafrost layer (Plate 1.4) (Mitrofanov et al. 2004; Feldman et al. 2008). The results indicate water ice content ranging from 10 to 55% by mass, depending on latitude, with the highest concentrations in and around the southern sub‐polar region (Mitrofanov et al. 2004).
The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument mounted on the Mars Express satellite analyses the reflection of active, low frequency radio waves to identify aquifers containing liquid water, since these have a significantly different radar signature to the surrounding rock. The initial findings of the MARSIS sensor effectively identified the basal interface of the ice‐rich layered deposits in the South Polar Region with a maximum measured thickness of 3.7 km, with an estimated total volume of 1.6 × 106 km3, equivalent to a global water layer of approximately 11 m thick (Plaut et al. 2007). However, more recent studies using the MARSIS instrument presented a lack of direct evidence for the existence of subsurface water resources on Mars, possibly as a result of the high conductivity of the overlying crustal material (a mix of water ice and rock) resulting in a radar echo below the detectable limit of the MARSIS sensor (Farrell et al. 2009).
Other studies based on groundwater modelling approaches to explain various topographic features on Mars, such as chaotic terrains thought to have formed owing to disruptions of a cryosphere under high aquifer pore pressure, have concluded that a global confined aquifer system, for example as proposed by Risner (1989), is unlikely to exist and, instead, regionally or locally compartmentalized groundwater flow is more probable (Harrison and Grimm 2009).
Interestingly, the discovery of recurrent slope lineae (RSL) may indicate the presence of seasonal brine water flow on the Martian surface (McEwen et al. 2014). From data acquired by the Mars Reconnaissance Orbiter, RSL appear as narrow, dark markings, typically extending downslope on steep slopes from bedrock areas, often associated with small gullies, and indicative of intermittent, flow‐like features (McEwen et al. 2014). It is conjectured that RSL emanate from bedrock outcrops and progressively lengthen during warm seasons and fade during cooler
