This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on August 22, 2012.
Groundwater is defined to be water that collects or flows beneath the Earth’s surface, filling the porous spaces in soil, sediment, and rocks. Groundwater originates from rain and from melting snow and ice and is the source of water for aquifers, springs, and wells. The water moves down into the ground because of gravity, passing between particles of soil, sand, gravel, or rock until it reaches a depth where the ground is filled, or saturated, with water. The area that is filled with water is called the saturated zone and the top of this zone is called the water table. The water table may be very near the ground’s surface or it may be hundreds of feet below. Most groundwater is clean, but groundwater can become polluted, or contaminated. It can become polluted from leaky underground tanks that store gasoline, leaky landfills, or when people apply too much fertilizer or pesticides on their fields or lawns. When pollutants leak, spill, or are carelessly dumped on the ground they can move through the soil.Here are some facts about groundwater. Groundwater is the primary water source wherever surface water is not readily available. Unlike surface water, which has been intensively developed in many parts of the world for thousands of years, groundwater has remained until less than a century ago a rather sparsely developed resource. According to a recent report produced by the Internal Atomic Energy Association (IAEA) groundwater is now a significant source of water for human consumption, supplying nearly half of all drinking water in the world and around 43 percent of all water effectively consumed in irrigation. Yet the relevance and socio-economic impacts of groundwater development are higher than these percentages may suggest.
According to an IAEA report, the UN World Water Development 2012, groundwater is crucial for the livelihoods and food security of 1.2 to 1.5 billion rural households in the poorer regions of Africa and Asia, but also for domestic supplies of a large part of the population elsewhere in the world. Furthermore, groundwater-fed irrigation is usually considerably less susceptible to water shortage risks than irrigation supplied by surface water. This is likely to result in higher economic returns per unit of water used, as demonstrated by studies in Spain and India. Consequently, the share of groundwater in the overall socio-economic benefit from abstracted water tends to be higher than its volumetric share in the total water abstraction.
2. GROUNDWATER CHALLENGES:
Rapid global population growth recognized to be instrumental in mounting demands for water, food, and income which instigated, during the twentieth century, an unprecedented revolution in groundwater abstraction across the globe. It was reported that intensive groundwater abstraction began in the first half of the twentieth century in a limited number of countries including Italy, Mexico, Spain and the USA, and then expanded worldwide since the 1960s. This fundamentally changed the role of groundwater in human society, in particular in the irrigation sector where it triggered an “Agricultural Groundwater Revolution”, significantly boosting food production and rural development. The use of groundwater has also considerably modified local and global water cycles, environmental conditions and ecosystems.The global groundwater abstraction rate has at least tripled over the last 50 years and continues to increase at an annual rate 1 to 2 percent. Excessive groundwater abstraction is resulting in falling water tables, water quality degradation and land subsidence, as is the case in several cities in Asia – including Bangkok, Beijing, Chennai, Manila, Shanghai, Tianjin and Xian. The supplying aquifer in Mexico City fell by 10 m as of 1992, resulting in land subsidence of up to 9 m. Over-abstraction in coastal areas results in saltwater intrusion: in Europe, 53 out of 126 groundwater areas show saltwater intrusion, mostly in aquifers that are used for public and industrial water supply. A growing number of large urban centre aquifers are also facing pollution from organic chemicals, pesticides, nitrates, heavy metals and water-borne pathogens.
Under conditions of reduced natural outflow and/or induced recharge, groundwater abstraction causes depletion of groundwater storage until a new dynamic equilibrium is established. In the world’s arid and semiarid zones, numerous groundwater systems are not resilient enough to accommodate storage depletion under intensive groundwater development. The result is a progressive depletion of groundwater, accompanied by steadily declining groundwater levels. Recently, the Gravity Recovery and Climate Experiment (GRACE) produced estimates of the current rate of groundwater depletion in a number of very large aquifers and a global simulation model produced estimates for the entire planet. Results so far show that significant groundwater storage depletion is taking place in many areas of intensive groundwater withdrawal. Physical exhaustion of groundwater storage is a threat in very shallow aquifers only. More commonly, the more important impacts of groundwater depletion are side-effects of the associated declining water levels, and include increasing cost of groundwater (due to larger pumping lifts), induced salinity and other water quality changes, land subsidence, degraded springs and reduced baseflows.
As far as the quality of groundwater is concerned, while the bulk of global groundwater resources at shallow and intermediate depths have adequate quality for most uses, gradual changes in local groundwater quality have been observed in zones scattered around the world. The most ubiquitous changes are caused by pollutants produced by humans such as liquid and solid waste, chemicals used in agriculture, manure from livestock, irrigation return flows, mining residues and polluted air. A second category results from the migration of poor quality water into aquifer zones, such as saltwater intrusion in coastal areas or upward migration of deep saline groundwater as a result of groundwater abstraction. Climate change and associated sea level rise are expected to constitute another threat to groundwater quality in coastal areas.
Groundwater systems around the world are coming under increasing stress from various anthropogenic and natural factors. In many areas, this threatens the future availability of good-quality groundwater at affordable cost or in situ environmental functions of groundwater. Sound water resources management based on scientific knowledge and paying due attention to groundwater is therefore crucial. It should strive for a balance between present and future benefits from groundwater, pay attention to deterioration of groundwater quality, control environmental impacts of groundwater abstraction, and mitigate such impacts in cases where reduced groundwater availability cannot be prevented.
3. EFFECTIVE GROUNDWATER MANAGEMENT THROUGH ISOTOPES:
According to reliable sources, more than 97 percent of the Earth’s available fresh water is located underground, yet this vital resource is often poorly understood and poorly managed. Stable and radioactive isotope techniques are cost effective tools in hydrological investigations and assessments, and are critical in supporting effective water management.
Because of the fact that many countries around the world lack a comprehensive assessment of the quality and availability of their resources, they are not able to manage their water resources in order to meet current as well as future demands. Nuclear approaches, in the form of isotope hydrology, help to address this shortcoming, and may be a much faster way of obtaining key information than traditional hydrological monitoring approaches.
3.1 The Use of ISOTOPES in Groundwater Hydrology:
As geochemical tools, stable and radioactive environmental isotopes such as 2H, 3H, 14C, and 18O can provide information on the physical and chemical processes operating on groundwater, on the hydrogeological characteristics of aquifers including origin, time, and rate of recharge, and on aquifer interconnections. This kind of information, often cannot be obtained other than isotope techniques, is valuable in the assessment and management of groundwater resources, especially in the developing countries and the mountainous areas where there is a scarcity of hydrogeological information and/or long-term series of observation data are generally missing.The following is derived from an IAEA report which demonstrates how ISOTPES are commonly employed to investigate groundwater.
3.1.1 Sources and Mechanisms of Groundwater Recharge:
A qualitative and quantitative characterization of groundwater recharge is essential to ensure the sustainable development and management of groundwater resources. Aquifers which receive little recharge exhibit only small fluctuations in groundwater levels; a reliable estimate of recharge rate cannot therefore easily be obtained on the basis of classical approaches alone, such as water level monitoring. Isotope techniques are virtually the only tools which can be used to identify and evaluate present day groundwater recharge under arid and semi-arid conditions.The isotopic composition of groundwater (expressed as abundance of oxygen18 and deuterium) is determined by the isotopic composition of recharge. If most of the recharge is derived from direct infiltration of precipitation, the groundwater will reflect the isotopic composition of that precipitation. However, if most of the recharge is derived from surface water (rivers or lakes) instead of from precipitation, the groundwater will reflect the mean isotopic composition of the contributing river or lake. This isotopic composition is expected to be measurably different from that of local precipitation. The difference arises from the fact that recharge via bank filtration may represent water originating from precipitation in a distant area, for instance in a high mountain region. In high mountain regions the isotopic content of precipitation is different to that of precipitation falling on plains. This difference in isotopic composition allows for differentiation of precipitation sources, and hence of recharge mechanisms. In addition to differences in isotopic composition of groundwater resulting from different recharge sources, there can be differences due to how recently recharge occurred. In hydrological settings in which groundwater is very old (>10 000 years), regional climatic conditions at the time of recharge may have been different from those existing today, and this is reflected in the isotope composition of the groundwater.
Under certain circumstances, the residence time and thus recharge rate of modern groundwater can also be estimated by measuring the seasonal variations of hydrogen and oxygen isotopes. The applicability of this method is limited to those areas where precipitation shows a pronounced seasonal variation, such as in mountainous areas.
Groundwater in shallow aquifers typically has a residence time of decades to hundreds of years. In contrast, deeper and less permeable aquifers that extend for many kilometres can have through-flow times of thousands of years. If the flow regime is simple and mixing is minimal, such aquifers can serve as archives of information about environmental conditions at the time of recharge. The stable isotopes of hydrogen and oxygen in palaeowaters (groundwater recharged under climate conditions different than today) reflect the air temperature at land surface and the air mass circulation (origin of moisture) at the time of precipitation and infiltration. While palaeotemperatures derived from oxygen–deuterium analyses are useful, recently developed noble gas analytical methods provide greater certainty and precision in palaeotemperature determination.
3.1.2 Groundwater Age and Dynamics:
The radioactive decay of environmental radioisotopes and the transient nature of some of these (bomb tritium, anthropogenic krypton-85, bomb carbon-14 and bomb chlorine-36) make such isotopes a unique tool for determining groundwater residence time. Residence time, also called groundwater age, is the length of time water has been isolated from the atmosphere.Recharge of unconfined aquifers usually results in a vertical gradient of groundwater ages (increasing age with depth), while in confined aquifers the dominating feature is a horizontal or lateral gradient (age increasing with distance from area of recharge). In the former case this gradient is approximately proportional to the inverse of the recharge rate (volume/time), while in the latter case the gradient is approximately proportional to the inverse of the flow velocity. Therefore, the hydrogeologically relevant parameters primarily addressed by groundwater dating with radioactive isotopes are the recharge rate and flow velocity of groundwater in unconfined and confined aquifers, respectively.
One of the approaches to determine groundwater flow rate is to estimate flow velocity by measuring the decrease in the radioisotope concentration along the flow path. If the mean porosity value of the aquifer is known, groundwater flow rate can be estimated. This simple approach requires access to at least two wells along the flow path of an aquifer and knowledge of the initial radioisotope concentration in the recharge area.
Under natural conditions, groundwater movement is generally very slow, often in the order of a few metres per year. Water that has moved a few kilometres along the flow path under these conditions is hundreds or thousands of years old; an age beyond the dating range of tritium, tritium and helium-3, and chlorofluorocarbons. Therefore, in large aquifers with long flow paths, the most common radiometric approach to determining groundwater residence times has been carbon14. Its half-life of 5730 years makes it a suitable tool for the dating of groundwater in an age range of about 2000 to 40 000 years. Very slow moving groundwater in deep confined aquifers extending over tens and, in some cases, several hundreds of kilometres can reach ages of tens and even hundreds of thousands of years. These ages are beyond the dating range of carbon14 and require the use of very long-lived radioisotopes. Of the three longlived radioisotopes used in water studies — krypton81, chlorine36 and iodine129 — only chlorine36 has been found to have wider practical use so far. However, interpretation of chlorine36 data to ascertain groundwater age is often hampered by insufficient knowledge of in situ production of the isotope owing to reactions in the aquifer matrix. Recent developments in sampling and analytical methods suggest that the use of krypton81 may grow substantially given that it is a reliable tool to date groundwater in the range of 40 000 to 1 million years old.
3.1.3 Interconnections between Aquifers:
Both groundwater dynamics and groundwater contamination can be influenced by hydraulic interconnections between aquifers. Environmental isotopes, especially stable isotopes, can be used to investigate such interconnections, provided the isotopic composition of groundwater in the aquifers being measured is different.Thus, isotopes can be used to prove a lack of hydraulic interconnections between aquifers based on contrasting compositions. In some settings, hydraulic connections exist naturally between aquifers, and this can be evaluated through variations in isotopic composition. Intense exploitation of an aquifer can induce leakage from overlying and underlying aquifers. Stable isotope data can be used to estimate the flow of groundwater from adjacent aquifers.
3.1.4 Interaction between Surface Water and Groundwater:
Groundwater often consists of a mixture of recharge from surface water (lakes or rivers) and local precipitation. It is important to know the proportions of these recharge components in order to increase the sustainable supply of drinking water through bank infiltration, and to prevent drinking water pollution by infiltration of water from a contaminated surface water source. Different recharge components can be identified through the stable isotope compositions of groundwater because evaporation of water in surface water bodies, in particular under semi-arid and arid conditions, leads to an increase in the proportion of the heavy isotopes deuterium and oxygen-18.
A simple isotopic balance equation can then be used to estimate the relative proportions of surface water and precipitation in recharge. The accuracy of this determination generally depends on the magnitude of the difference in isotopic composition of the two components and under ideal conditions is in the order of a few per cent.River water can show a seasonal variation in isotopic composition, usually observed with reduced amplitude and after a time lag in wells near the river. This time lag as well as the change in mean isotopic composition provides the minimum time (transit time) required for river water and possibly it’s dissolved pollutants to reach a groundwater supply well. Isotope composition also provides insight into the fraction of river water in recharge (possibly polluted) relative to other recharge sources.
In arid climates, river water may be enriched in deuterium and oxygen-18 relative to groundwater if it was replenished under historical conditions with greater humidity. The fraction of river water in groundwater can be estimated based on the differences in isotopic composition of the mixing components.
3.1.5 Groundwater Salinization:
- In areas where salinization of groundwater is occurring, it is necessary to identify the mechanism of salinization in order to prevent or alleviate the cause. Isotope techniques can be used to distinguish the importance of the following processes which may lead to the salinization of groundwater:
- Leaching of salts by percolating water;
- Concentration of dissolved salts through evaporation; and
- Intrusion, present or past, of salt water bodies such as sea water, brackish surface water or brines.
3.1.6 Groundwater Pollution:
Pollution of aquifers by anthropogenic contaminants is of great concern in the management of water resources. Environmental isotopes can be used to trace the pathways of pollutants in aquifers and predict spatial distribution and temporal changes. This information is critical in order to be able to understand the source of contaminants, assess their scale and migration, and to plan for remediation. Measurements of the concentration and stable isotope composition of sulphate and nitrate in groundwater have been widely used to identify sources of sulphate/nitrate pollution.The stable isotope composition of sulphate and nitrate has also been used to evaluate microbial sulphate reduction and denitrification processes, respectively. Concentration and stable isotope composition of hydrocarbons and their degradation products can together provide a powerful tool for pollution assessment and remediation. The combined use of the stable carbon isotopic composition of carbon dioxide and the oxygen isotope composition of molecular oxygen, nitrate or sulphate provides a robust tracer for the verification and quantification of microbiological processes associated with hydrocarbon contaminated groundwater.
There are numerous studies being conducted around the world using stable and radioactive isotopes in support of comprehensive groundwater management, focusing as the time and cost effectiveness of these techniques is recognized. For instance, in the Guaraní Aquifer System of South America, the Tadla Basin of Morocco and the Nubian Sandstone Aquifer System of northern Africa, interpretations of isotope data have been used to not only confirm traditional hydrological studies, but also to provide insight into groundwater flows and aquifer dynamics. In particular, isotopes have been used in these areas to define groundwater recharge sources and mechanisms, to determine groundwater age and rate of movement, and to quantify the mixing of groundwater between aquifers. The application of isotopic techniques in hydrological investigations in general and in the comprehensive management of groundwater resources in particular is expected to grow substantially in the coming years.