What is groundwater?
Groundwater is all water, that infiltrated into the subsurface and accumulated in small, connected voids and crevices of underground rocks and sediments, comparable to a sponge. Such rocks and sediments that contain groundwater are called aquifers. They store and transport large quantities of groundwater. Other rocks or sediments with very few, or poorly connected voids and crevices are called aquitards. They block the groundwater from flowing, limiting the extent of an aquifer laterally as well as vertically. 
For a more thorough understanding of the topic of groundwater we recommend the open-source publications of "The Groundwater Project", for example "Groundwater in our water cycle".

Groundwater in Jordan
Large parts of the groundwater in Jordan are extremely old and have accumulated several thousand years ago. Nowadays, rainfall in Jordan is scarce, with a maximum of 500 mm/a in the northern highlands (around Ajloun), 200-400 mm/a around Amman and Madaba, around 100-150 mm/a in the Southwest and less than 100 mm/a in the East of the country. Of this relatively low amount of rain only 5 % finds its way into the groundwater (summing up groundwater recharge of 280 Mio. m³/a) This means that every drop of water, which is abstracted from the underground, needs long time periods to be replenished.
However, in order to supply the municipalities with drinking water, 520 Mio. m³/a are abstracted for domestic supply Additional 580 Mio. m³/a are withdrawn for agricultural purposes (not including illegal abstractions, which are estimated to be between 200 and 400 Mio. m³/a). The different industries consume another 35 Mio. m³/a. This sums up to much more than 1000 Mio. m³/a, or in other words: 
Four times more groundwater is abstracted than recharged! 
BGR and MWI made an assessment of the overall groundwater situation in Jordan in 2017 with regard to quantity, which can be found here.
For more recent information on the Jordanian water sector, please refer here.

What is groundwater quality?
Groundwater quality can be affected directly or indirectly by human activities: The obvious deterioration in groundwater takes place, when anthropogenic contaminants from garbage, fertilizers or waste water trickle into the underground. The more subtle deterioration begins, when an aquifer is overpumped. This can trigger the intrusion of waters with a different chemistry from other aquifers or it can catalyze the mobilization of trace elements from the aquifer itself:
While in the underground, groundwater continuously interacts with the surrounding rocks and minerals. Especially when the hydrochemical regime (the main composition of the water) is changed - e.g. by the introduction of air into parts of the aquifer, that have been flooded for thousands of years - this interacton with the aquifer material can suddenly alter, possibly resulting in a release of trace elements, e.g. Selenium, Molybdenum, Iron, Nickel, Vanadium, etc.
This doesn't necessarily result in a release of toxic amounts of these elements, but due to the many unknowns, we currently face in the underground, it is crucial to monitor the concentration of these elements as closely as possible, in order to minimize their mobilization and develop countermeasures. All relevant parameters for domestic purposes are covered in the Jordanian Drinking Water Standard.

Whilst the groundwater was recharged "only" several thousand years ago, the material of the aquifers (mostly sandstone and limestone) has been deposited millions of years ago. It is hard to imagine that, before the opening of the Jordan Rift Vally, the area was lying below sea level most of the time and thick layers of sediments accumulated. The older sediments derive from Cambrian-Silurian times (540 - 420 Mio. years ago) and consist of sandstone and other clastic sediments. The younger sediments mainly derive from Cretaceous time (130-65 Mio. years ago) and consist of limestone, phosphate and shale (partly organic). They lie on top of each other, sometimes intersected by aquitards, sometimes connected to each other. When the Jordan Rift Valley started opening (which started 18 Mio. years ago) these layers were "torn open" and pushed into the position, where we find them today (check).

Even though the geology is complex, a set of main aquifers can be identified:

• The oldest layers consist of sandstone, of which the lowermost layer (the "Ram"-aquifer, often called "Disi"-aquifer) is the most popular and thickest one, and is heavily exploited for drinking water purposes in the south of the Kingdom. Often, however, it is summarized with the two sandstone units above (the "Kurnub" and the "Zarqa"-formations) to form the Deep sandstone aquifer.
• In the period of the Upper Cretaceous (100 - 65 Mio. years before today), shallow marine conditions prevailed in large parts of Jordan, favouring the deposition of huge layers of limestone. These layers have been categorized after geological conditions in five layers of the "Ajloun-group" (A-group) and five layers of the "Balqa-group" (B-group). Since groundwater can partly flow between these layers, they are summarized to three different aquifers:
  • A1/A6
  • A7/B2 (which can be considered the most important aquifer in large parts of Jordan)
  • B4/B5
• In younger years, the rift system of the Jordan valley was activated, which basically created a huge open crack in the geological layers, which had been formed in the years before: The Jordan Rift Valley (including the Dead Sea and Wadi Araba in the south). This deep crack acts like a drainage trench for large parts of the country and most of the surface and the groundwater flows towards here. Together with this water, sediments and solutes are transported here and the so-called Alluvium-aquifer formed, a sequence of sandy and silty sediments, which form a loose layer of sediment in these lowlands.
• Furthermore, the opening of the cracks allowed magma (=molten layers of stone) to find a way from the Earth's interior to the surface, where it exited in the form of vulcanic lava. This mainly happened in the southern parts of Syria (around Jebel al-Arab/Jebel al-Duruz) and thick layers of lava flowed into the area nowadays known as "Hauran" and into the area around al-Azraq. Due to this incident, we can find the Basalt-aquifer in this area now.

In the following map, you can get a better idea of their geographical distribution. Each aquifer can be selected and displayed as a layer on the map of Jordan. The areas where the aquifer is exposed on the surface are called outcrops, while the areas where they are hidden underground, are called subcrops. To complete the picture, also the outcrops of the aquitards can be selected.

In order to monitor the trends in groundwater quality over the coming years, BGR and WAJ established an "Annual Groundwater Sampling" campaign, which started in 2022. Every year, the AGS surveys the groundwater quality at a maximum of 88 selected monitoring locations throughout Jordan. The monitoring network aims to capture all major aquifers as well as all governorates of Jordan. The samples are analyzed by WAJlab for most inorganic parameters of the Jordanian Drinking Water Standard.

In order to create a baseline understanding of groundwater quality in the Kingdom, BGR and WAJ conducted the "Groundwater baseline study 2023", based on samples, which were taken between 2020 and 2023. It comprises in total 520 monitoring locations throughout Jordan, mostly governmental and private drinking water wells. The samples were analyzed by the laboratory of BGR in Hannover and by WAJlab for a comprehensive set of inorganic parameters, considering the Jordanian Drinking Water Standard.