By Daniel Collins
Several posts back I reported on a public perceptions survey that showed that New Zealanders seem to know less about groundwater and wetlands than about rivers and lakes. (Hydrogeologist Michael Campana, OSU, lamented a similar bias towards surface waters among US and international water resource management circles.) I expect this is because rivers and lakes are much more in the public eye — more noticeable when driving around the landscape or on a map, or easier to cover on prime time news. So I’d like to start shedding some light on a part of the water cycle that is basically always in the dark. This should put readers in good stead when reading about groundwater on Waiology or seeing it in the news.
So, groundwater — what is it? The name is almost a dead giveaway really. It’s water contained in the gaps in rock, soil or some other geological layer. But there’s a catch: to count as groundwater and not as soil moisture, all the gaps must be filled with water — that is, the geological material must be saturated.
And where does this water come from? Groundwater is just another part of the water cycle, recharged by water percolating from the soil, rock or rivers above. Over time, groundwater can return to the surface when the water table reaches the ground surface, when it discharges into a surface water body, or when tapped by plant roots or wells.
If a geological layer is particularly good at containing and conveying groundwater, it’s called an ‘aquifer’, from the Latin ‘aqui-‘ (water) and ‘ferre’ (to bear or convey). Its first known occurrence in the English language was in a 1901 article in the journal Science (PDF):
‘The artesian system shows four or five aquifers, or water-bearing strata, more or less completely separated from one another.’
In French, ‘aquifere’ seems to have been used nearly 100 years earlier by the biologist Lamarck when distinguishing between air-conducting passages and water-conducting passages in the body. It attained its hydrogeological meaning by 1835, when Arago described the physics of natural springs.
For a geological layer to be capable of bearing an appreciable amount of water, it must have enough gaps or fractures in it to allow water to flow. If there isn’t much space for water, or if it can only flow very slowly, the layer is called an aquiclude.
How conductive the layer is depends on how it was formed, geologically, and with what material. Gravel deposited by ancient rivers or glaciers tends to form good aquifers; fine sediments deposited beneath lakes or estuaries tend to become aquicludes. In some regions, like the coastal parts of the Canterbury Plains, you can actually find alternating layers of geological material: aquifer, aquiclude, aquifer, aquiclude, etc. In Canterbury’s case, this came about by sequential glacial advances and sea level rises over geological time.
The upper-most aquifer, when not disconnected from the soil by an aquiclude, is termed an ‘unconfined aquifer’. Aquifers that lie beneath aquicludes are ‘confined’. In some instances, the water in confined aquifers can be under such pressure that they give rise to artesian springs or wells (named after the region in France, Artois) — where water flows unaided above the top of the aquifer or even the ground surface. And as for the water table, this is the depth where the ground switches from being partly saturated to fully saturated, and only applies to unconfined aquifers.
In aquifers, groundwater flows from gap to gap, or from fracture to fracture. While water on the Earth’s surface flows downhill, water in aquifers flows down the pressure gradient which could be upwards. Compared with rivers, however, groundwater flow is much, much slower. It might take decades or centuries for water to cover the distance in an aquifer that river water might cover in a day.
So there you have it – the lowdown on groundwater. It’s an important resource, so we had better understand how it works.