- Published: Tuesday, 14 August 2012 15:33
How Old is our Groundwater?
The age of groundwater can span anywhere from two weeks for very local flow systems to millions of years for deep, regional flow systems. For example, a shallow flow system would constitute recharge in a small upland area followed by discharge to an immediately adjacent low-lying area.
The distance of groundwater travel may be on the order of 100's of metres. In contrast, groundwater flow in deeper regional flow systems can travel several 100's to 1,000's of kilometres.
A classic example of a deep regional flow system in Alberta is the general flow of groundwater from the recharge area in the western part of the province near the Rocky Mountain cordilleran and foothills region eastward across the plains areas towards the province of Saskatchewan (see chart - right). The time that it can take for recharged groundwater to travel this distance is in the order of several million years.
How old is our groundwater?
Work conducted by groundwater scientists and hydrogeologists in the province of Alberta has begun to establish the age of some of the waters collected from the various water-bearing formations beneath the province (Lemay 2003, Lemay and Konhauser 2006, WorleyParsons 2010). Although more work is needed to understand the full range of groundwater ages, initial results indicate that some of the water beneath our landscape has been there for a very long time.
Table 4 provides a very cursory review of some of the ages of groundwater measured in samples collected from near-surface bedrock and confined aquifers. These ages have been obtained using radiogenic isotopes of carbon and chloride, each with a half-life of 5,730 and 301,000 years, respectively. Simply put, the underlying principle with groundwater age-dating is that once water has recharged the subsurface, and is no longer in contact with the environment where the carbon-14 (14C) and chlorine-36 (36Cl) are generated naturally, the radioactivity of the isotopes will decay at a rate associated with the characteristics half-life. Therefore, collection of a groundwater sample and measurement of the remaining radioactivity will provide an idea of the how long the sample has been isolated from the surface environment.
|Aquifer type / Formation||Approx. Depth (meters)||Carbon-14 (est. years)||Chlorine-36 (est. years)|
|Alluvial (near surface)||Less than 15||<2,500||--|
|Buried Channels||17 to 230||1,560 to 30,900||--|
|Paskapoo||27 to 52||--||350,00 to 1,100,00|
|Scollard||20 to 39||--||700 to 1,300,00|
|Berlly River||15 to 91||500 to 42,500||--|
|Wapiti / Horseshoe Canyon||27 to 262||1,900 to 5.400||5,100 to 2,500,00|
What is most striking about the information provided above, is that groundwater samples that have been collected within relatively shallow depths (less than 60 meters) have attained ages in excess of 1,000,000 years. Although this may represent the amount of time since the water was recharged and subsequently flowed through the subsurface, mixing with older waters could also explain such results. Therefore, it is imperative that flow path phenomena and potential interactions between otherwise discrete formations be well understood before making such interpretations.