Isotopes help hydrologists combat contamination

Isotopes act as signatures for ‘fingerprinting’ – giving clues to trace the source of water and contamination technology helps hydrologists to determine factors affecting water quality, such as subsurface processes, geochemical reactions and reaction rates. It can also be used to better understand the relationship between surface water and groundwater, and even to detect leaks.

Identifying the source of water contamination can be achieved with the help of isotopes, a proven technology and an invaluable investigation tool. This is showcased by a recent SRK Consulting case study on a metals processing plant.

Aparticular environment will acquire a characteristic isotopic composition or signature by virtue of the hydrogeochemical processes involved. We can use this isotope signature for ‘fingerprinting’ – giving us clues to trace the source of water, and hence the contamination.

By definition, isotopes are atoms with the same atomic number – in other words, the same number of protons in their nuclei; however, they may differ in mass because of variation in the number of neutrons. For example, protium (hydrogen-1) is a stable isotope of hydrogen with one proton and no neutrons, while deuterium (hydrogen-2) has one neutron and tritium (hydrogen-3) two neutrons.

Isotope signatures

Spatial variations in deuterium and oxygen-18 occur in the hydrological cycle; there is a lower proportion of these isotopes in rain that falls inland than in rain falling at the coast. These are among the variables we can use to identify whether the source of water is from surface dams or from groundwater – as the signature from the rainfall will be retained

By Ismail Mahomed

in groundwater. Evaporation also has an impact on this signature, as preferentially lighter isotopes will evaporate from the oceans and surface water.

Case study analysis

One of the aspects of the metals processing plant case study was considering the evaporation signature of some of the ponds on the client’s site – a signature that was different to the groundwater. Data analysed during this project suggested that there was some mixing of water from different sources in the underdrain of one of the ponds.

This underdrain had a calcium-nitratedominant chemical signature that differed from the other wastewater impoundments, underdrains and groundwater. The isotope results further illustrated the uniqueness of this pond’s underdrain signature.

Water vulnerability

Hydrology can benefit greatly from using isotopes in the tracing of groundwater, to help determine the vulnerability and sustainability of water resources. The

Cost-effective

Oxygen-18 and deuterium are particularly useful and can be easily and costeffectively analysed. While the traditional isotopes are well established, there are more novel isotopes – such as lithium-7, mercury-200 and copper-65 – that are gaining traction as more research is conducted and applications developed. These could be used in South Africa, but the analyses would need to be done by laboratories abroad.

About The Author

Ismail Mahomed is a principal hydrogeologist at SRK Consulting. He has over 20 years of experience in hydrogeology, mainly in dewatering, due diligence, numerical groundwater modelling, isotope hydrology, environmental impact assessments and contaminated land assessments. He has also been involved in mine dewatering and pore pressure investigations, numerical groundwater modelling, feasibility studies and due diligence, assessing environmental impact and liability, subsurface contamination characterisation and assessments, water supply investigations, as well as surface water and groundwater quality monitoring and management.