What is chloride?
Chloride is a very common, naturally occurring chemical, probably best known as half of the chemical sodium chloride – table salt. It is the most abundant dissolved substance in sea water, and it is monitored by regional councils as an indicator of seawater contamination in groundwater. Chloride can also be derived from rocks and minerals in the aquifer, or from agricultural soils and human waste.
The concentration of chloride in sea water is roughly 19,000 milligrams per litre (mg/L). In contrast, fresh groundwater typically has chloride concentrations less than 100 mg/L. Therefore, when groundwater becomes contaminated with sea water, chloride concentrations increase dramatically.
At low concentrations, chloride can be used as an indicator of human wastewater discharges or other contamination. Natural groundwater derived from rain water may have chloride concentrations less than 10 mg/L. With chloride leached from chemical fertilisers on agricultural soils, or from waste water discharged onto the land surface, the chloride concentration in the groundwater may increase to 20 or 30 mg/L or more. These concentrations are still too low to affect the taste of the water, but they can be measured in groundwater samples and used to investigate contamination sources.
Chloride concentrations can also be useful indicator of groundwater age. The longer the water is in the ground, the more time it has to react with the surrounding aquifer material, and chloride is one of the common chemicals to be dissolved from the aquifer material over time.
How is it measured?
Chloride concentrations in groundwater are measured by collecting a sample from a well and sending it to an accredited laboratory. High chloride concentrations can also be estimated by measuring the electrical conductivity of the water, but at lower concentrations, the electrical conductivity is influenced by other dissolved substances as well, and the relationship with chloride is less clear.
Chloride concentrations are reported on LAWA in units of milligrams per litre (mg/L). Other units that may be used in the literature include parts per million (ppm) and grams per cubic metre (g/m3); both of these are dimensionally equivalent to mg/L.
What do the values mean?
Chloride in water has no adverse health effects, but the Ministry of Health has set a Guideline Value (GV) of 250 mg/L as the concentration above which chloride can start to affect the taste of the water.
The most common reason for measuring chloride levels in groundwater is to understand the risk to coastal aquifers from seawater intrusion. A sudden, sharp increase in chloride concentration in a coastal well may indicate that the well has started to draw in sea water. Knowing the potential risk from seawater intrusion is necessary for regulators to manage current level of abstraction, along with any future effects related to sea-level rise.