Chloride in groundwater
What is chloride?
Chloride is a naturally occurring ion that is present in both fresh and salt water. Chloride is most commonly derived from dissolved salts such as sodium chloride or magnesium chloride. Chloride is essential in small amounts for normal cellular function in plants and animals.
Natural sources of salts to freshwater resources include the oceans; the natural weathering of bedrock and soils; and volcanic activity.
Why monitor chloride?
Chloride concentrations in groundwater are monitored by regional councils across New Zealand as an indicator of contamination. This may include seawater contamination as well as contamination from human activities.
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. A sudden, sharp increase in chloride concentration in a costal well may indicate that the well has started to draw in sea water. Knowing the potential risk from sea water intrusion is necessary for regulators to manage current level of abstraction, along with any future effects related to sea-level rise.
At low concentrations, chloride can be used as an indicator of human wastewater discharges or other contamination. Natural Groundwater derived from rainwater may have chloride concentrations less than 10 mg/L. With chloride leached from chemical fertilisers on agricultural soils, or from wastewater discharged onto the land surface, the chloride concentration in the groundwater may increase to 20 or 30 mg/L or more. These concentrations are too low to affect the taste of water, but they can be measured in groundwater samples and used to indicate contamination and provide information about potential sources. Common contamination sources are animal waste, fertilizer and septic systems. For example, groundwater that is worsening due to excessive effluent application, or intensive land use, will almost invariably have an increasing chloride concentration.
High chloride concentrations in freshwater can harm aquatic organisms by interfering with osmoregulation, the biological process by which they maintain the proper concentration of salt and other solutes in their bodily fluids. Difficulty with osmoregulation can hinder survival, growth, and reproduction.
