Mercury is a naturally occurring heavy metal that has become a significant contaminant in water sources due to industrial activities and the burning of fossil fuels. Because it does not break down easily and can accumulate in the human body, prolonged exposure, even at low concentrations, can lead to serious adverse health effects. The primary risks involve neurological damage, particularly in developing fetuses and young children, and potential harm to the kidneys and other organ systems. Understanding the presence and concentration of this toxicant in your water supply requires precise testing procedures.
Preparing Samples for Accurate Testing
Obtaining a water sample that accurately represents the contamination level is the first and most practical step in mercury detection. The choice of container is critical because mercury is highly volatile and can easily sorb, or stick, to plastic surfaces, leading to falsely low results. Analysts recommend using pre-cleaned, acid-washed containers made of borosilicate glass or polytetrafluoroethylene (PTFE) to prevent the loss of the contaminant before it reaches the laboratory.
The sample collection method must account for whether the contamination originates from the water source itself or from the plumbing within a building. A “first draw” sample is collected immediately upon turning on the tap after the water has sat motionless in the pipes for at least six hours. This technique captures the highest concentration of metals that may have leached from the fixtures and internal plumbing materials.
Alternatively, a “fully-flushed” sample is collected after the water has been run for three to five minutes, effectively clearing the stagnant water from the internal pipes. This flush sample represents the quality of the water coming directly from the main supply line or well, providing a clearer picture of the source water contamination.
Once collected, the sample must be chemically preserved to stabilize the mercury and prevent it from escaping or adhering to the container walls. Preservation typically involves immediately acidifying the water sample by adding a mineral acid, such as hydrochloric acid (HCl), to reach a low pH below 2. For ultra-trace analysis, some methods may also use an oxidizing agent like bromine monochloride (BrCl) to ensure all mercury species remain in a dissolved, measurable form.
High-Precision Laboratory Detection Techniques
The analysis of mercury in water requires highly sensitive instrumentation due to the extremely low concentration levels that are still considered hazardous. Certified environmental laboratories utilize two primary methods for achieving the necessary precision: Cold Vapor Atomic Absorption Spectrometry (CVAAS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Both techniques are capable of measuring mercury in the parts-per-billion (ppb) or even parts-per-trillion (ppt) range.
Cold Vapor Atomic Absorption Spectrometry is a dedicated method where all mercury compounds in the sample are first converted to elemental mercury vapor. This is achieved by adding a strong reducing agent, such as stannous chloride, to the prepared water sample. The resulting elemental mercury vapor is then swept by an inert gas into an optical cell where it absorbs light at a specific wavelength, typically 253.7 nanometers. The amount of light absorbed is directly proportional to the concentration of total mercury present in the original water sample.
Inductively Coupled Plasma Mass Spectrometry offers a more versatile and often more sensitive approach to trace metal analysis. In this method, the water sample is introduced into a high-temperature plasma, which converts the mercury atoms into positively charged ions. These ions are then passed into a mass spectrometer, a device that separates and measures them based on their mass-to-charge ratio. ICP-MS can quantify mercury at much lower detection limits than CVAAS and has the added benefit of analyzing multiple elements simultaneously.
A significant distinction in mercury testing is the analysis of total mercury versus speciated mercury, a process that separates inorganic mercury from its organic form, methylmercury. Methylmercury is a potent neurotoxin that is formed when microbes in the environment convert inorganic mercury, and its toxicity is significantly higher. Speciated analysis, often accomplished by coupling a separation technique like chromatography with ICP-MS, provides a more detailed picture of the health risk.
Interpreting Mercury Levels and Taking Action
Once the laboratory analysis is complete, the numerical result must be compared against established public health standards to determine safety. In the United States, the enforceable safety standard for inorganic mercury in drinking water is set at two parts per billion (2 ppb), known as the Maximum Contaminant Level (MCL). Any measurement at or above this concentration indicates a potential health risk and requires a response.
A result exceeding the standard suggests the need for immediate action, which typically involves a temporary switch to an alternative source of drinking water, such as bottled water. The long-term solution involves installing a water treatment system designed to effectively remove heavy metals like mercury. The most effective point-of-use systems employ a multi-stage filtration process.
Reverse osmosis (RO) filtration systems are highly recommended, as they utilize a semi-permeable membrane to physically separate up to 97% of dissolved inorganic contaminants, including mercury, from the water. Another effective treatment option is activated carbon filtration, which uses adsorption to trap mercury compounds within its porous structure. Granular activated carbon (GAC) is often used in combination with RO systems to provide a comprehensive barrier.
Choosing the right remediation system depends on the specific mercury species and its concentration. While activated carbon can effectively remove certain forms of mercury, a reverse osmosis system is generally considered the more robust choice for consistently reducing inorganic mercury levels to below the safety standard. Periodic retesting of the water after installing a treatment system is necessary to confirm that the filtration unit is functioning correctly and maintaining a safe drinking water supply.