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Are You Getting what You Paid For? A Case Study in Forensic Geochemistry

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Dr. R. V . Krishnamurthy



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Isotopes are atoms of the same element that differ in atomic mass due to different number of neutrons in the nucleus. Some isotopes are stable isotopes and others are unstable, or radioactive, isotopes. Stable isotopes maintain constant concentrations in the environment over time. Unstable isotopes continue to decay into daughter elements. Stable isotopes of oxygen, hydrogen, carbon, sulfur, and nitrogen are the most commonly used in environmental and ecological research.

Forensic geochemistry is a relatively new discipline that uses chemical and isotopic markers to solve problems of forensic interest. These include fingerprinting oil spills, food adulteration and forgery in arts, drug abuse in sports etc. Carbon isotope ratios were used in the famous Tour de France case where the gold medalist was charged with steroid use. Carbon isotope ratios can be used to check adulteration of natural honey with corn syrup, reportedly a common process. Hydrogen isotopes enable one to test if a bottle of orange juice is pure juice from Florida or it is made from extract mixed with water from Michigan.

In this preliminary study we explore the possibility of using stable oxygen and hydrogen isotopes to verify the claims made by select vendors of bottled water in Egypt. In other words, we investigate, based on isotope ratios if the claimed source such as “water from deep well”, “water from mountain springs” etc. can be sustained. This may therefore qualify to be a study in “water adulteration”. This is an important issue not only in tourist hot beds such as Egypt but in many parts of the world where use of bottled water is becoming increasingly popular.

Our approach makes use of the observation that processes of evaporation and rain out causes the isotopic ratio to be specific in a given place, dictated by mean surface temperature, latitude, altitude and distance from the oceans which are the main source of water vapor in the atmosphere (Dansgaard, 1964). This is illustrated in Figure (1). As illustrated in the figure, the rain gets stripped more and more of the heavier isotopes (18O compared to 16O and 2H (Deuterium) compared to 1H (Hydrogen). Similarly, at the same latitude, rain at higher altitudes will have more of the lighter isotope compared to the heavier one. If one were to collect a water sample, say, from the top of Alps and also from a well at the base of the Alps, they will be isotopically distinct.

Additionally, the oxygen and hydrogen define a linear line and serves as the foci of all natural waters (Figure 5). Values from this locus will shift if the water body were to undergo secondary effects such as evaporation.

In the specific case of Egypt, one of the extensively used ground water reservoirs is the Nubian Sandstone Aquifer System (NSAS). One finds them at great depths (300 m to 2000 m). This was formed thousands or even millions of years ago and has a distinct range of isotopic values, shown by the blue circle in Figure (5). On the other hand, surficial waters such as that from the Nile River, lakes or shallow wells are far removed as shown by the red circle.

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