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Natural organochlorines in groundwater

08/2008

In groundwaters, halogenated organic compounds can be found practically everywhere. Typical AOX concentrations found in uncontaminated Danish groundwaters are 1-15 mg Cl/L (ppb Cl). The AOX (adsorbable organic halogens or total organic halogens (TOX)) of a groundwater sample is the sum of the halogens (chlorine, Cl, bromine, Br, and iodine, I) bound in almost all halogenated organic compounds. The dominant halogen contributing to groundwater AOX is chlorine, but lower concentrations of organic bromine and iodine are always found as well. Very high and dominant iodine contents have been found in dissolved organic matter (humic substances, fulvic acids) from one groundwater in old marine sediments.

Natural origin of groundwater AOX?

Whereas groundwater contamination with organohalogens (e.g. chlorinated pesticides and solvents) is a serious threat to our drinking water resources in many regions of industry and agriculture, contamination is not the only source of groundwater AOX. Concentrations of AOX as high as 80 mg Cl/L have been detected in deep aquifers (i.e. the section of the subsoil where the groundwater is accumulated) that are well protected against contamination from the surface by thick layers of clay.

These aquifers are not yet recharged with water from the period after 1955 associated with intensified industrial production of organhalogens. In one aquifer, organically bound chlorine was found in groundwater originating from just after the last glaciation. The high age of the groundwater itself was demonstrated by thorough investigation of the hydrology, hydrogeochemistry and isotope chemistry of the aquifer. The organic matter isolated from groundwater of one deep aquifer has been found to originate from aquifer sediments deposited in the Miocene period more than 5 million years ago. This organic matter was found to be at least older than 35,000 years by the carbon-14 dating method, but still contained organically bound chlorine, bromine and iodine. Organically bound halogens have also been found in organic matter from aquifer sediments such as peat (deposited approximately 1,000 years ago) and a marine clay (older than 4,000 years).

Chemical structures of groundwater AOX

In contrast to the variety of halogenated organic compounds found as contaminants, the AOX of uncontaminated groundwaters generally exhibits structures resembling those of the dissolved organic matter contained therein. These structures are formed during partial degradation of the remains from terrestrial plants or marine algae, chemical transformations and release to the groundwater. The resulting complex chemical structures of comparatively high molecular weights, such as the humic substances, typically contain 0.3 ‰-2 ‰ of halogens, predominantly chlorine. Still, some individual chlorinated compounds can be identified. Chloroform, a chlorinated solvent and a common by-product in chlorination of drinking waters, has been found (up to 2.7 mg/L) in uncontaminated groundwater below a spruce forest.

Processes behind naturally produced groundwater AOX

Organohalogens are released to the groundwater from buried geological sediments. The organohalogens found in even very old sediments can be explained partly by a content of organic halogens in both terrestrial and marine plants, and partly by microbial halogenation occurring during the partial degradation (humification) of dead plant material. Incorporation of inorganic iodine into humic substances has been demonstrated to produce organically bound iodine, and this process is probably causing the high concentrations or organic iodine found in an aquifer in old marine sediments. A similar incorporation of inorganic chloride and bromide into humic substances has not been reported.

High concentrations of AOX are present in the water charging the aquifers. In a spruce forest, AOX was found to be washed off the trees by the rain and could be found in water reaching the ground (the "throughfall"). Among the haloorganic compounds found in "throughfall" sampled in the spruce forest were chlorinated phenols and acetic acids. Also, evidence was found that further halogenation (i.e. incorporation of inorganic halide, such as chloride) of organic matter occurred in the soil, and the halogenation was ascribed to the action of enzymes produced by soil microorganisms. The AOX released from vegetation or produced in the soil was transported with the infiltrating water (precipitation seeping down from the surface to the groundwater) but with partial degradation or retention on the subsoil. In one study, arable land was demonstrated to contribute less to groundwater AOX than a spruce forest, and the impact on groundwater AOX of applied pesticides and degradation products analysed for was less important than the natural processes.

Chloroform found in the shallow groundwater below a spruce forest is probably the result of natural chlorination processes of the microorganisms in the soil. Activity of a group of enzymes, chloroperoxidases, has indeed been found in a spruce forest soil with high chloroform concentrations in the soil air and the groundwater beneath. The chloroperoxidases can be excreted by microorganisms, in particular by fungi, and can produce reactive halogen, thus causing the formation of chloroorganic compounds such as chloroform from inorganic chloride and soil organic matter. It has recently been reported that isotopically-labelled chloride, added to spruce forest soil, is incorporated into the chloroform released into soil air.

Degradation of groundwater AOX

In deep, uncontaminated aquifers, where the natural microbial activity has resulted in oxygen depletion (anaerobic conditions) the dissolved organic matter is generally much less halogenated than in shallow (near the surface) aquifers with oxygen rich groundwater (aerobic conditions). Field studies of uncontaminated aquifers do imply the occurrence of processes such as AOX dehalogenation (cleavage of organohalogens yielding inorganic halides and non-halogenated organic compounds) and degradation (both aerobic and anaerobic conditions), and release of less halogenated organic matter from sediments to groundwaters (anaerobic, methane rich aquifers). Degradation of some haloorganic contaminants and persistence of others in different aquifer environments may, thus, reflect the capacity and selectivity of natural dehalogenation/degradation processes in aquifers.

Perspectives

In acknowledging the natural background of organic halogens in groundwaters, we should not forget that many organohalogens found as groundwater contaminants are indeed of major environmental concern. Also, the presence of a harmful compound in drinking water is no more acceptable to the consumer if it is of natural origin than if it is a contaminant. On the other hand, if natural haloorganic compounds are degraded/dehalogenated in uncontaminated aquifers, this suggests that a potential for natural remediation of groundwater contaminated with haloorganic compounds is an inherent property of many natural aquifer systems. Therefore, the existence of natural processes capable of both production and degradation of organic halogens should be considered carefully in future evaluations of groundwater contamination.