Emerging contaminants in groundwater (2011)

The term ‘emerging contaminants’ is generally used to refer to compounds previously not
considered or known to be significant to groundwater (in terms of distribution and/or
concentration) which are now being more widely detected. As analytical techniques improve,
previously undetected organic micropollutants are being observed in the aqueous
environment. Many emerging contaminants remain unregulated, but the number of regulated
contaminants will continue to grow slowly over the next several decades. There is a wide
variety of sources and pathways for these compounds to enter the environment and these
include agriculture and urban areas. Some of these contaminants can have human or
ecological health effects and there is a need for better understanding of their fate in
environmental systems.
This report provides a short review of the types of organic micropollutants which can be
found in the aqueous environment. These include nanomaterials, pesticides, pharmaceuticals,
industrial additives and by-products, personal care products and fragrances, water treatment
by-products, flame/fire retardants and surfactants, as well as caffeine and nicotine metabolites
and hormones. Many of the compounds are relatively small polar molecules which are not
effectively removed by conventional drinking water treatment using activated carbon.
Pesticides and some industrial compounds are presently covered by the Water Framework
Directive, the Groundwater Regulations and the Drinking Water Directive. Additional
parameters, such as bisphenol A and nonyl-phenol are anticipated to be covered by revisions
to the Drinking Water Directive. Others are currently unregulated.
In order to assess the hazards presented by such compounds, information on usage,
persistence, leachability and a robust sensitive analytical method is required. The UK
metaldehyde problem was not originally discovered due to lack of an analytical method and
was exacerbated by recalcitrance in water treatment. For many pesticides these requirements
are fulfilled and an assessment of risk of leaching to groundwater can be made. However, for
pesticide metabolites this information can be sparse and for compounds such as
pharmaceuticals it can be lacking.
A simple hazard assessment for currently approved pesticides was made from information on
UK usage, persistence, sorption to soil carbon and published leaching indices. The following
compounds were assessed as having the greatest potential for leaching to water: 2,4-D,
amidosulfuron, bentazone, clopyralid, dicamba, florasulam, fosthiazate, imazaquin,
iodosulfuron-methyl-sodium, maleic hydrazide, MCPA, MCPP-P, metribuzin, metsulfuronmethyl,
quinmerac, oxamyl, and triclopyr with a further 46 also having potential. Of these, 19
had an octanol/water partition coefficient (Kow) less than that of metaldehyde and therefore
are likely to be incompletely removed by water treatment.
A simple assessment for pesticide metabolites, based only on organic carbon/water partition
coefficient (Koc) and persistence data, in this study gave results which agreed in principle with
other studies. The different approaches indicate that the metabolites of chlorothalonil,
cyanazine, diflufenican, flufenacet, iodosulfuron-methyl, metaldehyde, metazachlor and
metsulfuron-methyl are likely to pose the greatest risk to drinking water. In many cases these
metabolites are derived from parents which have a lesser risk.
Other organic micropollutants, such as pharmaceuticals, cannot as yet be assessed in the same
way due to a lack of persistence data since the majority of persistence studies have been
directed at water treatment. A range of organic micropollutants from urban settings have been
detected in ground and surface water. Commonly detected compounds include: bisphenol A, carbamazepine, galaxolide, ibuprofen, iopamidol, phthalates, phenyl ethoxylates, and
sulfamethoxazole. Case studies show that a small number of contaminants may be used to
characterise the contaminant loading and also be used to assess the migration pathways in
urban areas.
Data interpreted by BGS from the Environment Agency’s monitoring programme for organic
pollutants indicates that the 30 most frequently detected compounds comprise both
established and emerging compounds and include a number of polyaromatic hydrocarbons,
petroleum compounds, triazine herbicides, chlorinated solvents, degradation products and
THMs, caffeine, DEET and industrial compounds such as bisphenol A and tributyl phosphate.
Specific determinands include a range of currently licensed and phased out pesticides with a
few metabolites, pharmaceuticals including carbamazepine and triclosan, caffeine, nicotine
and food additives and alkyl phosphates. These data exhibit hot spots which may indicate
possible research areas.
Future research should focus on a compound identified in the literature and detected by
Environment Agency monitoring. Possible topics could be a study of migration through the
unsaturated zone. In many cases the mechanism for migration of emerging contaminants from
the surface to groundwater is very unclear.