Assessing the Persistence and Mobility of Organic Substances to Protect Freshwater Resources

Persistent and mobile organic substances are those with
the highest
propensity to be widely distributed in groundwater and thereby, when
emitted at low-levels, to contaminate drinking water extraction points
and freshwater environments. To prevent such contamination, the European
Commission is in the process of introducing new hazard classes for
persistent, mobile, and toxic (PMT) and very persistent and very mobile
(vPvM) substances within its key chemical regulations CLP and REACH.
The assessment of persistence in these regulations will likely be
based on simulated half-life, t1/2, thresholds;
the assessment of mobility will likely be based on organic carbon–water
distribution coefficient, KOC, thresholds.
This study reviews the use of t1/2 and KOC to describe persistence and mobility, considering
the theory, history, suitability, data limitations, estimation methods,
and alternative parameters. For this purpose, t1/2, KOC, and alternative parameters
were compiled for substances registered under REACH, known transformation
products, and substances detected in wastewater treatment plant effluent,
surface water, bank filtrate, groundwater, raw water, and drinking
water. Experimental t1/2 values were rare
and only available for 2.2% of the 14 203 unique chemicals
identified. KOC data were only available
for a fifth of the substances. Therefore, the usage of alternative
screening parameters was investigated to predict t1/2 and KOC values, to assist
weight-of-evidence based PMT/vPvM hazard assessments. Even when considering
screening parameters, for 41% of substances, PMT/vPvM assessments
could not be made due to data gaps; for 23% of substances, PMT/vPvM
assessments were ambiguous. Further effort is needed to close these
substantial data gaps. However, when data is available, the use of t1/2 and KOC is considered
fit-for-purpose for defining PMT/vPvM thresholds. Using currently
discussed threshold values, between 1.9 and 2.6% of REACH registered
substances were identified as PMT/vPvM. Among the REACH registered
substances detected in drinking water sources, 24–30% were
PMT/vPvM substances.