Persistence, according to the guidelines of the Stockholm Convention, is evident where, chemicals have a half-life in the aquatic environment of over two months (i.e. it takes over two months for the chemical to break down to 50% of its original concentration).
There are ongoing concerns over the potential persistence of medical products such as animal and human drugs which may readily infiltrate and persist in the aquatic environment as a result of being excreted within urine. It is becoming increasingly apparent that countries and organisations around the globe are being faced with pollutants that have not previously been considered a risk. This is an area where we are seeing growth in research as we grapple to understand the occurrence, extent and significance of the problem in terms of the environment, ecosystems and ultimately human health.
The European Commission Science for Environment Policy Issue 480 reported the results of two studies looking at two different types of issue:
- Which factors make drugs persistent? A look at Sulphonamides (anti-bacterials) in Polish rivers: and
- Whether banned pesticides continue to affect toxicity in streams in Denmark.
The Polish study assessed the breakdown of sulphomides in samples from two rivers in Poland. As with any biological process it was perhaps not surprising that the researchers found that the factor with the biggest effect on biodegradation was temperature, with rates of breakdown significantly lower during the colder, winter season. Other factors the study found to affect degradation were the type of sulphonamide, pH, salinity and the presence of heavy metals. Sulpha-methoxazole was the most resistant to biodegradation, with an average half-life of 72 days making it a persistent pollutant.
The Danish study looked at water samples collected between 2010 and 2012 during the main periods when pesticides are applied to crops and took samples of sediment from the stream bed and suspended sediment. They also looked at both the influence of surface run -off and groundwater on concentrations in streams. Interestingly two of the four most commonly detected pesticides were dinitro-ortho-cresol and trichloroacetic acid, were last sold in Denmark in the 1980s, with three of the commonly detected pesticides in groundwater mecoprop, dichloprop and dichlobenil being banned in the late 1990s. Such persistence can have a significant impact on aquatic ecotoxicity and it was estimated by the study that this could increase by up to 10000 times when legacy pesticides were included.
Further information on these studies can be downloaded at http://ec.europa.eu/environment/integration/research/newsalert/pdf/which_factors_make_drugs_persistent_sulphonamides_polish_rivers_480na4_en.pdf for the Polish Study and at
http://ec.europa.eu/environment/integration/research/newsalert/pdf/banned_pesticides_continue_to_affect_toxicity_in_streams_480na3_en.pdf for the Danish Study.
If you are wondering how this might all relate to our UK rivers then you might be interested in Professor Andrew Johnson’s presentation to the Society of Environmental Toxicology and Chemistry (SETAC) Europe 26th Annual Meeting reporting on research into which chemical poses the greatest risk in rivers in the video below.
 “Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by SCU, The University of the West of England, Bristol