3.2 Persistence in Soil
The main route for degradation of veterinary medicines in soils is via aerobic soil
biodegradation. Degradation of veterinary medicines is affected by environmental
conditions such as temperature and pH and the presence of specific degrading
bacteria that have developed to degrade groups of medicines (Gilbertson et al.
1990 ; Ingerslev and Halling-Sørensen 2001 ). As well as varying significantly
between chemical classes (e.g. see Fig. 2 ), degradation rates for veterinary medi-
cines also vary within a chemical class. When manure is combined with soil,
degradation may be enhanced for selected medicines. Depending on the nature of
the chemical, other degradation and depletion mechanisms may occur, including
soil photolysis and hydrolysis (e.g. Wolters and Steffens 2005 ). The degradation
processes may well result in the formation of degradation products (e.g. Kolz et al.
2005 ). In some instances, these degradation products may be of greater environ-
mental concern than the parent compound as some have similar or greater toxicity,
some are more persistent and some are more mobile (Boxall et al. 2003 ). It is
therefore important that the fate of the degradation products in soils is considered
when assessing the impact of a veterinary medicine on the environment.
3.3 Transport in Soil Systems
Contaminants applied to soil can be transported to aquatic systems in surface
runoff, subsurface flow and drainflow. The extent of transport via any of these
processes is determined by a range of factors, including: the solubility, sorption
behaviour and persistence of the contaminant; the physical structure, pH, organic
carbon content and cation exchange capacity of the soil matrix, and climatic
conditions such as temperature and rainfall volume and intensity. Most work to
date on contaminant transport from agricultural fields has focused on pesticides,
nutrients and bacteria, but recently a number of studies have explored the fate and
transport of veterinary medicines. Lysimeter, field-plot and full-scale field studies
have investigated the transport of veterinary medicines from the soil surface to field
drains, ditches, streams, rivers and groundwater (e.g. Aga et al. 2003 ; Kay et al.
2004 ,2005a,b,c; Burkhard et al. 2005 ; Hamscher et al. 2005 ; Kreuzig and Holtge
2005 ; Blackwell et al. 2007 , 2009 ). A range of experimental designs and sampling
methodologies has been used. These investigations are described in more detail
below.
3.3.1 Leaching to Groundwater
The movement of sulfonamides and tetracyclines in soil profiles has been investi-
gated at the field scale using suction probes (Hamscher et al.2000a; Blackwell et al.
2007 ). In these studies, sulfonamides were found at depth but the tetracyclines were
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