## Materials and Methods
### Method Validation Procedure  
There are currently no legislative guidelines available for validation of veterinary residues in water matrix; therefore, a method validation approach was implemented based on a combination of criteria set out in SANTE/11813/2017 guidelines [38] relating to pesticides in food and European Legislation 2002/657/EC [37], pertaining to veterinary residues in food. As part of this validation the following performance parameters were examined: identification, selectivity, sensitivity/linearity, trueness, within-laboratory repeatability (WL\({}_{\text{r}}\) or RSD\({}_{\text{r}}\)) and within-laboratory reproducibility (WL\({}_{\text{R}}\) or RSD\({}_{\text{wR}}\)). Further to this, method recovery, limits of detection (LOD) and limits of quantification (LOQ) were also assessed as part of method validation. Validation was performed at concentration levels equivalent to a low, medium and high concentration across the calibration curve, to be consistent with the method sensitivities for the different analytes (described in Table 2).  
The selectivity of the method was assessed by individually injecting standards and internal standards to check for isobaric interferences by monitoring all transitions. In addition, blank groundwater samples (\(n\) = 30, all from different sources) were analysed along with reagent blanks (both spiked with IS and non-spiked with IS) to determine any matrix interferences co-eluting with analytes. To assess linearity, matrix matched calibration curves, with at least seven points, were prepared by fortification of negative controls over a range of concentrations as described above (Table 4).  
Trueness and Precision were both assessed in terms of within lab repeatability conditions (WL\({}_{\text{r}}\)) and within lab reproducibility (WL\({}_{\text{R}}\)) conditions, using fortified negative control samples, given that no certified reference material is available for these analytes in water. The WL\({}_{\text{r}}\) study involved a negative control sample fortified at each of three validation levels in replicates of \(n\) = 6. For WL\({}_{\text{R}}\) a similar experiment was carried out at the same three concentration levels, with a total of \(n\) = 18 replicates analysed over 5 different days (3 days with \(n\) = 4 replicates and 2 day with \(n\) = 3 replicates). In this case, negative control samples from different sources were used on each of the different days, with different negative controls also used for each of the three validation levels.  
The dependence of recovery on analyte concentration was assessed whereby blank water samples were fortified pre- and post-extraction (\(n\) = 3) at two different concentrations; 20 ng L\({}^{-1}\) and 200 ng L\({}^{-1}\) for all analytes, except BITH, CLOR, MOR and OXY, which were at concentrations of 40 and 400 ng L\({}^{-1}\). Recovery was determined by comparison of analyte response in the pre-extraction spiked samples (spiked at the beginning, immediately prior to extraction) to that in the samples spiked post-extraction (spiked at the end, immediately prior to instrumental determination). Use of such approach allowed the effects of matrix on analyte response to be considered in calculating the recovery.  
LODs and LOQs were determined by fortification of blank samples at concentrations equivalent to the lowest calibrant level. The chromatographs of each analyte, on five different occasions, were visually inspected and the LOD and LOQ were given as the estimated analyte concentration that achieved a signal to noise (S/N) of 3 and 10, respectively, with consideration given to both quantifier and qualifier ions. The LOQ was assessed as the lowest spiking level which satisfied the method performance criteria set out by SANTE for trueness and precision, in combination with the minimum S/N.