*2.1. Reagents*

Ultrapure water (≥18 MΩ-cm resistivity; ≤<sup>5</sup> <sup>μ</sup>g L−<sup>1</sup> TOC) was obtained from a Milli-Q IQ 7000 purification system (Merck Millipore, Darmstadt, Germany). Elemental calibration solutions for inductively coupled plasma mass spectrometry (ICP-MS) were obtained from HPS (North Charleston, Charleston, SC, USA) and were prepared as 1gL−<sup>1</sup> solutions in 1% nitric acid. All dilutions were performed with analytical-grade 65% nitric acid (Suprapur®) or 30% hydrochloric acid (Merck, Darmstadt, Germany). Our analytical procedures were validated using the following Certified European Reference Materials (IRMM, Geel, Belgium): "ERM-CC141 Loam soil", "ERM-CD281 Rye grass", and "ERM-BD151 Skimmed milk powder".

#### *2.2. Sample Collection*

Soil, milk, and forages (fresh or silage, depending on the farm stock) were sampled at 16 dairy farms, each of which had no more than 40 heads of cattle. As the studied area comprises multiple pollution sources, 10 farms were classified as close to industries (having one or more pollution sources less than 5 km away), while 6 farms were grouped as far (located 5 km or more from each pollution source). This 5 km distance was chosen to provide a compromise between the total farms sampled—those at maximum distance (classified as far) and those near to industries. Sampling on each farm was performed in autumn, spring, and summer in order to assess reproducibility. The soil samples from the upper layer soil (20 cm) were collected in three random points per cropland with a Dutch auger of 5 cm inner diameter. Once in the laboratory, the samples were air-dried at room temperature, crumbled, finely crushed, and sieved through a 2 mm screen [9,11,24]. Forage samples consisted of grass-based fodder and they were either fresh or preserved as silage (grass chopped and packaged without air to facilitate the fermentation process and minimize nutrient losses, for use as animal feed). Forage samples were collected at three points in the trough, then pooled to a total mass of around 1 kg. Milk samples (1 L) were collected directly from the tank after stirring. Figure 1 summarizes the sample collection and processing.

**Figure 1.** Sampling collection and processing of soil, forage, and tank milk from the selected farms.
