Environmental Forensics is a scientific methodology developed for identifying sources, the timing of release, and transport pathways for potentially hazardous environmental contaminants. It combines a variety of analytical methods with principles derived from disciplines such as chemistry, geology, geochemistry, hydrogeology, and statistics, with the purpose to provide objective scientific and legal conclusions on the source and/or time of a contaminant release. Instrumental development and refining separation schemes have allowed higher quality data to be obtained and played a major role in the recent progress of the field. The use of modern techniques such as inductively coupled plasma sector field mass spectrometry (ICP-SFMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for trace and ultra-trace element concentrations and isotope ratio measurements provides Environmental Forensics with new opportunities. The work described in the present thesis has been focused on closing knowledge gaps in the field of Environmental Forensics, including analytical method development as well as processes- and source(s)t-tracing using multiple isotopes in environmental studies. Paper I is dedicated to the assessment of performance of double-focusing, sector field mass spectrometry (ICP-SFMS) for determination of analytes (including technology critical elements (TCE)) at ultra-trace levels in complex matrixes, with a special emphasis on the determination of Au, Ag, Ir, Os, Pd, Pt, Re, Rh, Ru, Sb, and Te. Instrumentation development was performed by optimization and testing different configurations of the ICP-SFMS introduction system as well as various sample preparations, pre-concentration, and matrix separation methods. Factors affecting instrumental sensitivity, contamination risks, reagent purity, spectral interferences, matrix effects, and analyte recovery are discussed. Optimized matrix specific methods were applied to a range of reference and control materials (riverine and brackish waters, seawater, whole blood, serum, and urine). Samples included brackish water and seawater from the Laptev Sea, venous blood, tap water, and snow samples collected in Luleå, northern Sweden. In Paper II an analytical procedure was developed, consisting of high pressure/temperature acid digestion using an UltraCLAVE system followed by a one pass, single column matrix separation allowing the first Cr isotope study in epiphytic lichens and mosses, as well as analysis of chromite and soils by ICP-SFMS and MC-ICP-MS. The overall reproducibility of the method, which was found to be ±0.11‰ (2σ), was assessed by replicate preparation and Cr isotope ratio measurements performed by different operators in multiple analytical sessions over a few months. Results indicated no correlation between soil concentrations and isotopic compositions (R2=0.2), while a strong negative correlation (R2=0.7) between Cr concentrations in lichens and mosses and δ53Cr signatures indicates airborne Cr contribution from local anthropogenic source(s) depleted in heavy isotopes. Chromium isotope data obtained for lichens and mosses indicate the potential of using this approach for tracing and quantifying airborne Cr pollution caused by stainless steel foundries.  Paper III evaluates heavy metal contamination in brackish water, groundwater, and sediments collected close to a deposit facility at the Rönnskär Cu–Pb–Zn smelter in Skellefteå, northern Sweden. This study investigates the ranges of isotopic compositions of four elements (Cd, Cu, Pb, and Zn) in smelter process materials (ores, products, and waste), as well as in polluted groundwater and sediments of the affected area. The study’s objective was to evaluate the isotope variability of the polluting source and identify possible isotope fractionation between a source and a sink. This study further assesses the viability of using isotopic information to identify the source of the pollutant in various matrices. Isotope composition data were used as a compliment to multi-element screening analysis and multivariate statistical techniques. Expanding the number of elements utilized in isotope tracing empowers our abilities to decipher the source(s) and the extent of environmental exposure from contamination events related to mining and refining operations. Results show clusters of elements with elevated concentrations and significant inter-element correlations that can be traced practically in all matrices tested (from dust samples to sediments), confirming a link between the source and the polluted environmental compartments. Differences in the relative mobility in the environment for different elements (shown in the example of Re and Mo distribution in sediments) may however affect the usefulness of the elemental ratios in reconstructing the extent and timing of pollution events. Among the isotopes evaluated in this study, radiogenic Pb and stable Zn isotope systems offer the most promising source identification in the area close to the smelter. However, temporal variability in the isotopic composition of the source adds complexity for the Pb isotopes. Numerous post-deposition fractionating processes alter the original source ratios for Cu, Zn, and to a lesser extent, Cd. At larger distances from the source, additional fractionation during element migration and dilution of source-specific signatures with background components makes source tracing more challenging. To fully realize the great promise offered by expanding the number of elements utilized in isotope tracing as a powerful way to decipher sources and fate of environmental exposure, a comprehensive evaluation of both source(s) and background variability, as well as post-depositional fractionation, needs to be an integral part of any Environmental Forensics investigation. Paper IV combines uranium (U) and other trace element concentrations with iron and uranium isotope measurements as a proxy to reconstruct historical changes of U release and accumulation in one tailings pond and two lakes (Mettä-Rakkurijärvi and Rakkurijärvi) receiving deep mine waters in northern Sweden, Kiruna. Uranium is deposited in lake sediments downstream of the mine, with elevated U concentrations in the surface sediments exceeding 55 mg kg-1, a >20-fold increase from the pre-industrial years. The distribution of anthropogenic U between the lakes does not follow the distribution of other contaminants reaching the system with mine waters, with a higher relative proportion of U accumulating in sediments of the second lake. Vertical concentration profiles for redox-sensitive elements as well as Fe isotopic composition were used to re-construct past redox-conditions potentially controlling early diagenesis of U in surface sediments. The isotopic signature of U in surface sediments (activity ratio AR=2.5) is far from that of secular equilibrium. These signatures are a function of time and weathering-induced fractionation, used here as a source signature of U originating in the deep groundwater in the mine. Linear regressions of inverse U concentration in water (dissolved, particulate, and total) versus AR reaffirms a simplified mixing situation with two isotopically distinct sources: 1) a natural source (low U concentration, AR 2.64), and 2) an anthropogenic source (high U concentration, AR ≈1.95). After mixing with mine water from the Rakkurijoki system, the AR of receiving Kalix River water decreases from 2.66 to 2.24. Monitoring data on the surface waters demonstrate the effects of the tributary waters of the Rakkurijoki systems as it discharges into the Kalix River, where the U concentration of the river downstream is more than doubled.