In England and Wales, two-thirds of drinking water comes from surface water. Rising dissolved organic carbon (DOC) concentrations in surface waters have been reported in the past few decades, commonly attributed to climate and land use change. Peatlands have historically been drained to create new grazing land and this has been suggested as one of the causes of the rising DOC trend. The experimental work in this thesis was undertaken in North and West Wales catchments and primarily examines the variations in DOC concentrations and how these are linked with trihalomethane formation potential (THMFP) in different field and laboratory settings. Chapter 2. From July 2012 – Feb 2013 water samples were collected from three contrasting sites in North Wales: Llyn Conwy (“lake”) a natural oligotrophic drinking water reservoir, Nant-y-Brwyn (“stream”) a natural Stream outside the catchment of the reservoir but analogous to other upland streams within the area and four man-made drainage ditches (“ditches”) within the predominately peat catchment of Llyn Conwy. The results of this study demonstrate that the drainage ditches have the potential to contribute more carbon per unit volume of water to the oligotrophic Llyn Conwy than the natural stream feeding into Llyn Conwy (equivalent to Nant-y-Brwyn). Mean DOC values were 6.32 mg/L higher in the drainage ditches when compared to Llyn Conwy. However, there were no statistically significant differences in the propensity to form trihalomethanes between the sites (p>0.05). Chapter 3: From March 2014 until September 2014 monthly water sampling was performed to compare the effect of drought on a discontinuous peat-accumulating wetland experimental site in mid-Wales, UK. Porewater samplers were installed at both the droughted and control sites. The results demonstrated that peatland exhibited reduced porewater DOC and phenolics concentrations and a shift in quality to lower molecular weight and less aromaticity. The mean DOC concentration of the control being 14.1 ± 0.77 mg/L and the drought being 11.9 ± 0.45 mg/L (F=4.93, p<0.05). Despite this, a significant change in the mean standardised trihalomethane formation potential (THMFP) values were not observed (p>0.05). Chapter 4: Twenty peat cores were collected from two locations in North Wales in June 2013; 10 each from Migneint bog and Cors Erddreiniog fen. The Mignent is the largest blanket bog in North Wales and Cors Erddreiniog is the largest area of fen on the island of Anglesey. These peat cores were transferred to the laboratory and kept under controlled temperature and levels of water table over a period of 160 days. The results indicated is that artificially exposing peat cores to a drought treatment reduced DOC concentrations compared to control cores (DOC: mean of all controls (71.68 ± 53.30 mg/L), mean of all droughts (39.69 ± 38.66 mg/L)). The values determined for the correlated THMFP values were mean drought 57.75 ± 20.33 μg CHCl3/mg DOC and mean control 54.59 ± 23.63 μg CHCl3/mg DOC, but these differences were not significant Chapter 5: The effect of constructed wetlands on DOC characteristics and THMFP of water sample from Llyn Cefni were measured twice a month from 2014 until March 2015. It was concluded that in the wintertime the constructed wetland is having a positive impact on water quality of the reservoir because it is still sequestering some nitrate and phosphate, but also reducing the DOC concentration of the water flowing into the reservoir by 18% on average. Chapter 6: In order to investigate the effect of storage of water samples that contain THMs after disinfection treatment, samples were collected from two selected surface waters; Nant-y-Brwyn stream (oligotrophic) and Cefni reservoir (eutrophic) in North Wales, subject to experimental chlorination in the laboratory and then stored at 4°C and at room temperature for an 11 week period. The majority of the weekly measurements of the concentrations of key DBPs showed no statistical differences from the initial week 0 values, regardless of water source types or storage temperatures (p<0.05).