Dissolved organic matter (DOM) is found in all freshwaters globally, by dissolving in rainwaterduring its path through soil and on to oceans via. rivers and streams. To provide potable water fit for human consumption, selected streams and rivers are used by either direct abstraction, or by diversion into reservoirs prior to treatment. For ca.100 years, chlorine and its compounds have been used by water treatment companies to disinfect water. However, research has shown that reactions between chlorine and DOM can produce compounds (disinfection by-products, or DBPs) which may be hazardous to human health. This thesis explores the relationship between catchment character, organic matter concentration, and the potential formation of DBPs. In particular, trihalomethanes (THMs) were measured as these are currently the only regulated DBPs in the UK. To achieve this, water samples were collected quarterly over one year from two contrasting catchments, to study seasonal variations in DOM concentration and character. A third catchment was also sampled, with similar catchment characters to the first two catchments, to determine whether geographical location and land use types affected the data. Each catchment was studied to see if catchment characteristics (e.g. class of vegetation, soil type or bedrock) could be mapped using a Geographical Information Systems (GIS) approach), to observe any effects on DOM and/or the DBPs found in treated water, with the aim of producing a risk assessment map to aid the choice of future abstraction locations for drinking water. Hence, samples were chlorinated and chloraminatedin the laboratory before being analysed for DBP formation and residual chlorine concentrations were measured. Catchment specific GIS derived data were statistically analysed with water chemistry data, and detected relationships were explored statistically. Major findings include medium to strong positive correlations between the standardised THM4 (STHM4 – the concentration of THM4 formed from 1 mg L-1 dissolved organic carbon (DOC)) concentration and geology, where an increase of the area of inland rock in a catchment increases STHM4 concentration. Medium strength positive correlations were found between STHM4 and vegetation classes, where, as the area of acid grassland, and heather increase, so does the concentration of STHM4. Negative relationships were discovered showing the obverse, where, as loamy and clayey floodplain soils with naturally high groundwater increased in area, STHM4 concentration dropped (at the Hampshire Avon ii and Conwy catchments combined). The occurrence of coniferous woodland in a catchment was found to correlate with the CHCl3 formation potential of waters (Pearsons, f=0.530, p=<0.05, n=20), supporting findings in published literature. Laboratory based chlorination and chloramination of sample waters, followed by gas chromatography provided DBP data, specifically THM4. These data show that more chloroform was formed after chlorination than chloramination, and that chloramination formed 3 times more CHBr3 (another THM4 compound) than chlorination, under laboratory formation potential conditions. Results showed that the chlorination of water prior to DOM removal could result in a THM4 concentrations 5 times greater than the current UK regulatory limit, per mg L-1 dissolved organic carbon (DOC), whereas chloramination forms ca.5 times less than the current UK regulation per 1 mg L-1 DOC. However, chlorination of water prior to DOM removal is never done in practice, so this data provides information on the composition of the organic matter and whether DOM from a specific catchment contains specific components that are responsible for an increase in a specific DBP. Data also show that increasing organic nitrogen or organic carbon does not necessarily increase nitrogenous or carbonaceousDBPs (N-DBPs or C-DBPs). However, importantly, data shows that an increase in the area of land use classed as ‘urban’, results in an increase in DON (likely due to human influences) in the water draining from them, posing potential issues for eutrophication in downstream water bodies and the formation of N-DBPs at water treatment works. Whilst N-DBP detection was explored from several different angles, the development of a definitive method was not possible due to very low N-DBP concentrations, time and financial constraints. However, various methods were adapted to aid in the detection of them, showing promising initial results, providing the background for future projects into the discovery of a suite of N-DBPs such as haloacetonitriles and halonitromethanes. Finally, the data in this thesis have been inputted into maps for each major catchment to present data with a high visual impact, but also to illustrate land use types that have been found to correlate with increases in DBPs and specific nutrients in the water draining from them. However, the high variation in DOM concentration and character from site to site make extrapolation of these risk assessment data, to other catchments, unsafe. Nevertheless, collection of data from a catchment (similar to the work presented here) where a new water abstraction location is desired can prove advantageous in providing information to utility companies of what difficulties they may encounter when treating the water. Though this can be done by grab sampling at each site of interest, this can prove costly and timely and involves both field and laboratory based work aspects, wheras the method presented here requires less cost and time, once the method is initialised, to derive data of similar value. Despite the fact that disinfection performance would always trump DBP minimalisation, this is likely to be a vital tool in ensuring the provision of safe and healthy water fit for the consumption of an ever increasing human population.