Water quality is impacted by chemical compounds from a range of anthropogenic sources. Some chemicals are routinely monitored because their risk is well known, while many others called contaminants of emerging concern (CECs) are not. This thesis includes the development and application of LC-MS/MS methods for monitoring groups of CECs, including Watch List chemicals, and selected pesticides. The first comprehensive investigation into Watch List chemicals in Ireland, spanning four years and two Watch Lists, is presented. Occurrence frequencies for 14 out of 34 compounds studied was ≥50%, showing widespread contamination of CECs in surface waters. Risk quotient determination showed high risk occurrences of neonicotinoid pesticides, estrogen hormones and antidepressant venlafaxine. These results are fed straight to the EU and has a direct influence on EU policy decisions. In Ireland >67% of the land mass is agricultural, making pesticides a group of interest. A year- long study was conducted to examine pesticides through water treatment in two areas. This is the first study of this kind targeting pesticides to be executed in Ireland. It was indicated that only 4 studied analytes were removed with an average efficiency of ≥50%, highlighting the need for improved treatment. Acid herbicides were determined predominantly in receiving waters only, indicating alternative pollution sources than WWTP effluent. A catchment-based approach for pesticide monitoring using multiple sampling methods was performed. Catchments identified as at-risk for agricultural pressures were studied during pesticide spraying season by passive and grab sampling. Passive samplers identified analytes which were undetected in grab samples, demonstrating the benefit of employing multiple sampling methods. A catchment previously not thought to be at risk for herbicide pollution was found to contain high (>100 ng L-1) levels of acid herbicides in grab samples. The findings of these studies contributes to informing future policy, and aids in protecting waters internationally.