Groundwater contamination by pesticides is a widespread environmental problem and a major threat to drinking water supplies. Diffuse source contamination of groundwater that enters from an extensive area is characterized by low pesticide concentrations (nanogram-microgram per liter) in large volumes of water. It is regarded as one of the major threats to groundwater quality originating from agriculture, roads and railways. These large volumes of water in combination with the low concentration cause difficulties in preventing contamination of drinking water supplies and this is a challenge to develop remediation solutions. Abstraction fields often include several wells. Even if only one of the wells is contaminated, this water mixes with uncontaminated groundwater from the other wells and causes excessive volumes of water to be treated at the waterworks. An alternative approach to the treatment of contaminated groundwater at waterworks is enhanced in situ aerobic bioremediation in the anaerobic aquifers at the contaminated well. This may be achieved by increasing the oxygen content in aquifers with pumping strategies in or around drinking water abstraction wells. With that approach, aerobic groundwater from the upper part of the aquifer may mix with deeper anaerobic groundwater, increasing the oxygen content and creating favorable conditions for biodegradation. However, factors such as oxygen consumption related to organic matter, reduced inorganic species present in the sediment and precipitation of iron followed by the growth of iron bacteria leading to bioclogging reduces the efficiency of bioremediation of contaminated aquifers. The overall scope of this PhD study was to investigate biostimulated degradation potentials of pesticides at low concentrations in groundwater contaminated by diffuse sources in or around water abstraction fields. This approach could lead to more efficient in situ remediation solutions and protection of groundwater as a drinking water supply. Herbicides are generally expected to be difficult to be degraded under anaerobic conditions, but prone to biodegradation under aerobic conditions. Laboratory batch experiments were conducted with anaerobic aquifer material and groundwater collected near an operating drinking water abstraction field to study the potential for stimulating biodegradation of pesticides (bentazone, mecoprop and dichlorprop) at environmentally relevant concentrations (1 μg L-1) by addition of oxygen. Addition of oxygen stimulated mineralization not only at high oxygen concentrations but also at substantially lower concentrations (< 2mg L-1). Biostimulation in terms of enhanced oxygen concentrations around abstraction well fields was found to be a potential remediation solution for pesticides. Furthermore, bentazone mineralization was first time found in aquifer sediments at low oxygen concentrations. Enhanced biostimulation by adding nitrate or nutrients was also seen as potential technologies, however, in the case of nitrate, it was suggested that a mixed oxygen/nitrate system could be used for the contaminants to be degraded under microaerophilic conditions. A literature review regarding degradation of bentazone in topsoils and aquifer sediments supported laboratory experiments that were performed with groundwater and aquifer sediment. Bentazone was degradable in aquifer sediments under aerobic conditions. Furthermore, we were able to transfer bacteria with the capability to degrade bentazone over a wide range of bentazone concentrations from the aquifer material to laboratory experiments. Laboratory microbial studies, aiming to support the observations of transformation of dichlorprop to 4-CPP by different analytic methods at an old landfill, were conducted to investigate anaerobic degradation of 14C-labeled dichlorprop. Concentrations of dichlorprop and 4-CPP with groundwater from the same field site under anaerobic conditions were also measured by analytical chemical analysis. Dichlorprop was recalcitrant in groundwater samples and 4-CPP concentration remained constant during the incubation period. This illustrates the complexity of the field site, characterized by clay-till, with varying redox conditions and residence times providing a higher degradation potential in the field than could be transferred to the laboratory. In conclusion, this PhD has developed our understanding on degradation processes of pesticides in aquifer systems. We have shown that biostimulation by oxygen addition even at relatively low concentrations is a promising remediation technology for groundwater contaminated by pesticides at abstraction well fields. The results of this thesis contribute to our understanding of the development of treatment strategies to protect drinking water wells and suggest an alternative solution to closure of abstraction wells due to pesticide contamination.