Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:The contamination of ground and surface water by As from soils and sediments represent significant threats to human health (WHO 2004) and, consequently, it is essential to fully understand the transport characteristics of solid phase As (Zhang and Selim 2005).
Category: