4.15. In many cases, direct measurements of key parameters or phenomena cannot be made. Instead, inferential methods will need to be used. Monitoring of one parameter may involve interpretation of the results of many separate measurement activities (e.g. groundwater flow rates, moisture content of soils, daily precipitation) over different periods of time. For instance, regional groundwater flow velocities and material properties such as transmissivity and hydraulic conductivity are deduced from head measurements and point measurements. Additional challenges arise for measurements that need to be taken at greater depths below the surface, in high radiation fields or in other situations that make access more difficult. Similarly, it may be easier to monitor a temperature gradient than relative saturation of a swelling clay buffer or host rock. Thermal conductivity, and ultimately relative saturation, can then be deduced from measurement of the temperature gradient. Another example of non-intrusive monitoring is the application of wireless signal transmission, although this technology is still at a developmental stage for application to waste disposal facilities. Monitoring at an alternative facility with similar characteristics or at a demonstration or ‘pilot’ disposal facility may also provide useful information.