Waste disposals are covered with high density and highly saturated clay layers, which prevent rain penetration through waste into groundwater. These layers without monitoring are overdesigned to achieve long term performance. In case of a containment control, which means water content control, the construction can be less expensive and defects can be selectively repaired. We developed a new subsurface moisture sensor system, which locates a water content change of 2% with an accuracy of 4 m in a clay layer on a field of 2000 m2. It consists of a network of unshielded high-frequency transmission lines, installed during the disposal construction. The electromagnetic propagation along these lines is affected by the dielectric coefficient of the surrounding media which is strongly related to its water content. Usually, soil moisture sensors with Time-Domain- Reflectometry (TDR) are much shorter than 1 m. By dielectric measurements on several soil types and by an advanced model of the effective line parameters in soil, we could design transmission lines with lengths up to 20 m. Additional coupling zones between lines are used to economize connecting cables and to enhance the space resolution. TDR methods have been found less accurate than appropriate frequency domain methods performed with a network analyzer. The demonstrated moisture distribution maps of this test field show small local and seasonal variation since 1997.