A new approach is developed to insert fiber optic cables vertically into the ground with direct push equipment. Groundwater temperatures may be measured along the cables with high spatial and temporal resolution using a Distributed Temperature Sensing system. The cables may be inserted up to depths of tens of meters in unconsolidated sedimentary aquifers. The main advantages of the method are that the cables are in direct contact with the aquifer material, the disturbance of the aquifer is minor, and no borehole is needed. This cost-effective approach may be applied to both passive and active heat tracer experiments. An active heat tracer experiment was conducted to estimate horizontal groundwater velocities in a managed aquifer recharge system in the Netherlands. Six fiber optic cables and a separate heating cable were inserted with a 1 m spacing at the surface. The heating cable was turned on for 4 days and temperatures were measured during both heating and cooling of the aquifer. Temperature measurements at the heating cable alone were used to estimate the magnitude of the groundwater velocity and the thermal conductivity of the solids. The direction of the velocity and heat capacity of the solids were estimated by including temperature measurements at the other fiber optic cables in the analysis. The latter analysis suffered from the fact that the cables were not inserted exactly vertical. The three-dimensional position of the fiber optic cables must be measured for future active heat tracer experiments.