(222)Rn is a natural radionuclide that is commonly used as tracer to quantify groundwater discharge to streams, rivers lakes, and coastal environments. The use of sporadic point measurements provides little information about short- to medium-term processes (hours to weeks) at the groundwater-surface water interface. Here we present a novel method for high-resolution autonomous, and continuous, measurement of (222)Rn in rivers and streams using a silicone diffusion membrane system coupled to a solid-state radon-in-air detector (RAD7). In this system water is pumped through a silicone diffusion tube placed inside Sari outer air circuit tube that is connected to the detector. (222)Rn diffuses from the water into the air loop, and the (222)Rn activity in the air is measured. By optimizing the membrane tube length, wall thickness, and water flow rates through the membrane, it was possible to quantify radon variations over times scales of about 3 h. The detection limit for the entire system with 20 min counting was 18 Bq m(-3) at the 3 sigma level. Deployment of the system on a small urban stream showed that groundwater discharge is dynamic, with changes in (222)Rn activity doubling on the scale of hours in response to increased stream flow.