A new 222Rn passive-sampler to determine the residence time in-situ in the hyporheic zone

Abstract

<p>The exchange of stream water and groundwater in the hyporheic zone (HZ) plays an important role for the hydrological and biogeochemical functioning of rivers. Hydrological parameters of the hyporheic zone such as storage volume and water fluxes, as well as biogeochemical turnover are a function of the residence times (t<sub>h</sub>) of water since infiltration. Accurate determination and good resolution of residence times with depth is strongly coupled to the choice of tracer and the quality of the field data. In particular, radon (<sup>222</sup>Rn), a radioactive noble gas of the <sup>238</sup>U decay chain that is enriched in groundwater by contact with minerals in the aquifer matrix and degasses to the atmosphere in surface water, is a suitable residence time tracer as activities increase with time along the hyporheic flow path. Unlike many other environmental tracers traditionally used to trace water age, <sup>222</sup>Rn-based residence time estimates can resolve residence times on the order of a few hours, allowing fast and small-scale exchange processes to be studied. However, sampling of HZ water to measure <sup>222</sup>Rn introduce uncertainties due to difficulties of extracting sufficient water volume required for the measurement of <sup>222</sup>Rn. In the study presented here, we demonstrate the applicability of a novel passive-sampler that has never been applied in hydrologic studies before and which is based on tracing the tracks of alpha particles formed during radioactive decay of <sup>222</sup>Rn and can be directly installed in the riverbed sediment. Laboratory experiments where passive samplers were installed in water samples of known <sup>222</sup>Rn activities were conducted to calibrate the number of tracks per passive sampler [tracks cm<sup>-2</sup>] with <sup>222</sup>Rn activities [Bq m<sup>-3</sup>] to traditional measurement techniques. Results showed a good correlation between number of tracks and radon activities (R² = 0.97). Passive-samplers were installed in a vertical profile of a losing stream segment in the streambed of a first-order stream close to Tübingen (southwestern Germany). Temperature was used as comparative measurement. Number of tracks increased with depth (between 20 and 240 tracks cm<sup>-1</sup>) and, after converting tracks to <sup>222</sup>Rn activities using preceding calibration, led to activities between 450 and 5390 Bq m<sup>-3</sup>. Depth profiles of temperature had lower temperatures at the bottom and higher water temperatures in the upper sediment layers, which matched to <sup>222</sup>Rn increase and reflect the expected longer hyporheic flowpaths at depth. Evaluation of t<sub>h</sub> based on measured tracks led to residence times between 1.9 hours in the upper 5 cm of the sediment and 31.2 hours in 20 cm depth. The residence times are reasonable given the expected slow flow velocity in fine-grained riverbed sediment material. These results suggest that the new-passive samplers have a great potential for hydrologic studies, and especially in the hyporheic zone as <sup>222</sup>Rn activities can be determined <em>in-situ</em> at high spatial resolution, at relatively low cost while avoiding common difficulties of obtaining water samples of gaseous tracers from the streambed sediment.</p>