Quantifying residence times of bank filtrate: A novel framework using radon as a natural tracer.

Bank filtration is a cost-effective and sustainable method of improving surface water quality for drinking water production. During aquifer transit, natural biodegradation and physiochemical filtration improve the quality of the raw water by removing sediments, pollutants, and pathogens. Strict regulations prohibit the use of substances that can be used to estimate aquifer residence times to define water protection areas for bank filtration. In this study, we present a novel measurement and modeling framework for deriving mean aquifer residence times for bank filtrate using the natural tracer radon-222. The method is intended for application in the drinking water sector, where extraction wells are screened over the entire aquifer and pumps are operated at high production rates. Mean aquifer residence times are estimated using composite residence time distributions that account for flow path mixing and non-uniform residence times with multiple components including bank filtrate, shallow groundwater, and deep groundwater. The mathematical framework is demonstrated for a drinking water production facility. Radon activities for the six monitored extraction wells ranged between 4,400 and 8,400 Bq/m³. Estimated mean aquifer residence times for the wells range from < 5 days to 110 days and strongly depend on i) the type of residence time distribution model (exponential, gamma or piston flow), ii) the mixing ratio between bank filtrate and local groundwater, and iii) the heterogeneity in the groundwater endmember. By accounting for mixing processes, we can show that radon can be used beyond the "5-fold half-life" (~20 days) commonly described in the literature as the upper limit for age dating purposes for radon. This method provides a simple and cost-efficient way to quantify residence times of bank filtrate on a regular basis without any addition of external substances to the aquifer.

A method for long-term high resolution 222Radon measurements using a new hydrophobic capillary membrane system.

Radon (R86222n) as a hydrological tracer offers a method for studying short to medium term groundwater - surface water interactions. These high frequency processes play an important role in wetland hydrology and biogeochemistry and may influence their contribution to the global carbon cycle. Therefore, there is a definite need for robust methods to measure high resolution 222Rn time series in-situ. In this study we adapted and improved a membrane system to measure 222Rn continuously with a primary focus on a rapid response time and low power consumption. The membrane system was constructed using a hydrophobic capillary membrane and laboratory experiments were conducted to quantify the systems' response time to predefined 222Rn pulses. It was then deployed in a stream draining a riparian wetland. The new membrane system could reduce the response time by ≈ 60 % in comparison to the established silicone membrane. We could identify the behaviour of the system in response to dynamically changing 222Rn activities and suggest a new method using simple linear regression to quantify the systems' response when the response time concept is inapplicable. Finally, we were able to measure high temporal resolution 222Rn activities reliably over an extended field deployment (68 d). We conclude that the improved system fills a gap ensuring high temporal resolution while maintaining extended maintenance intervals. This allows the user to study high frequency hydrological processes in remote areas. This new membrane system can be used to detect fast changes in 222Rn activities improving the comprehension of the underlying hydrological processes.

Solar Output Controls Periodicity in Lake Productivity and Wetness at Southernmost South America.

Cyclic changes in total solar irradiance (TSI) during the Holocene are known to affect global climatic conditions and cause cyclic climatic oscillations, e.g., Bond events and related changes of environmental conditions. However, the processes how changes in TSI affect climate and environment of the Southern Hemisphere, especially in southernmost South America, a key area for the global climate, are still poorly resolved. Here we show that highly sensitive proxies for aquatic productivity derived from sediments of a lake near the Chilean South Atlantic coast (53 °S) strongly match the cyclic changes in TSI throughout the Holocene. Intra-lake productivity variations show a periodicity of ~200-240 years coherent with the time series of TSI-controlled cosmogenic nuclide 10Be production. In addition TSI dependent periodicity of Bond events (~1500 years) appear to control wetness at the LH site indicated by mineral matter erosion from the catchment to the lake assumingly through shifts of the position of the southern westerly wind belt. Thus, both intra-lake productivity and wetness at the southernmost South America are directly or indirectly controlled by TSI.