Evaluation of dissolved 4He as a groundwater age tracer in shallow fractured aquifers

<p>Anthropogenic gas tracers such as CFC, SF<sub>6</sub>, <sup>85</sup>Kr, <sup>36</sup>Cl or <sup>3</sup>H have been widely used to study shallow groundwaters with residence time of less than 70 yr. For longer groundwater residence time (100- x1000 yr), <sup>39</sup>Ar, <sup>14</sup>C, <sup>36</sup>Cl and <sup>4</sup>He have been used. <sup>4</sup>He can cover a dating range of 10 to thousands of years (Solomon et al., 1996). The main difficulty is to estimate the production rate through U and Th decay and the others fluxes: atmosphere, lithosphere and asthenosphere. In many cases U-Th production is not sufficient to explain the <sup>4</sup>He concentrations observed in the aquifer. Other <sup>4</sup>He fluxes can then be estimated through the use of other tracers: <sup>14</sup>C, <sup>36</sup>Cl or modeling. Fracturing may also enhance <sup>4</sup>He concentrations in groundwater.</p><p>We present here the evaluation of <sup>4</sup>He in a crystalline fractured aquifer in the Northwest of France (H+ national hydrogeological network), in order to investigate the range of groundwater residence time in this complex shallow aquifer. Previous studies on this aquifer reveal mixing between young (<70 yrs) and old waters (>1000 yrs) (Ayraud et al., 2008). The Helium radiogenic production rate is then evaluated through in situ production (U, Th, porosity), calibration with CFC and <sup>14</sup>C, and modelling of the diffusion processes affecting <sup>14</sup>C and <sup>4</sup>He through physical characteristics of the aquifer (porosity, fracture spacing and aperture). Young groundwater residence times estimated by <sup>4</sup>He agree with those estimated by CFC and <sup>3</sup>H/<sup>3</sup>He. In this fractured media, old groundwater residence times (> 100 yr) are better estimated through the integration of the mass transfer between the fractures and the porous rock matrix through diffusion processes. <sup>4</sup>He proves to be a valuable tool to characterize groundwater mixing processes and groundwater residence times from a decade to thousands of years.</p><p>Solomon, D. K., Hunt, A., & Poreda, R. J. (1996). Source of radiogenic helium 4 in shallow aquifers: Implications for dating young groundwater. Water Resources Research, 32(6), 1805-1813.</p><p>Ayraud, V., Aquilina, L., Labasque, T., Pauwels, H., Molenat, J., Pierson-Wickmann, A. C., ... & Fourre, E. (2008). Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses. Applied geochemistry, 23(9), 2686-2707.</p>

Variability of Cosmogenic 35S in Rain—Resulting Implications for the Use of Radiosulfur as Natural Groundwater Residence Time Tracer

Information about groundwater residence times is essential for sustainable groundwater management. Naturally occurring radionuclides are suitable tools for related investigations. While the applicability of several long-lived radionuclides has been demonstrated for the investigation of long residence times (i.e., years, decades, centuries and more), studies that focus on sub-yearly residence times are only scarcely discussed in the literature. This shortage is mainly due to the rather small number of radionuclides that are generally suitable for the purpose and show at the same time adequately short half-lives. A promising innovative approach in this regard applies cosmogenic radiosulfur (35S). 35S is continuously produced in the stratosphere from where it is conveyed to the troposphere or lower atmosphere and finally transferred with the rain to the groundwater. As soon as the meteoric water enters the subsurface, its 35S activity decreases with an 87.4 day half-life, making 35S a suitable time tracer for investigating sub-yearly groundwater ages. However, since precipitation shows a varying 35S activity during the year, setting up a reliable 35S input function is required for sound data evaluation. That calls for (i) an investigation of the long-term variation of the 35S activity in the rain, (ii) the identification of the associated drivers and (iii) an approach for setting up a 35S input function based on easily attainable proxies. The paper discusses 35S activities in the rain recorded over a 12-month period, identifies natural and anthropogenic influences, and suggests an approach for setting up a 35S input function applying 7Be as a proxy.

Re-evaluation of groundwater residence time using a combined 3H/3He, 14C and 222Rn approach

<p>The TMG aquifer is one of the largest aquifer systems in South Africa and is currently targeted as a potential source of potable water for the City of Cape Town (CoCT) which recently experienced a period of extreme water stress. Groundwater in the TMG aquifer typically has very low total dissolved salts, on the order of 50 mg/L of less, making it challenging to constrain the groundwater residence time. However, residence time is a key parameter to provide proper constraints on turnover time of groundwater in the aquifer system before large-scale abstraction takes place, in order to evaluate the sustainability of the resource. This study used the <sup>3</sup>H/<sup>3</sup>He system to date modern water (<100 years) and <sup>14</sup>C to date older groundwater (>500 years). Groundwater residence times were determined for the TMG aquifer and five associated aquifer systems in the Western Cape of South Africa, namely the alluvial, Witteberg, Bokkeveld, Cape Granite Suite (CGS) and Malmesbury aquifers. Good correlation between <sup>3</sup>H/<sup>3</sup>He and <sup>14</sup>C ages indicate relatively short residence times for the alluvial and TMG aquifers whereas groundwater from the Witteberg, Bokkeveld, CGS and Malmesbury aquifers indicate mixing of older water bodies with modern recharge resulting in distinctly different ages derived from the two dating systems. In an attempt to better constrain the mixing relationship with modern precipitation, <sup>222</sup>Rn was used to assess the interaction between precipitation and groundwater after rainfall events. The basis for this approach comes from the assumption that precipitation has little <sup>222</sup>Rn in it, with groundwater <sup>222</sup>Rn derived from interaction with the groundwater host rocks. This should result in groundwater <sup>222</sup>Rn activity being diluted through recharge with precipitation. However, since the half-life of <sup>222</sup>Rn is only 3.82 days, <sup>222</sup>Rn activities should respond rapidly to recharge, and should also recover rapidly from this recharge. Three behavioural characteristics were established; (1) groundwaters where the <sup>14</sup>C activity was of ≥ 100 pMC (TMG and alluvial aquifers), and where an immediate dilution in radon’s activity was recorded due to direct recharge. (2) groundwaters where the <sup>14</sup>C activity was 80% – 90% pMC (Malmesbury aquifer) where a delayed response in the dilution of radon’s activity was recorded; and (3) groundwaters where the <sup>14</sup>C activity was ≤ 70% and radon activities were stable indicating little or no recharge. <sup>222</sup>Rn thus proved an important mechanism for evaluating the validity of residence times derived from both <sup>3</sup>H/<sup>3</sup>He and <sup>14</sup>C.</p>