Dating of streamwater using tritium in a post nuclear bomb pulse world: continuous variation of mean transit time with streamflow

272

443

Abstract. The complex interactions of runoff generation processes underlying the hydrological response of streams remain not entirely understood at the catchment scale. Extensive research has demonstrated the utility of tracers for both inferring flow path distributions and constraining model parameterizations. While useful, the common use of linearity assumptions, i.e. time invariance and complete mixing, in these studies provides only partial understanding of actual process dynamics. Here we use long-term (< 20 yr) precipitation, flow and tracer (chloride) data of three contrasting upland catchments in the Scottish Highlands to inform integrated conceptual models investigating different mixing assumptions. Using the models as diagnostic tools in a functional comparison, water and tracer fluxes were then tracked with the objective of exploring the differences between different water age distributions, such as flux and resident water age distributions, and characterizing the contrasting water age pattern of the dominant hydrological processes in the three study catchments to establish an improved understanding of the wetness-dependent temporal dynamics of these distributions.The results highlight the potential importance of partial mixing processes which can be dependent on the hydrological functioning of a catchment. Further, tracking tracer fluxes showed that the various components of a model can be characterized by fundamentally different water age distributions which may be highly sensitive to catchment wetness history, available storage, mixing mechanisms, flow path connectivity and the relative importance of the different hydrological processes involved. Flux tracking also revealed that, although negligible for simulating the runoff response, the omission of processes such as interception evaporation can result in considerably biased water age distributions. Finally, the modeling indicated that water age distributions in the three study catchments do have long, power-law tails, which are generated by the interplay of flow path connectivity, the relative importance of different flow paths as well as by the mixing mechanisms involved. In general this study highlights the potential of customized integrated conceptual models, based on multiple mixing assumptions, to infer system internal transport dynamics and their sensitivity to catchment wetness states.

Section: Hrachowitz Et Al: What Can Flux Tracking Teach Us Aboutmentioning

confidence: 99%

What can flux tracking teach us about water age distribution patterns and their temporal dynamics?

Hrachowitz

Savenije

Bogaard

et al. 2013

272

443

“…Objective 14 C dating requires recognition and quantification of these processes. A number of models based on both major ion and stable C isotope geochemistry have been proposed to correct apparent 14 C ages (Han and Plummer, 2013) With a significantly shorter half-life (12.33 years), 3 H can be used to date groundwater with residence times of up to 100 years (Vogel et al, 1974).With the decay of the 1960s 3 H bomb-pulse peak in the Southern Hemisphere to near background levels, unique ages may now be determined from single 3 H measurements (Morgenstern et al, 2010). As 3 H is part of the water molecule, there is negligible change to 3 H activities other than decay, and 3 H is an excellent tracer for the movement of water through hydrological systems (Michel, 2004).…”

Section: Introductionmentioning

confidence: 99%

Using <sup>14</sup>C and <sup>3</sup>H to understand groundwater flow and recharge in an aquifer window

Atkinson

Cartwright

Gilfedder

et al. 2014

Abstract. Knowledge of groundwater residence times and recharge locations is vital to the sustainable management of groundwater resources. Here we investigate groundwater residence times and patterns of recharge in the Gellibrand Valley, southeast Australia, where outcropping aquifer sediments of the Eastern View Formation form an "aquifer window" that may receive diffuse recharge from rainfall and recharge from the Gellibrand River. To determine recharge patterns and groundwater flow paths, environmental isotopes (3H, 14C, δ13C, δ18O, δ2H) are used in conjunction with groundwater geochemistry and continuous monitoring of groundwater elevation and electrical conductivity. The water table fluctuates by 0.9 to 3.7 m annually, implying recharge rates of 90 and 372 mm yr−1. However, residence times of shallow (11 to 29 m) groundwater determined by 14C are between 100 and 10 000 years, 3H activities are negligible in most of the groundwater, and groundwater electrical conductivity remains constant over the period of study. Deeper groundwater with older 14C ages has lower δ18O values than younger, shallower groundwater, which is consistent with it being derived from greater altitudes. The combined geochemistry data indicate that local recharge from precipitation within the valley occurs through the aquifer window, however much of the groundwater in the Gellibrand Valley predominantly originates from the regional recharge zone, the Barongarook High. The Gellibrand Valley is a regional discharge zone with upward head gradients that limits local recharge to the upper 10 m of the aquifer. Additionally, the groundwater head gradients adjacent to the Gellibrand River are generally upwards, implying that it does not recharge the surrounding groundwater and has limited bank storage. 14C ages and Cl concentrations are well correlated and Cl concentrations may be used to provide a first-order estimate of groundwater residence times. Progressively lower chloride concentrations from 10 000 years BP to the present day are interpreted to indicate an increase in recharge rates on the Barongarook High.

“…Lumped-parameter models are commonly used to describe groundwater flow in shallow unconfined and semi-confined aquifers Zuber, 1991, 1982;Morgenstern et al, 2010;Zuber et al, 2005). Piston flow models assume that no mixing takes place between recharge and water in the aquifer and are suitable for settings where dispersion is low.…”

Section: Aquifer Interactionsmentioning

confidence: 99%

“…Conversely, exponential flow models assume a vertical stratification of groundwater ages in an aquifer and are suitable for the sampling of fully penetrating wells or surface water bodies fed by aquifers receiving homogeneous recharge. This study uses the exponential piston flow model (EPFM) which combines a portion of piston flow followed by a portion of exponential flow and is appropriate for bores in unconfined to semi-confined aquifers screened below the water table that do not sample the shallowest groundwater that has very short residence times (Morgenstern et al, 2010;Cartwright and Morgenstern, 2012). For the EPFM the activity of 3 H or 14 C at time t (C t ) is given by…”

Section: Aquifer Interactionsmentioning

confidence: 99%

Residence times and mixing of water in river banks: implications for recharge and groundwater–surface water exchange

Unland

Cartwright

Cendón

et al. 2014

Abstract. Bank exchange processes within 50 m of the Tambo River, southeast Australia, have been investigated through the combined use of 3 H and 14 C. Groundwater residence times increase towards the Tambo River, which suggests the absence of significant bank storage. Major ion concentrations and δ 2 H and δ 18 O values of bank water also indicate that bank infiltration does not significantly impact groundwater chemistry under baseflow and post-flood conditions, suggesting that the gaining nature of the river may be driving the return of bank storage water back into the Tambo River within days of peak flood conditions. The covariance between 3 H and 14 C indicates the leakage and mixing between old (∼ 17 200 years) groundwater from a semiconfined aquifer and younger groundwater (< 100 years) near the river, where confining layers are less prevalent. It is likely that the upward infiltration of deeper groundwater from the semi-confined aquifer during flooding limits bank infiltration. Furthermore, the more saline deeper groundwater likely controls the geochemistry of water in the river bank, minimising the chemical impact that bank infiltration has in this setting. These processes, coupled with the strongly gaining nature of the Tambo River are likely to be the factors reducing the chemical impact of bank storage in this setting. This study illustrates the complex nature of river groundwater interactions and the potential downfall in assuming simple or idealised conditions when conducting hydrogeological studies.