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

Abstract

Tritium measurements of streamwater draining the Toenepi catchment, a small dairy farming area in Waikato, New Zealand, have shown that the mean transit time of the water varies with the flow of the stream. Mean transit times through the catchment are 2–5 years during high baseflow conditions (in winter), becoming older as streamflow decreases (in summer), and then quite dramatically older during drought conditions, with ages of more than 100 years. Older water seems to be gained in the lower reaches of the stream, compared to younger water in the headwater catchment. The groundwater store supplying baseflow was estimated from the mean transit time and average baseflow to be 15.4×10<sup>6</sup> m<sup>3</sup> of water, about 1 m water equivalent over the catchment and 2.3 times total annual streamflow. Nitrate from recent intensified land use is relatively high at normal streamflow, but is low at times of low flow with old water. This reflects both lower nitrate loading in the catchment several decades ago, and active denitrification processes in older groundwater. Silica, leached from the aquifer material and accumulating in the water in proportion to contact time, is high at times of low streamflow. There was a good correlation between silica and streamwater age, which potentially allows silica concentrations to be used as a proxy for age when calibrated by tritium measurements. This study shows that tritium dating of stream water is possible with single tritium measurements now that bomb-test tritium has effectively disappeared from hydrological systems in New Zealand, without the need for time-series data

Concentration versus streamflow trends of major ions and tritium in headwater streams as indicators of changing water stores

“…Monthly variations of rainfall 3 H activities in southeast Australia in any given year are <1 TU, and the 3 H activities in summer rainfall are close to the annual average (Cartwright, Irvine, et al, ; Cartwright & Morgenstern, ; Tadros et al, ). With such a seasonal distribution of 3 H activities, the uncertainties resulting from using the average annual 3 H activity are minor (cf., Morgenstern et al, ). The analytical uncertainty of 2% to 3% also results in a minor uncertainty in MTT.…”

“…Determining the transit times at different streamflows is also invaluable in distinguishing between the possible causes of chemostatic behaviour. Whereas the use of 3 H to estimate MTTs from individual samples is currently only possible in the southern hemisphere, the decrease of the bomb‐pulse peak will allow a similar approach in northern hemisphere catchments in the next few years to decades (Morgenstern et al, ).…”

“…In the southern hemisphere, the higher 3 H activities of water recharged during atmospheric nuclear tests in the 1950s and 1960s (the “bomb‐pulse” 3 H) have largely decayed to below those of recent rainfall (Clark & Fritz, ; Morgenstern et al, ). This permits MTTs to be estimated from individual samples, which in turn allows the variation of MTTs with streamflow to be determined (Morgenstern et al, ). Unlike dissolved gas tracers (e.g., the chlorofluorocarbons or SF 6 ) or other radioisotopes such as 14 C, 3 H is part of the water molecule and its activities are not impacted by exchange with the atmosphere or biogeochemical reactions in the soils or aquifers.…”

“…Because of the decline of the bomb‐pulse 3 H, older southern hemisphere waters have lower 3 H activities than younger waters. This permits relative MTTs of water contributing to streamflow may be readily determined even where calculating precise MTTs is difficult (Cartwright, Irvine, Burton, & Morgenstern, ; Cartwright & Morgenstern, , ; Morgenstern et al, ; Stewart, Morgenstern, Gusyev, & Maloszewski, ).…”

A multi-tracer approach to quantifying groundwater inflows to an upland river; assessing the influence of variable groundwater chemistry

“…Tritium ( 3 H) is another powerful tracer for determining the movement and residence time of water in the hydrosphere (Morgenstern et al, 2010). 3 H has a half life of 12.43 years and is generally used as a technique for dating recently (<100 years) recharged groundwater (Han et al, 2012;Manning et al, 2012).…”