Depth to water table correction for initial carbon-14 activities in groundwater mean residence time estimation

Carbon-14 (14C) is routinely used to determine mean residence times (MRTs) of groundwater. 14C-based MRT calculations typically assume that the unsaturated zone is in equilibrium with the atmosphere, controlling the input 14C activity. However, multiple studies have shown that unsaturated zone 14C activities are lower than atmospheric values. Despite the availability of unsaturated zone 14C data, no attempt has been made to generalise initial 14C activities with depth to the water table. We utilise measurements of unsaturated zone 14C activities from 13 studies to produce a 14C–depth relationship to estimate initial 14C activities. The technique only requires the depth to the water table at the time of sampling or an estimate of depth to water in the recharge zone to determine the input 14C activity, making it straightforward to apply. Applying this new relationship to two Australian datasets (113 14C measurements in groundwater) shows that MRT estimates were up to 9250 years younger when the 14C–depth correction was applied relative to conventional MRTs. These findings may have important implications for groundwater samples that suggest the mixing of young and old waters and the determination of the relative proportions of young and waters, whereby the estimated fraction of older water may be much younger than previously assumed. Owing to the simplicity of the application of the technique, this approach can be easily incorporated into existing correction schemes to assess the sensitivity of unsaturated zone 14C to MRTs derived from 14C data.

Depth to water table correction for initial carbon-14 activities in groundwater mean residence time estimation

Carbon-14 (14C) is routinely used to determine mean residence times (MRTs) of groundwater. 14C-based MRT calculations typically assume that the unsaturated zone is in equilibrium with the atmosphere, controlling the input 14C activity. However, multiple studies have shown that unsaturated zone 14C activities are lower than atmospheric values. Despite the availability of unsaturated zone 14C data, no attempt has been made to generalise initial 14C activities with depth to the water table. We utilise measurements of unsaturated zone 14C activities from 13 studies to produce a 14C-depth relationship to estimate initial 14C activities. The technique only requires the depth to the water table at the time of sampling, or an estimate of depth to water in the recharge zone to determine the input 14C activity, making it straightforward to apply. Applying this new relationship to two Australian datasets (113 14C measurements in groundwater) shows that MRT estimates were up to 9250 years younger when the 14C-depth correction was applied relative to conventional MRTs. These findings may have important implications for groundwater samples that suggest the mixing of young and old waters and the determination of the relative proportions of young and waters, whereby the estimated fraction of older water may be much younger than previously assumed. Owing to the simplicity of the application of the technique, this approach can be easily incorporated into existing correction schemes to assess the sensitivity of 14Cuz to MRTs derived from 14C data.

Site-level simulations of measurable soil fractions with the Millennial V2 soil model

<p>Soil carbon (C) models are used to predict C sequestration responses to climate and land use change. Yet, the soil models embedded in Earth system models typically do not represent processes that reflect our current understanding of soil C cycling, such as microbial decomposition, mineral association, and aggregation. Rather, they rely on conceptual pools with turnover times that are fit to bulk C stocks and/or fluxes. As measurements of soil fractions become increasingly available, it is necessary for soil C models to represent these measurable quantities so that model processes can be evaluated more accurately. Here we present Version 2 (V2) of the Millennial model, a soil model developed in 2018 to simulate C pools that can be measured by extraction or fractionation, including particulate organic C, mineral-associated organic C, aggregate C, microbial biomass, and dissolved organic C. Model processes have been updated to reflect the current understanding of mineral-association, temperature sensitivity and reaction kinetics, and different model structures were tested within an open-source framework. We evaluated the ability of Millennial V2 to simulate total soil organic C (SOC), as well as the mineral-associated and particulate fractions, using three independent data sets of soil fractionation measurements spanning a range of climate and geochemistry in Australia (N=495), Europe (N=176), and across the globe (N=716). Considering RMSE and AIC as indices of model performance, site-level evaluations show that Millennial V2 predicts soil organic carbon content better than the widely-used Century model, despite an increase in process complexity and number of parameters. Millennial V2 also reproduces between-site variation in SOC across gradients of climate, plant productivity, and soil type. By including the additional constraints of measured soil fractions, we can predict site-level mean residence times similar to a global distribution of mean residence times measured using SOC/respiration rate under an assumption of steady state. The Millennial V2 model updates the conceptual Century model pools and processes and represents our current understanding of the roles that microbial activity, mineral association and aggregation play in soil C sequestration.</p>

Effect of sediment-organism interactions on hyporheic exchange in streams: role of sediment reworking time

<p>In-stream faunal organisms constantly interact with their habitat to modify its physical and hydraulic properties. However, little is known about how sediment-organism interactions could modify the hyporheic exchange. Previous experimental work investigating the effects of the activities of faunal organisms on exchange across the sediment-water interface has been largely conducted in small mesocosms or infiltration columns that do not represent the lotic environment adequately. Therefore, the experimental findings from these studies may not be transferable to flowing water environments (e.g., streams). Our previous experimental work demonstrated that sediment reworking by macroinvertebrates could significantly alter the hyporheic flux, mean residence times, and depth of exchange in streambeds. In this work, we explore how sediment-organism contact time influence the effect of the activities of model organisms, Lumbriculus variegatus, on the hyporheic flow regime. We conduct laboratory experiments in re-circulating flumes subject to different sediment reworking times (5 and 10 days). The hyporheic flow characteristics in these flumes were studied by conducting dye tracer tests after the bed sediments were reworked. Deposition of fecal pellets and holes/burrows dug by sample organisms were visible at the bed surface in both the experimental flumes. The flume reworked for a longer time exhibited higher hyporheic flux, longer median/mean residence times, and deeper depth of solute penetration compared to the flume reworked for a shorter period. The modification of hyporheic flow regime to different degrees depending on the sediment reworking times has direct relevance to the biogeochemistry in hyporheic zones, and thus on the overall quality of surface and sub-surface waters. We advocate that more intensive laboratory experiments and field investigations must be conducted to support the findings from our study and advance our understanding of the role of the activities of faunal organisms on fluvial ecosystem functioning.</p>

Renewed thinking on groundwater age

<p>Groundwater age and mean residence times have been invoked as measures of groundwater sustainability, with the idea that old or "fossil" groundwater is non-renewable. This idea appears to come from the link between groundwater age and background recharge rates, which are also of questionable use in assessing the sustainability of groundwater withdrawals. The use of groundwater age to assess renewability is further complicated by its relationship with flow system geometry. Young groundwaters near recharge areas are not inherently more renewable than older groundwaters down gradient. Similarly, there is no reason to preferentially use groundwater from smaller aquifers, which will have smaller mean residence times than larger aquifers for the same recharge rate. In some cases, groundwater ages may provide some information where groundwater recharge rates were much higher in the past and systems are no longer being recharged. However, there are few examples where the relationship between depletion and changes in recharge over long time periods has been rigorously explored. Groundwater age measurements can provide insights into the functioning of groundwater flow systems and calibration targets for numerical models and we advocate for their continued use, but they are not a metric of sustainable development. Simple metrics to assess groundwater sustainability remain elusive and a more holistic approach is warranted to maintain water levels and environmental flows.</p>

Exploring controls of timber stock residence times in storage after severe storm events

The storage of significant amounts of timber from thrown or dead trees after natural disturbances is an established practice for forest enterprises. Timber storage mitigates economic losses caused by supply-driven timber price falls after natural disturbances. We use a forest accounting database to explore the controls of residence times of coniferous timber stocks in storage following severe storm events. We characterize forest enterprises’ timber stock outflow distributions from storage over several years by mean residence times and their variances. We conduct regression analyses on the expected residence times and their variances. We assess the significance of several explanatory variables representing economic, institutional and tree species-related factors on these metrics using multiple linear regression analyses. Illustrating the effect of these variables on timber storage residence time distributions we reanalyze the database by grouping the FADN data sets with regard to the identified control variables and determine their mean timber storage outflow distributions after the storm events as well as associated expected residence times and their variances. Applying the resulting parameters with the continuous gamma distribution to simulate TSO residence time distributions clearly illuminates the effect of the control variables on storage management. We show that besides market price dynamics, species groups, ownership categories and forest worker capacities are statistically significant controls for mean residence times of timber stock in storage and their variances. We find that stronger timber price falls correlate with shorter mean residence times of timber stocks in storage. We relate this to liquidity maintenance of forest enterprises. We model duration times parameterizing the Gamma distribution. The application of the Gamma distribution to characterize storage management behavior offers the potential to describe differences in timber stock quantities even on shorter timescales than the mean storage residence times. According to our results, we propose to assess timber stocks in storage over a multi-year period in order to improve related national and international accounting schemes.

Isotope-age-dating of alpine spring water and global change : Evidence from temperature, chemistry and tritium data

<p>Global air-temperature changes over the last 150 years and in particular during the last 30 – 40 years are well documented world-wide. In alpine areas in Europe the increase in air-temperature is even higher in the range of 2° C. Very few studies exist about groundwater temperature changes due to global warming. The increase or decrease in temperature at the point of discharge depends besides the air temperature at the time of infiltration on the amount of precipitation, the local meteorological conditions, the mean residence time, the land use, and the natural and anthropogenic heat flow during the passage underground.</p><p>Nearly no papers exist about the water quality changes due to global change impacts and Mean Residence Times (MRT). This is very difficult to evaluate due to missing long-term quality measurements and strong impacts by anthropogenic activities and land use changes. To avoid the complication by anthropogenic land use changes and activities the authors investigated the on-line discharge, temperature, and electric conductivity measurements as well as quarterly hydro-chemical and isotope analyses of 40 Alpine springs from a monitoring network all over the Austrian Alps (approx. 60,000 km<sup>2</sup>). All the selected springs have a recharge area with no or minimal anthropogenic impacts during the last 30 – 40 years. About 235,000 on-line measurements and 11,000 chemical analyses were evaluated for trends and compared to daily measurements at meteorological and surface water stations close to the recharge areas of the springs. To show the connection to the paleoclimatology changes of existing δ<sup>18</sup>O measurements on precipitation and spring water was evaluated as well indicating altitudes of recharge areas in range of 500 – 2400m.</p><p>Forty springs with a minimum record of 16 years have been selected for trend analysis over a period of 20 years (1993 – 2013). 28 (74%) of the selected spring show a significant mean increase in water temperature of 0.34 °C in the range of 0.06 to 1.03 °C. This increase is half of the air- and water temperature increase in meteorological stations and surface waters close to the recharge areas of the investigated springs. The electric conductivity linearly increased in 21 (55%) of the investigated springs at about 4%. The discharge stayed the same in most springs. In 23 (72%) springs the content of dissolved oxygen decreased over these 20 years at about 9% percent.</p><p>The reasons of the changes in water-temperature, dissolved load and the oxygen content as well as the impact of different Mean Residence Times (MRT) will be discussed and interpreted.</p>

Groundwater mean residence times of a subtropical barrier sand island

Fresh groundwater on barrier islands is affected by changing sea levels and precipitation variability due to climate change and is also vulnerable to anthropogenic processes, such as contamination and groundwater over-abstraction. Constraining groundwater mean residence times (MRTs) and flow paths is essential for understanding and managing these resources. This study uses tritium (3H) and carbon-14 (14C) to determine the MRTs of groundwater along a transect across subtropical North Stradbroke Island, south-east Queensland, Australia. Hydraulic properties, major ion geochemistry and stable isotopes are used to validate residence times and to identify the processes responsible for their variability. 3H activities range from less than 0.01 to 1 TU (tritium units), which are values lower than those of local average rainfall (1.6–2.0 TU). 14C concentrations range from 62.5 to 111 pMC (percent modern carbon). Estimated MRTs determined using lumped parameter models and 3H activities range from 37 to more than 50 years. Recharge occurs over the entire island, and groundwater MRTs generally increase vertically and laterally towards the coastal discharge areas, although no systematic pattern is observed. MRTs estimated from 14C concentrations display similar spatial relationships but have a much greater range (from modern to approximately 5000 years). Water diversion and retention by lower-permeability units in the unsaturated parts of the dune systems are the most likely course for relatively long MRTs to date. The results indicate that the internal structures within the dune systems increase MRTs in the groundwater system and potentially divert flow paths. The structures produce perched aquifer systems that are wide-spread and have a significant influence on regional recharge. The geochemical composition of groundwater remains relatively consistent throughout the island, with the only irregularities attributed to old groundwater stored within coastal peat. The outcomes of this study enhance the understanding of groundwater flow, recharge diversion and inhibition for large coastal sand masses in general, especially for older sand masses that have developed structures from pedogenesis and dune movement. With respect to south-east Queensland, it allows the existing regional groundwater flow model to be refined by incorporating independent MRTs to test models' validity. The location of this large fresh groundwater reservoir, in dry and populous south-east Queensland, means that its potential to be used as a water source is always high. Background information on aquifer distribution and groundwater MRTs is crucial to better validate impact assessment for water abstraction.