![]() The mean age (b) uses the average of that distribution as age of the sample. Thus, a water sample does not have an age, but a distribution of ages. While this definition is convenient to estimate fluxes, advective age is unmeasurable: samples often include water from different flow paths moving at different speeds that mix through diffusion and dispersion. The idealized or advective age (a) is the time needed by a molecule of water to reach the sampling location from the recharge zone. Recharge in particular can be estimated by deriving an age of groundwater from the concentration of environmental tracers.Īge is an abstract concept in the case of water, and three main definitions can be formulated (McCallum et al., 2015 Suckow, 2014). These chemical compounds or isotopes get into aquifers naturally with recharge water, and their variations in concentration help characterize fluxes in aquifers (Cook & Herczeg, 2000 Kazemi et al., 2006). Environmental tracers play a substantial role in that perspective (Green et al., 2011). In this context, ensuring a sustainable management of groundwater resources becomes even more critical, but remains inconceivable without reliable estimates of fluxes in groundwater systems. Simulating so many models highlights how river evolution affects recharge estimates, pointing out the importance of considering the geological context.Īs anthropogenic and climatic stresses keep increasing, they limit inflows into groundwater systems while prompting their over-exploitation (e.g., de Graaf et al., 2019 Döll, 2009 Vörösmarty et al., 2000). Local errors range from close to zero to several thousand percent, so sampling from a few locations - a common setting in hydrogeological studies - could lead to high discrepancies. Then, we simulated groundwater flow and age and computed the error when estimating recharge. We started by simulating the evolution of meandering rivers and their deposits over tens of thousands of years using different parameters to obtain 20,000 3D models, each with different distribution and extent of heterogeneities. This assumption is unlikely to be met, so we created virtual aquifers to assess the impact of geological heterogeneities on recharge estimates. It assumes a homogeneous system, so the rocks or sediments bearing the water must have the same properties everywhere. One way to estimate this recharge uses groundwater age: the time since a parcel of water entered a system. Its sustainability depends on reliable estimates of the amount of water replenishing groundwater systems. But population growth and global warming put an ever-increasing stress on this vital resource. ![]() ![]() Key Pointsĭespite being hidden underground, groundwater is a major source of freshwater worldwide. This work paves the way for a more reliable estimation of recharge by considering the sedimentary context. For instance, a high bank erodibility and small coarse grain size under low aggradation rates favor the reworking of deposits, which increases heterogeneity and leads to high errors. A sensitivity analysis using the δ-importance measure and CUSUNORO curves exposes how fluvial processes influence the range and distribution of the error. But fluvial heterogeneities can lead to local errors ranging from close to zero to several thousand percent, which means that estimates may vary drastically depending on sampling locations. For most models, the mean estimate of the recharge rate remains below an absolute error of 25%. Our results are based on 20,000 realizations from varying inputs: grain sizes, aggradation rate of the river, porosity, recharge, etc. This reproduces the boundary conditions underlying Vogel's analytical model to estimate recharge rates. Finally, we use PFLOTRAN to simulate groundwater flow and mean age over two thousand years of spatially uniform recharge. Then, we use the fractional packing model to compute porosity and permeability of unconsolidated sediments assuming a mixture of a coarse and a fine fraction. First, we use CHILD, a landscape evolution model, to generate plausible deposits after tens of thousands of years of river evolution. We propose to assess how this assumption affects recharge estimates in fluvial deposits. As such, recharge can be estimated from groundwater age using analytical models assuming a homogeneous aquifer. Sustainable management of groundwater relies on robust estimates of aquifer fluxes.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |