Unsaturated Zone Flow Model Research Articles

Pore water isotope fingerprints to understand the spatiotemporal groundwater recharge variability in ungauged watersheds

AbstractReliable groundwater recharge quantification at the regional scale (e.g., watershed or subwatershed) is fundamental to sustainable water resource management. Although modeling at the watershed scale is gaining wide support, the long‐term monitoring needed for model calibration is often not readily available, as many watersheds worldwide remain ungauged. In response to this situation, we propose a new approach to estimate groundwater recharge at the watershed scale. This approach is fast and accurate and takes into account the existing variability without requiring long‐term monitoring. Only a single field campaign to acquire soil water content and pore water isotopic composition depth profiles is needed. The principle is to extend a physically based, one‐dimensional unsaturated zone flow model from the local (i.e., profile) to the watershed scale, using an index method for distributed recharge based on a GIS. The methodology was validated in a gauged watershed, where previous studies have estimated recharge using a spatialized water balance model calibrated using long‐term discharge monitoring data. Scaling was investigated by comparing recharge values obtained using the local‐scale approach at 10 study sites within the watershed with coinciding values obtained at the watershed scale. Recharge values were similar in terms of both dynamics and quantity. Using the pore water isotopic fingerprint of ungauged watersheds is therefore confirmed to be a suitable approach for understanding spatiotemporal recharge variability.

Capillary geochemistry in non-saturated zone of soils. Water content and geochemical signatures

Abstract The unsaturated zone (UZ) retains aqueous solutions against gravity by capillary forces. This suction state corresponds to a decreasing internal pressure of the water, which modifies its thermodynamic properties. Accordingly, the speciation of solutes and the solubility of solids and gases in such capillary solutions change. The volumetric capillary water content of the soil at high suction can be calculated extrapolating the water retention curves (WRC) with the Rossi–Nimmo model. Interestingly, several tens of liters per cubic meter of soil can be thus suctioned, a sufficiently large volume to support that: (1) capillary water is not restricted to nanosized pores, which means it disobeys the Young–Laplace law and is metastable with respect to vapor (superheating); and (2) the geochemistry of capillary solutions might significantly influence the subsurface mass transfer. Two field situations are here interpreted using the capillary thermodynamic properties: (1) the trapping of sand grains during the growth of desert roses (gypsum), and (2) the development of abnormal paragenetic sequences in some saprolites. The capillary approach is extended to the soil solids, so that the micro-mineralogy can be explicitly (though sketchily) integrated in the calculations. The key conclusion is that capillarity changes the saturation indexes (and so the reaction rates) at given solution composition, in a way consistent with the field observations. This perspective amounts to geochemically distinguishing the capillary and percolating solutions, which is interestingly analogous to the immobile and mobile water distinction already often integrated in UZ flow models.

A modeling study of perched water phenomena in the unsaturated zone at Yucca Mountain

The purpose of this paper is to illustrate the use of a multiphase subsurface flow model to study perched water phenomena in vadose zones. Modeling studies are based on field investigations of the unsaturated zone (UZ) at Yucca Mountain, Nevada. Perched water data have been compiled, analyzed, and incorporated into a three-dimensional (3-D) UZ flow model developed to investigate perched water phenomena at Yucca Mountain. A conceptual model of perched water occurrences is discussed, and a series of comprehensive computer modeling studies on perched water at the site are presented. The model has been calibrated using perched water data observed in six boreholes, and reproduces water-perching conditions in the unsaturated zone of the mountain. Both steady-state and transient simulations have been conducted. The steady-state simulation results are in agreement with the observed perched water data, including water saturation and potential profiles and perched water elevations. Transient numerical pumping tests were performed using pumping testing data collected in the field from two boreholes. The numerical pumping test results match observed water levels collected during pumping and recovery periods.