The 2014 water release into the arid Colorado River delta and associated water losses by evaporation

Groundwater recharge sites and pollution sources in the wine-producing Guadalupe Valley (Mexico): Restrictions and mixing prior to transfer of reclaimed water from the US-México border

Rapid depletion of aquifers in semiarid and arid regions threatens water security. This holds true especially in emerging countries where insufficient knowledge about aquifer systems precludes the implementation of advanced management measures, such as managed aquifer recharge. This study deals with the generation of baseline knowledge for the assessment of aquifers in arid and semiarid regions where artificial recharge with reclaimed water gains increasing impetus. The Guadalupe aquifer in Baja California provides water to 57% of the Mexican wine industry. Recent plans foresee a partial replenishment of its depleted groundwater reserves by transferring treated waste water from the Mexico-USA border for irrigation. The aquifer demonstrated to have a rapid response by rising the water table of about +20 m in relation to natural recharge under an intense rainfall period of 236 mm. Two predominant recharge sources were identified based on a geochemical multi-tracer approach: (a) water of modern age (<5 yr, >1.8 TU) and mixed water of recent-submodern age (³H 0.8-1.8 TU), and (b) sub-modern waters that were recharged before 1952 (³H < 0.5 TU). Water of the first type originate in the main Guadalupe stream, which has a more depleted average δ¹⁸O isotope value (-7.8‰) than average local rainwater (-2.0‰). The stream water initially has a Na-HCO₃ composition and recharges the entire Calafia zone and most groundwater along the riverbed across the valley. Water of the second type is mostly derived from hill-slope groundwater that has a stable isotope composition of mixed local rainwater and a NaCl composition. High total dissolved solids >2 g l^-1 together with enriched NO₃^- and Se concentrations characterize groundwater in the downstream the Porvenir zone. The geochemical age of this older, hill-slope groundwater suggests that its replenishment takes at least several decades when it becomes exhausted.

What Discharge Is Required to Remove Silt and Sand Downstream from a Dam? An Adaptive Approach on the Selves River, France

An increasing number of scientific studies are tackling the management of discharges downstream of dams for environmental objectives. Such management is generally complex, and experiments are required for proper implementation. This article present the main lessons from a silt sand removal experiment on a bypassed reach of a dam on the Selves River (164 km²), France. Three four-hour operational tests at maximum discharge (10, 15, and 20 m3/s) were carried out in September 2016 to determine the discharge required for transporting as much silt and sand as possible without remobilizing coarser sediments. In September 2017, an additional flow release was performed over 34 h at 15 m3/s. Suspended sediment concentration and water level were recorded throughout the releases. Monitoring at the reach scale was supplemented by morphological measurements. The results demonstrate that a discharge of approximately 10 m3/s enables significant transport of suspended sediments (SS), whereas a discharge of 15 m3/s enables significant sand transport. The results provide operational information on silt and sand transport applicable to other small rivers. This study represents an important contribution to the relatively sparse existing body of literature regarding the effects of water releases and sediment state. Our study also demonstrates that it is possible to successfully undertake water releases in small rivers with an adaptive management approach.

Evaluation of the ecological protective effect of the "large basin" comprehensive management system in the Tarim River basin, China

It is very important to construct a reasonable and efficient basin management system to meet the ecological water demand in arid areas with natural vegetation, and to maintain the integrity and stability of fragile ecosystems. However, how to assess the effect of basin management on ecological protection in arid areas as well as how to achieve the optimal control and efficient use of ecological water are major issues for many researchers and river basin managers. To address these two questions, we investigated the comprehensive management system for the Tarim River basin in China as a typical case study. The results showed that the natural vegetation coverage degree, the ecological water supply, temperature vegetation dryness index (TVDI), and the tree-ring chronology of Populus euphratica increased, whereas the disturbance of water resources by human activities decreased. Therefore, the effects of ecological protection were obvious after comprehensive "large basin" management. Based on an innovative application of tree-ring chronology to estimate the water leakage from the river, we determined the minimum runoff level (43.1 × 10⁸ m³) when the natural vegetation needs to overflow. To further improve the effect of comprehensive management, the optimal regulation mode (i.e. maintaining the groundwater depth at 2-6 m, and the frequency and duration of overflowing at 2-3 times per year for a duration of 15-20 days during July to September) for the ecological sluices was formulated from the perspective of the efficient utilization of ecological water. These results provide a scientific reference for constructing reasonable management systems for similar river basins in arid areas.

Arsenic-rich shallow groundwater in sandy aquifer systems buffered by rising carbonate waters: A geochemical case study from Mannar Island, Sri Lanka

Major ion, trace elements, and stable isotope analyses were performed on groundwater samples collected from Mannar Island in the northern Indian Ocean. Arsenic concentrations up to 34μg/L have been observed in groundwater samples from the island. In addition, 23% of extensively used shallow drinking water wells showed higher arsenic contents than the recommended value by the World Health Organization (10μg/L). Groundwater in the island showed pH values between 6.9 and 8.9 and was dominated by Na⁺, K⁺, Ca²⁺, Mg²⁺, HCO₃^-, Cl^- and SO₄^2-. The δ¹⁸O_H2O and δ²H_H2O composition of most groundwater plotted very close to the local meteoric waterline, however, some wells showed enriched isotope compositions that are most likely due to evaporation. Sea water intrusion in this island was likely of minor importance as indicated by the major ion composition. An approximated mass balance calculation using chloride concentrations indicated that out of the 35 investigated wells only 6 near-shore wells were influenced by sea water intrusion up to about 15%. Even though this is a sandy aquifer, groundwaters were characterized with higher contents of dissolved inorganic carbon (DIC) (2.11-10.9mmol/L). The corresponding δ¹³C_DIC values varied from -19.4‰ to -6.5‰. Except for a few samples with values approaching -20‰, these isotope values are more typical for carbonate dissolution and equilibration of CO₂ in the aquifer. This study shows that the underlying carbonate system may buffer the aqueous geochemistry of the groundwater on the island. The high arsenic content in groundwater may have been mobilized through reductive dissolution of Fe-Mn oxides and oxy-hydroxides that are coated on sandy aquifer materials. The lower content of DOC (0.2-1.5mmol/L) provides evidence for the restricted formation of pyrite in the aquifer.

Groundwater Flow Processes and Human Impact along the Arid US-Mexican Border, Evidenced by Environmental Tracers: The Case of Tecate, Baja California

With the increasing population, urbanization and industry in the arid area of Tecate, there is a concomitant increase in contaminants being introduced into the Tecate River and its aquifer. This contamination is damaging the usable groundwater supply and making local residents and commercial enterprises increasingly dependent on imported water from the Colorado River basin. In this study we apply a suite of chemical and isotopic tracers in order to evaluate groundwater flow and assess contamination trends. Groundwater recharge occurs through mountain-block and mountain-front recharge at higher elevations of the ranges. Groundwater from the unconfined, alluvial aquifer indicates recent recharge and little evolution. The increase in salinity along the flow path is due to interaction with weathering rock-forming silicate minerals and anthropogenic sources such as urban wastewater, residual solids and agricultural runoff from fertilizers, livestock manure and/or septic tanks and latrines. A spatial analysis shows local differences and the impact of the infiltration of imported waters from the Colorado River basin. The general trend of impaired water quality has scarcely been documented in the last decades, but it is expected to continue. Since the groundwater system is highly vulnerable, it is necessary to protect groundwater sources.

Will demography defeat river rehabilitation efforts? The case of the River Jordan

In the age-old debate between technological optimists and pessimists, desalination has been hailed as a "game changer" that can fundamentally alter the dynamics of water management under conditions of scarcity. While water from desalination facilities can reduce uncertainties in municipal supply, they are unlikely to replace the missing flow required to rehabilitate rivers and streams. The Jordan River is an iconic, but highly degraded water body whose restoration has been the subject of extensive research as well as numerous proposals and strategies. A review of the present state of the River and prospects for successful rehabilitation efforts reveals that neither the increase in the riparian population nor the reduced water supply due to climate change in the Jordan basin has been considered sufficiently in restoration strategies. Demographic pressures produce acute water shortages which make the provision of future environmental flows highly unlikely. While much can and should be done to improve its environmental condition, the Jordan River offers a cautionary tale for water scarce regions about the challenge of stream restoration initiatives in the face of accelerated population growth, notwithstanding the potential benefits of desalination as a source of drinking water.