Groundwater recharge in an arid grassland as indicated by soil chloride profile and multiple tracers

Abstract

Previous studies have shown that shallow groundwater in arid regions is often not in equilibrium with near‐surface boundary conditions due to human activities and climate change. This is especially the case where the unsaturated zone is thick and recharge rate is limited. Under this nonequilibrium condition, the unsaturated zone solute profile plays an important role in estimating recent diffuse recharge in arid environments. This paper combines evaluation of the thick unsaturated zone with the saturated zone to investigate the groundwater recharge of a grassland in the arid western Ordos Basin, NW China, using the soil chloride profiles and multiple tracers (2H, 18O, 13C, 14C, and water chemistry) of groundwater. Whereas conventional water balance and Darcy flux measurements usually involve large errors in recharge estimations for arid areas, chloride mass balance has been widely and generally successfully used. The results show that the present diffuse recharge beneath the grassland is 0.11–0.32 mm/year, based on the chloride mass balance of seven soil profiles. The chloride accumulation age is approximately 2,500 years at a depth of 13 m in the unsaturated zone. The average Cl content in soil moisture in the upper 13 m of the unsaturated zone ranges from 2,842 to 7,856 mg/L, whereas the shallow groundwater Cl content ranges from 95 to 351 mg/L. The corrected 14C age of shallow groundwater ranges from 4,327 to 29,708 years. Stable isotopes show that the shallow groundwater is unrelated to modern precipitation. The shallow groundwater was recharged during the cold and wet phases of the Late Pleistocene and Holocene humid phase based on palaeoclimate, and consequently, the groundwater resources are nonrenewable. Due to the limited recharge rate and thick unsaturated zone, the present shallow groundwater has not been in hydraulic equilibrium with near‐surface boundary conditions in the past 2,500 years.

Determining Regional-Scale Groundwater Recharge with GRACE and GLDAS

“…We selected the Ordos Basin (280,000 km 2 ) in the middle of the Loess Plateau as our study area because a large body of work has been conducted to estimate GR from environmental tracers (tritium, chloride) and models [17,[50][51][52][53][54][55][56][57]. In addition, the Ordos Basin is one of the largest-scale groundwater basins in the world, and is composed of different interconnected aquifer systems, including the Cambrian-Ordovician Karst Aquifer System, Cretaceous Aquifer System, Carbonate-Jurassic Aquifer System and Quaternary Aquifer System [58].…”

“…In addition, the Ordos Basin is one of the largest-scale groundwater basins in the world, and is composed of different interconnected aquifer systems, including the Cambrian-Ordovician Karst Aquifer System, Cretaceous Aquifer System, Carbonate-Jurassic Aquifer System and Quaternary Aquifer System [58]. Moreover, there has been little irrigation [52,54,55], and precipitation is the main source of GR [58]. Figure 1.…”

Identifying evaporation fractionation and streamflow components based on stable isotopes in the Kaidu River Basin with mountain–oasis system in north‐west China

“…The loose sedimentary rock structure, the widely spread grassland, and the abundant meltwater make infiltration relatively easy in this area (Kendall & Coplen, ). Second, the mountain groundwater was recharged by precipitation with more depleted water isotopes at higher altitudes (Li et al, ), and third, the mountain groundwater was recharged by old water with more depleted water isotopes (Huang, Pang, Liu, Yin, & Edmunds, ).…”

Estimating groundwater recharge for a freshwater lens in an arid region: Formative and stability assessment

“…Integrating different approaches of estimating recharge can be complementary and thus help refine the conceptual model of the recharge processes. For example, various tracers (such as Cl, 36 Cl and 3 H) have been used in combination to estimate recharge in unsaturated zones (Huang, Pang, Liu, Yin, & Edmunds, ; Li, Chen, Liu, & Si, ; Wang, Jin, Nimmo, Yang, & Wang, )), and other tracers (such as chlorofluorocarbons (CFCs)) have been used in saturated zones (Raiber, Webb, Cendón, White, & Jacobsen, ). Also, the combination of multiple environmental tracers and numerical modeling has been used for estimating recharge (Liu, Yamanaka, Zhou, Tian, & Ma, ).…”