Study on Groundwater Recharge Based on Chloride Mass Balance and Hydrochemistry in the Irrigated Agricultural Area, North China Plain

In the North China Plain, water shortage seriously restricts economic development, and agricultural irrigation depends heavily on groundwater extraction. Irrigation water and precipitation may directly recharge to groundwater in the irrigated agricultural region. Therefore, calculating the recharge of precipitation and irrigation to groundwater is essential for the sustainable utilization of water resources. Furthermore, determining the transformation relationship of precipitation-soil water-groundwater is helpful to understand the hydrological cycle process better. The average groundwater recharge calculated by the chloride mass balance method is between 66 and 144mm/yr, accounting for only 7%–17% of the total precipitation and irrigation water. The hydrogen and oxygen isotopes reveal that precipitation only affect soil water in topsoil, and soil water in deep soil is recharged upward by groundwater. Hydrochemical composition of soil water shows high concentrations of solutes in unsaturated zones. Infiltration water dissolves solutes through the unsaturated zone and brings them into the shallow groundwater, causing the deterioration of shallow groundwater quality. Therefore, reducing the recharge of precipitation and irrigation to groundwater by controlling the groundwater level and the intensity of single irrigation is recommended to protect groundwater quality. These results contribute to the effective management of groundwater resources and the control of agricultural pollution in groundwater.

Constraining probabilistic chloride mass-balance recharge estimates using baseflow and remotely sensed evapotranspiration: the Cambrian Limestone Aquifer in northern Australia

Regional-scale estimates of groundwater recharge are inherently uncertain, but this uncertainty is rarely quantified. Quantifying this uncertainty provides an understanding of the limitations of the estimates, and being able to reduce the uncertainty makes the recharge estimates more useful for water resources management. This paper describes the development of a method to constrain the uncertainty in upscaled recharge estimates using a rejection sampling procedure for baseflow and remotely sensed evapotranspiration data to constrain the lower and upper end of the recharge distribution, respectively. The recharge estimates come from probabilistic chloride mass-balance estimates from 3,575 points upscaled using regression kriging with rainfall, soils and vegetation as covariates. The method is successfully demonstrated for the 570,000-km2 Cambrian Limestone Aquifer in northern Australia. The method developed here is able to reduce the uncertainty in the upscaled chloride mass-balance estimates of recharge by nearly a third using data that are readily available. The difference between the 5th and 95th percentiles of unconstrained recharge across the aquifer was 31 mm/yr (range 5–36 mm/yr) which was reduced to 22 mm/yr for the constrained case (9–31 mm/yr). The spatial distribution of recharge was dominated by the spatial distribution of rainfall but was comparatively reduced in areas with denser vegetation or finer textured soils. Recharge was highest in the north-west in the Daly River catchment with a catchment average of 101 (61–192) mm/yr and lowest in the south-east Georgina River catchment with 6 (4–12) mm/yr.

Discovery of a large subsoil nitrate reservoir in an arroyo floodplain and associated aquifer contamination

In an area of elevated nitrate (NO3) groundwater concentrations in the northern Chihuahuan Desert in central New Mexico (United States), a large reservoir of nitrate was found in the subsoil of an arroyo floodplain. Nitrate inventories in the floodplain subsoils ranged from 10,000 to 38,000 kg NO3-N/ha—over twice as high as any previously measured arid region. The floodplain subsoil NO3 reservoir was over 100 times higher than the adjacent desert (59–95 kg NO3-N/ha). Chloride mass balance calculations of subsoils indicate arroyo floodplain subsoils have undergone negative recharge since 2600–8600 yr ago, while the surrounding desert has had negative recharge since 13,000–17,000 yr ago. Compared to the adjacent desert, plant communities are larger and more abundant in the floodplain, though subsoil NO3 is apparently not utilized. We demonstrate that NO3 accumulates in the subsoil of the floodplain through evaporation of monsoon season precipitation funneled into the arroyo. Through a one-dimensional vadose zone model, we show that the NO3 inventories in the arroyo floodplain could be acquired 8 to 75 times faster than through atmospheric deposition through the lateral movement

Supplemental Material: Discovery of a large subsoil nitrate reservoir in an arroyo floodplain and associated aquifer contamination

Additional information on site background, sampling methods, chloride mass balance calculations, and vadose zone modeling.<br>

Supplemental Material: Discovery of a large subsoil nitrate reservoir in an arroyo floodplain and associated aquifer contamination

Additional information on site background, sampling methods, chloride mass balance calculations, and vadose zone modeling.<br>

Groundwater recharge estimation in Undai watershed area, southern Mongolia

&lt;p&gt;Recharge estimation in arid and semi-arid areas is complicated. As for the country where the potable water for both the people and livestock is supplied from shallow unconfined aquifer due to the lack of existing ground water, the recharge estimation is crucial to water source management. However, since the deficiency of available data, such estimation has not been completed in the Gobi desert of Mongolia. Water-bearing rock units of the Undai river basin consist of Upper Quaternary alluvial-proluvial sands, gravels and pebbles. In this paper, direct recharge was estimated using chloride mass balance (CMB) and rainfall infiltration breakthrough (RIB) model in shallow unconfined aquifer, Undai watershed area Southern Mongolia.&amp;#160; As a result of groundwater recharge estimation survey conducted in 2018, the annual mean recharge of the groundwater along the Undai dry riverbed is calculated to be 13.7mm/year according to RIB model based on the water level fluctuation, which makes up 6.3% of total precipitation and 21.7mm/year according to CMB (chloride mass balance), which comprises 10% of the total annual precipitation. The largest recharge estimates were determined using the daily basis RIB method and the smallest estimates were determined using the chloride-mass-balance method.&lt;/p&gt;