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In 2001, a borehole was drilled to 400 m depth below the ground surface of a tableland in northern Japan, where horizontal porous sedimentary rock strata are deposited. Fresh drill-core and groundwater samples were collected from the borehole for estimating global fallout bomb pulses of 3H, which becomes tritiogenic 3He (tri3He) through β-decay, and 36Cl. In the northern hemisphere, the maximum fallout deposition of 36Cl from nuclear testing was observed during 1954–1957, and that of 3H in 1963. Most 3H (half-life 12.3 y) in the rock porewater had decayed to tri3He during the 38 years between 1963 and 2001. In the borehole, bomb pulses were observed in profiles of tri3He and 36Cl/Cl at 101 m and around 138 m depth below the ground surface, respectively. The downward groundwater velocity was estimated to be 2.9–3.1 m/y from the depth of the 36Cl/Cl bomb pulse and 2.7 m/y from the depth of the tri3He bomb pulse. Although the two velocity estimates differ by 10–15%, it is possible to model groundwater flow in the tableland as downward piston flow without diffusion. [ABSTRACT FROM AUTHOR]

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Groundwater is the primary water resource for human use on the Loess Tableland of China; however, the depletion of groundwater has been very severe in recent years. To understand the reasons for groundwater depletion, it is necessary to understand its recharge mechanisms. The objective of this study is to reveal the mechanisms of groundwater recharge on the Loess Tableland using deuterium and oxygen-18 in precipitation, in soil water under three land use types (natural grassland, wheat field, and apple orchard), and in groundwater. Results indicate that piston flow and preferential flow coexist in rainfall infiltration; however, their occurrences are greatly influenced by land use type. They both occurred in the natural grassland and wheat field; however, only piston flow was detected in the apple orchard. Groundwater is likely to be recharged by both piston flow and preferential flow through the unsaturated zone, while preferential flow should be the main recharge mechanism, as the isotopic composition of the groundwater is very close to that of precipitation while quite different from that of deep soil water. However, groundwater recharge by preferential flow only occurred in the natural grassland or wheat field, but not in apple orchard due to its thick dry soil layer. Regarding groundwater recharge by piston flow, it possibly occurs in most years for the natural grassland and in wet years for the wheat field; however, it is possibly difficult for the apple orchard to recharge groundwater by piston flow. The above results suggest that the conversion of farmland to apple orchard can affect the natural mode of the water cycle and reduce groundwater recharge on the Loess Tableland; therefore, attention should be paid to adjustments of land use. [ABSTRACT FROM AUTHOR]

Piston and preferential water flow are viewed as the two dominant water transport mechanisms regulating terrestrial water and solute cycles. However, it is difficult to accurately separate the two water flow patterns because preferential flow is not easy to capture directly in field environments. In this study, we take advantage of the afforestation induced desiccated deep soil, and directly quantify piston and preferential water flow using chloride ions (Cl−) and soil water profiles, in four deforested apple orchards on the Loess Plateau. The deforestation time ranged from 3 to 15 years. In each of the four selected orchards, there was a standing orchard that was planted at the same time as the deforested one, and therefore the standing orchard was used to benchmark the initial Cl− and soil water profiles of the deforested orchard. In the deforested orchards, piston flow was detected using the migration of the Cl− front, and preferential flow was measured via soil water increase below the Cl− front. Results showed that in the desiccated zone, Cl− migrated to deeper soil after deforestation, indicating that the desiccated soil layer formed by the water absorption of deep-rooted apple trees did not completely inhibit the movement of water. Moreover, there was an evident increase in soil water below the downward Cl− front, directly demonstrating the existence of preferential flow in deep soil under field conditions. Although pore water velocity was small in the deep loess, preferential water flow still accounted for 34–65% of total infiltrated water. This study presented the mechanisms that regulate movement of soil water following deforestation through field observations and advanced our understanding of the soil hydrologic process in deep soil. [ABSTRACT FROM AUTHOR]