Huatai Liu

(College of Oceanography & Environmental Science, Xiamen University, Xiamen, Fujian, 361005)

Abstract: In the plain at the northern foot of the Tianshan Mountain, the freezing process each year is far longer than the thawing process. On the groundwater balance test field in Changji, three typical soils (silty light clay, fine sand and sand gravel) were selected for the observation of groundwater recharge and depletion during the freezing-thawing period with the lysimeter. Different groundwater embedment depths were designed according to the extreme depth of groundwater evaporation of different lithologies. In the freezing period, areas with sand gravel and packsand were favorable for the protection of shallowly-embedded groundwater resources. In the thawing period, regardless of the depth of groundwater, groundwater of all of the three lithologies received recharge from thawing water; however, groundwater with fine particle lithology received a relatively small amount of recharge and suffered a lagging effect, whereas coarse particle lithology were more favorable for the formation of groundwater resources. The maximum groundwater depletion with a one-month freezing period did not exceed 25.3mm (sand gravel) whereas the maximum groundwater recharge with a one-month thawing period reached 113.3mm (fine sand), indicating that the thawing period is the an important period of groundwater recharge in the year.

Keywords: Groundwater Recharge; Groundwater Depletion; Freezing-Thawing Period; Inland Basins in Northwest China

Inland basins in Northwest China lie in a typical continental dry climate zone, featuring strong evaporation and little precipitation, with the plain in the basin normally receiving an annual precipitation of less than 100mm. Additionally, inland basins in Northwest China have a relatively long freezing-thawing period, generally starting from early November each year and ending in mid-March the next year, lasting approximately five months. The characteristics of aridity, limited precipitation, intense precipitation and extended freezing-thawing period have given rise to the uniqueness of groundwater recharge. In a non-freezing period, sub-precipitation has a very limited effect on the effective recharge of groundwater, whereas in the thawing period, thanks to the accumulation of moisture in the frozen soil stratum during the freezing period, groundwater receives a concentrated recharge during the thawing period; the amount and characteristics of groundwater recharge in the thawing period are sharply different from those in the non-freezing period ^[1]. As such, it is particularly important to conduct observation and research on groundwater recharge in the freezing-thawing period as they hold tremendous practical significance for understanding groundwater recharge in inland basins in the freezing-thawing period and for correctly evaluating groundwater resources.

1 Test Methods

1.1 Test site

        The tests were conducted on the groundwater balance test field of Xinjiang Geological & Mineral Bureau. The test field is located on the suburbs of Changji City; geographically, it’s a fine-soil flatland belt in the alluvial-proluvial plain at the northern foot of the Tianshan Mountain.

1.2 Instrument & samples

The instrument used for testing and observation was a common lysimeter, which consisted of an infiltrating (evaporating) utensil, an aqueduct and an underground observation chamber, with an observation accuracy of ±0.1mm. The infiltrating (evaporating) utensil contained undisturbed soil samples, which were the three typical soils on the plain at the northern foot of the Tianshan Mountain, namely, sand gravel, fine sand and silty light clay. Different groundwater embedment depths were designed according to the extreme depth of groundwater evaporation of different lithologies. Test samples of sand gravel were embedded at three different depths at 0.5m, 1m and 1.5m, respectively; test samples of fine sand were embedded at five depths at 0.5m, 1m, 1.5m, 2m and 3m, respectively; test samples of silty light clay were embedded at seven depths at 0.5m, 1m, 1.5m, 2m, 3m, 4m and 5m.

1.3 Observation methods

The amounts of groundwater recharge and depletion were observed with the Marriott bottle and infiltration catching bottle of the lysimeter. Observation was made every five days and the amount observed at the end of each month was taken as the amount of groundwater recharge of the month. A positive observed amount indicated that groundwater recharge had occurred while a negative observed amount indicated the groundwater had been depleted.

2 Test Results and Analysis

2.1 Process of the Growth and Disappearance of Frozen Soil and Snow Cover

The plain at the northern foot of the Tianshan Mountain typically enters the freezing period in early November each year and remains in the freezing period until early March the next year, and enters the thawing period in mid-March and remains in the thawing period until late March. During the freezing-thawing periods, due to the growth and disappearance of frozen soil and snow cover, the underlying surface conditions of groundwater recharge were significantly different from those in the non-freezing-thawing periods, which affected the recharge of groundwater. Following is an analysis of the growth and disappearance process of frozen soil and snow cover in 1999 and 2000.

2.1.1 Formation and Thawing Process of Frozen Soil

On November 3, 1999, with the advent of strong winds and low temperatures, frozen soil began to form on the ground surface, initially with a mere thickness of about 1cm. Under the effect of temperatures at the ground surface, the initial thickness of frozen soil fluctuated; but, overall, the thickness of the frozen soil was rising, with the maximum thickness reaching 102cm (February 6, 2000). When it reached its maximum thickness, the frozen soil would stay stable for approximately 30 days. As the temperature rose, the frozen soil began to thaw at its base (March 9, 2000); shortly afterwards, the top layer of the frozen soil also started it thawing process (March 13, 2000); the thawing front rapidly approached the center of the frozen soil and eventually penetrated to a depth of 30cm below the ground surface (March 19, 2000). Clearly, the freezing process was much longer than the thawing process as the former lasted 126 days while the latter took only 11 days (Fig. 1).

2.1.2 Formation and Thawing Process of Snow Cover

On November 10, 1999, with dropping temperature and snowfall, snow cover began to take shape, reaching a thickness of about 3cm; but a few days later, it thawed. On November 17, 1999, another snowfall hit the area, again reaching a thickness of around 4cm. Afterwards, the thickness of the snow cover experienced a few fluctuations. On December 3, 1999, another heavy snowfall took place, bringing the thickness of the snow cover to 18cm. Subsequent snowfall on January 4, 2000 further increased the thickness of the snow cover to 42cm, making it the thickest snow cover in the year. After reaching its maximum thickness, the snow cover retained its thickness for a certain length of time until March 8, 2000. With rising temperature, the snow cover thawed rapidly and melted completely by March 16, 2000. Similar to the growth and thawing process of the frozen soil, the formation process of the snow cover was far longer than its thawing process as the former lasted 118 days while the latter took merely eight days (Fig. 1).

Fig. 1 Formation and Thawing Process of Frozen Soil and Snow Cover

2.2 Underground Recharge and Depletion of Sand Gravel

On November and December of 1999 during the freezing process, when the groundwater embedment depth was less than and equal to 0.5m, the groundwater recharge of sand gravel was a negative value, indicating that groundwater at this particular embedment depth primarily suffered depletion; however, the depleted amount was small, totaling 2.9mm in November and 1.6mm in December. In contrast, on November and December of 1999 and January and February of 2000 during the freezing process, when the groundwater embedment depth was larger than or equal to 1m, the groundwater recharge of sand gravel was a positive value, indicating that groundwater at this particular embedment depth primarily received recharge; however, the recharged amount was small, totaling merely 0.1-0.5mm (Table 1).

During the thawing period in March 2000, as the frozen soil layer and snow cover melted, groundwater received a concentrated recharge, and sand gravels of different groundwater embedment depths all received considerable groundwater recharge in March, with sand gravel at lower depth (0.5m) receiving a recharge of up to 83.2mm and sand gravels at depths of 1m and 1.5m receiving recharges of 64.9mm and 61.8mm respectively, which indicated that the recharged amount showed a declining tendency as the embedment depth increased (Table 1).

Table 1 Amount of Groundwater Recharge of Sand Gravel at Different Groundwater Embedment Depths (mm)

Note: ND means that the soil at this particular embedment depth is completely frozen,

with neither groundwater recharge nor depletion.

2.3 Groundwater Recharge and Depletion of Fine Sand

In November and December of 1999 during the freezing process, when the groundwater embedment depth was less than and equal to 2m, the groundwater recharge of fine sand was a negative value, indicating that groundwater primarily suffered depletion, with the depleted amount ranging from 0.3m to 25.3cm, the smaller the depth, the greater the depletion. In November and December of 1999 and January and February of 2000, when the groundwater depth was larger than or equal to 3m, the groundwater recharge of fine sand became a positive value, indicating that groundwater primarily received recharge; however the recharged amount was small, ranging from 3.0mm to 4.8mm (Table 2).

During the thawing period in March 2000, regardless of the embedment depth, fine sand was primarily for groundwater recharge, and groundwater received considerable recharge, with the maximum recharge reaching 113.3mm (at an embedded depth of 0.5m). As the groundwater embedment depth increased, the amount of recharge received by groundwater dropped, with the recharge at an embedment depth of 3m being 27.0mm, merely 23% of the recharge at an embedment depth of 0.5m (Table 2).

Table 2   Amount of Groundwater Recharge of Fine Sand at Different Groundwater Embedment Depths (mm)

2.4 Underground Recharge and Depletion of Silty Light Clay

In November and December of 1999 and January and February of 2000 during the freezing process, when the groundwater embedment depth was less than and equal to 4m, the groundwater recharge of silty light clay was a negative value, indicating that groundwater primarily suffered depletion, with the depleted amount ranging from 1.6m to 11.8cm, the greater the depth, the smaller the depletion. In contrast, when the groundwater embedment depth was larger than or equal to 5m, the groundwater recharge of silty light clay became a positive value; however, the recharged amount was small, ranging from merely 0.7mm to 1.7mm (Table 3).

During the thawing period in March 2000, at groundwater embedment depths of 0.5m and 1m, silty light clay also received considerable groundwater recharge from infiltration, but still less than sand gravel and fine sand, totaling 66.4mm and 40.9mm, respectively. In stark contrast to sand gravel and fine sand, the groundwater recharge during the freezing period at embedment depths of 1.5m, 2m, 3m and 4m remain a negative value, which means that groundwater still primarily suffered depletion. This is because at a groundwater embedment depth of 1m, the kinestate of soil moisture in the freezing period changed quickly from an evaporating type to an infiltrating type, allowing groundwater to receive enormous recharge rapidly; in contrast, at a groundwater embedment depth of 1.5m~4m, the kinestate of soil moisture in the thawing period first changed from an evaporating type to an infiltrating-evaporating type, which means that the kinestate of soil moisture on adjacent groundwater surface remained in an evaporating state and required considerable time before it was changed from the infiltrating-evaporating type to a completely infiltrating type^[2] . This phenomenon also showed that silty light clay featured poor infiltration but strong moisture holding capacity; in March, time was no sufficient to turn the moisture kinestate of the entire soil profile into an infiltrating type; moreover, as the embedment depth increased, the process of the change from an infiltrating-evaporating type to an infiltrating type required longer time; the results showed that the infiltration front in late March had yet to reached the 1.5m embedment depth (Table 3).

Table 3   Amount of Groundwater Recharge of Silty Light Clay at Different Groundwater Embedment Depths (mm)

3  Comparison of Groundwater Recharge and Depletion of Different Lithologies

In the freezing period (November, December, January and February), at the same embedment depth, sand gravel underwent the lowest groundwater

depletion, silty light clay experienced the highest groundwater depletion, and fine sand was in between (fine sand underwent greater depletion than silty light clay only when the embedment depth was smaller than 1m) (Fig. 2)

In the freezing period, where the groundwater embedment depth was lower than 1m, sand gravel experienced groundwater depletion; where the groundwater embedment depth was lower than 3m, fine sand underwent depletion; where the groundwater embedment depth was lower than 5m, silty light clay underwent depletion (Fig. 2). Obviously, areas with sand gravel and fine sand lithology are favorable for the protection of shallowly-embedded groundwater resources.

In the thawing period, at the same embedment depth, fine sand received the highest recharge, followed by sand gravel, with silty light clay receiving the lowest recharge (Fig. 2, March). The groundwater recharge of silty light clay had a lagging effect. Obviously, areas with fine sand and sand gravel lithology are favorable not only for the protection of groundwater resources but also for the formation of groundwater resources.

In the entire freezing-thawing period, the highest depletion of groundwater over a month-long freezing period was merely 25.3mm (sand gravel), whereas the highest depletion of groundwater over a month-long thawing period reached 113.3mm (find sand), indicating that the freezing-thawing period is an important period of recharge for groundwater.

November December January

February March

Fig. 2     Amount of Groundwater Recharge of Different Lithologies in Different Months

4 Conclusions

The freezing-thawing period of inland basins in Northwest China lasts nearly five months. In this period, due to the impact of frozen soil and snow cover, the recharge and depletion of groundwater is significantly different from those in the non-freezing period. Thus, strengthening the observation and research on groundwater recharge in this particular period will enhance our understanding of the formation of groundwater resources in inland basins in Northwest China and improve the accuracy of the evaluation of groundwater resources. Tests and studies over the 1999-2000 period have allowed the author to come to the following conclusions:

(1) In the freezing period, for sand gravel, where the groundwater embedment depth was lower than 1m, groundwater primarily suffer depletion; for fine sand, where the groundwater embedment depth was lower than 3m, groundwater primarily suffer depletion; for silty light clay, where the groundwater embedment depth was lower than 5m, groundwater primarily suffer depletion as well. Under opposite conditions, groundwater primarily receives recharge. Areas with sand gravel and fine sand are favorable for the protection of shallowly-embedded groundwater resources.

(2) In the thawing period, regardless of the groundwater embedment depth, groundwater of sand gravel, fine sand and silty light clay all receive recharge from thawing water (including thawing water from snow cover and frozen soil layer), but the recharge of groundwater of fine particle lithology is small and has a lagging effect. In contrast, coarse particle lithology is more favorable for the formation of groundwater resources.

(3) In the entire freezing-thawing period, the highest depletion of groundwater over a month-long freezing period is not larger than 25.3mm (sand gravel) whereas in the thawing period, the highest recharge of groundwater over a month-long period reaches 113.3mm (fine sand). The thawing period is an important period of recharge for groundwater during the year.