Using geochemistry and environmental tracers to study shallow unconfined aquifer recharge and mineralization processes in the Yinchuan Plain, arid Northwest China

Irrigation water extracted from the Yellow River plays a key role in water resource management in the Yinchuan Plain (YCP), arid Northwest China. Investigating the soluble matters (ion and gas) of groundwater provides information to explain the unconfined shallow aquifer recharge and groundwater mineralization processes after long-term flood irrigation activity. Environmental tracing with the elements, H, O, H, and CFCs, combining geochemistry using major ions and selected trace elements, was conducted for 43 water samples from September to October 2019 in the YCP. Evaporite and silicate weathering dominate the shallow unconfined groundwater geochemical compositions. Water–rock interactions control the mineralization characteristics regularly along the groundwater flow paths from the southwest toward the northeast. Stable isotopes suggest that Yellow River water and precipitation in winter and/or from Helan Mountainous area are the main recharge sources. The shallow unconfined aquifer mixed young (post-1940) and old (pre-1940) water with young water ratios from 53.1 to 73.5% inferred from the CFC concentrations and H activities. Water reinfiltrations extracted from the Yellow River and from the old groundwater are confirmed. Lateral flow recharge for the shallow unconfined aquifer is less indistinctive than that from the water re-infiltration in the plain areas.


INTRODUCTION
Groundwater is an essential but invisible resource and even acts as a unique water source in arid areas. Ascertaining groundwater sources usually proves elusory for people due to their complexity and invisibility. Investigating hydrogeological and geochemical processes such as aquifer recharge, groundwater mixing, and mineralization processes is an efficient way for ensuring sustainability, Another molecule of water is tritium ( 3 H), which is radioactive with a half-life of 12.32 years, and is a powerful tracer to study groundwater recharge years and recharge sources (Tadros et al. ; Ma et al. ). 3 H is more widely used in the Northern Hemisphere due to its much higher activity caused by synthetic rather than natural conditions. The continual atmospheric thermonuclear tests in the Northern Hemisphere during the 1950s and 1960s have abundantly generated 3 H in the atmosphere, supplying us artificial tracers to study modern water recharge and water mixing features. Nowadays, rainfall 3 H activities are still affected by the tail end of the bomb pulse in the Northern Hemisphere, which is particularly seen in the arid northwest of China due to both the continental effect (Tadros et al. )  there is a possibility for some ambiguity in regard to the CFC ratio plot (Darling et al. ). The different atmospheric CFC ratios that exist, that is those of today and those before the 1990s make investigating groundwater recharge years and mixing ratios by CFCs possible. The altitudes in the plain area vary from 1,100 to 1,200 m, where in the Helan Mountain, they vary from 1,500 to 3,200 m to the summit of 3,500 m. The plain area is consistent with the piedmont plain, alluvial-proluvial plain, and lacustrine plain with slopes of 10-30, 5, and 0.2-1.8‰, respectively. The lacustrine plain has witnessed cultivation for nearly 2000 years, which is still an important production area for agriculture, forestry, animal husbandry, and fisheries. The porous aquifers are divided into two types, which are the single unconfined aquifer and multilayer aquifers.
The single unconfined aquifer distributes along the Helan Mountain in the west and south of YCP, and the thickness varies from 10 to 300 m with sandy gravel and some pebbles.
The water depth varies from 0.5 to 4.0 m, and the water abundance varies from 2,000 to 5,000 m 3 /d. Detailed data are given in Figure 1. The multilayer aquifers consist of an unconfined aquifer, the first confined aquifer, and the second confined aquifer from top to bottom within 250 m under the surface. What can be noticed is that the three aquifers are obstructed by a discrete aquitard, which indicates that there are hydraulic connections among each other.
The unconfined aquifer has a thickness of 20-60 m and distributes in the alluvial-proluvial plain and lacustrine plain. Figure 2 shows that the water abundance decreases to 1,000-3,000 m 3 /d in the central area and to <1,000 m 3 /d in

Field sampling
A total of 43 water samples were taken in the YCP, including three Yellow River water samples (collected along the river flow direction), one spring sample, and 39 groundwater samples. Groundwater was pumped from agricultural and domestic wells ( Figure 1 and Table 1), in which the wells were pumped for a minimum of 10 min prior to sampling.
Water temperature (T ), electrical conductivity (EC), and pH values were measured in the field using calibrated Hach (HQ40d) conductivity and pH meters (Table 1), which had been calibrated before use. Bicarbonate was determined by titration with 0.05 N HCl on-site. Samples to be analyzed for geochemistry and environmental isotopes (δ 2 H, δ 18 O, and 3 H) were filtered on-site through 0.45 μm millipore syringe filters and stored in pre-cleaned polypropylene bottles at 4 C until analysis. Water samples for 3 H analysis were collected and stored in 500 mL airtight polypropylene bottles. For cation analysis, the samples were acidified to pH <2 with ultrapure HNO 3 .
CFCs are dissolved gases, and, thus, extreme precautions are needed to be taken to avoid contamination from equipment such as pumps and tubing for CFC samples (Han et al. ; Cook et al. ). After purging the wells, the water samples were collected directly from the borehole using a copper tube sampling pipe for CFC analysis. One end of the pipe was connected to the well casing, and the other end was placed at the bottom of a 120 mL borosilicate glass bottle, inside a 2,000 mL beaker. The well water was allowed to flow through the tubing for 10 min, leading to a thorough flushing of the tubing. The bottle was submerged, filled, and capped underwater till no bubbles appeared in the bottle, following the protocols described by Ma et al. (). In this study, six bottles were collected at each well and three of them were analyzed. A total of eight wells were collected for CFC  analysis.

Laboratory analyses
The calculations of the stable isotopic values (δ 2 H and δ 18 O) and the geochemical calculations for water were performed at the Institute of Geological Survey, China University of Geosciences (Wuhan). δ 2 H and δ 18 O analyses were done using a Liquid Water Isotopic Analyzer (LGR, IWA-45EP, USA).

Mechanisms controlling mineralization processes
Geochemical sources of groundwater Ion ratios and their relationship between each other provide essential information for identifying geochemical sources in groundwater. It is seen from Figure 3    explains the high molar Na/Cl ratios of the groundwater: It is seen from Figure 4(b) that the molar ratios of Si/(Na-Cl) for groundwater (except for groundwater No. 25) and Yellow River water totally show low values with a mean value of 0.47, which is largely smaller than 2. When considering only the albite weathering followed by Equation (1), it would result in a molar Si/(Na-Cl) ratio of ±2 for the groundwater. Therefore, we could make a hypothesis that the excess Na probably did not originate solely from silicate weathering, as no silicate weathering reaction could explain such a low ratio (Stallard & Edmond ). Figure 4(c) shows a good linear relationship between Na and SO 4 (R 2 ¼ 0.8996, meq./L), indicating that sodium sulfate (Na 2 SO 4 ) dissolution is likely be another potential source of excess Na. Figure 4( It is seen in Figure 4   intersection point of the LMWL and evaporation line is roughly overlapped with the Yellow River water, suggesting the same recharge sources for both the Yellow River water and groundwater. The slope of evaporation line is as low as 5.5 due to the dry air humidity, which is reasonable in the arid areas. Therefore, we can hypothesize that the Yellow River supplies water to the shallow unconfined aquifer in the YCP during which the water molecule fractionate leads to enriched isotope values in the residual groundwater.
Irrigation water is being extracted from the Yellow River for the last more than 60 years in the YCP; moreover, precious hydrogeological studies have given a consistent opinion that irrigation water infiltration dominates the shallow unconfined aquifer recharge. As a result of the irrigation infiltration and local rainfall recharge, the groundwater recharge sources might be relatively young and the recharge year could be estimated based on the CFCs.
CFCs were first synthesized in the 1930s, and they can be dissolved in water with various solubilities. The CFC concentrations and 3 H activities in groundwater have been widely used to study the young water recharge (post 1940).
As can be seen in Tables 1 and 2   which reduces the CFC-11 concentration in the groundwater and then leads to the relatively old recharge years based on CFC-11. It is unlikely that a groundwater aerobic environment contributes to CFC degradation under anoxic conditions. However, the fine soil textures in the lacustrine plain ( Figure 2) lead to a relatively low infiltration process, and, thus, CFC-11 has shown a greater propensity for degradation and/or contamination than CFC-12. Therefore, the CFC-12-based recharge year is discussed in the following text.
It is seen in Figure 7   distinguished. Secondly, it helps us to explore quantified data to study the groundwater mixing in which both young water and old (CFC-free) water coexist. Thirdly, the abnormally high CFC concentration indicating urban industrial contamination can be recognized.
As shown in Figure 8 much higher concentrations, which likely suggests that contamination of these two groundwater samples has occurred during sampling or measuring.
As a molecule of water, groundwater 3 H activity does not vary largely after being exposed in the air, in contrast to CFC concentrations. The combined use of CFCs and 3 H may further help to resolve even more complicated mixing scenarios due to the large difference in the temporal pattern of the input functions between CFCs and 3 H, such as groundwater mixed with irrigation water and/or young water mixed in different decades.
It is seen in Figure 9  Figure 9 also displays that groundwater No. 6, 10, and 11 have high CFC-12 concentrations but relatively low 3 H activities. We can then infer that these water sources are mixed by young water and re-infiltration water (extracted from old groundwater or Yellow River water). Northern hemisphere atmospheric 3 H concentrations have been elevated radically since the 1950s, and, thus, old water recharge can be identified by anomalously low 3 H activities in comparison with CFCs. Groundwater No. 6 has the lowest 3 H activity and the highest CFC-12 concentration, likely indicating that this water sample is extracted from the old groundwater, which, meanwhile, would have been exposed to the air and which finally re-infiltrates into the shallow unconfined aquifer. contain both high CFC-12 concentrations and 3 H activities, which could be possible due to mixing with Yellow River water re-infiltration. Irrigation from Yellow River water has been going on unhindered for more than 60 years in the YCP, during which re-infiltration to the shallow unconfined aquifer is possible.