Investigating groundwater residence time and recharge sources is crucial for
water resource management in the alluvium aquifers of arid basins.
Environmental tracers (chlorofluorocarbons,
Groundwater is the largest available freshwater resource. It supplies freshwater to communities around the world, and it plays an essential role in energy and food security, human health, and ecosystem conservation (Gleeson et al., 2016). Investigating the residence time of groundwater (i.e. the period from recharge to drainage in pumping wells, springs, or streams) reveals information about water storage, mixing, and transport in subsurface water systems (Cartwright et al., 2017; Dreuzy and Ginn, 2016; McGuire and McDonnell, 2006). This is particularly important in alluvium aquifers where groundwater renewability is generally strong (Huang et al., 2017), thus functioning as potable water sources in arid areas. Moreover, alluvium aquifers are increasingly vulnerable to anthropogenic contaminants and land-use changes (Morgenstern and Daughney, 2012).
Because the residence time distribution in subsurface water systems cannot be
empirically measured, a commonly used approach is parametric fitting of trial
distributions to chemical concentrations (Suckow, 2014).
The widely used lumped parameter models (Małoszewski and Zuber, 1982;
Jurgens et al., 2012), which commonly assume that the hydrologic system is at
a steady state, have been applied to subsurface water systems (Cartwright et
al., 2018; McGuire et al., 2005; Morgenstern et al., 2015; Stewart et al.,
2010). The groundwater residence time tracers can be classified into three
types depending on the time span they measure. The first, isotopes of water
(
The age of water can be determined through
In contrast to
Additionally, mixing between water of different ages, which occurs within the aquifer or during pumping from long-screened wells (Cook et al., 2017; Custodio et al., 2018; Visser et al., 2013), poses difficulties for estimating MRTs using tracer data. The calculated MRTs will be less than the actual values in the mixed water due to aggregation errors (Cartwright and Morgenstern, 2016; Kirchner, 2016; Stewart et al., 2017). MRT estimation using a multi-model approach based on incorporated residence time tracers would reduce the calculation uncertainty (Green et al., 2016; Visser et al., 2013) and indicate whether MRTs can be realistically estimated (Cartwright et al., 2017).
Maps showing
Mixing within the aquifers and during the pumping process from the
long-screened wells is expected to be common in the fault-influenced
hydraulic drop alluvium aquifers of the Manas River Basin (MRB) in the arid northwest of China (Fig. 1a, b). In particular, pumping from long-screened
wells (of which there are over 10 000 boreholes; Ma et al., 2018) makes
groundwater mixing most likely to occur. MRTs that result from a deep
unsaturated zone (with water table depths of up to 180 m) and contrasting
geological settings (hydraulic head drops of as much as 130 m caused by the
thrust fault) are still insufficiently recognised in the alluvium aquifer
(Fig. 1c). We aim to provide the first estimation of MRTs from borehole
groundwater drainage (e.g. well withdrawal) using CFCs and
The bedrock of the upper Manas River catchment in the mountain area of northwestern China consists of granites, sedimentary formations of Devonian and Carboniferous age, and Mesozoic limestone (Jelinowska et al., 1995). Pyroclastic rock is exposed in relatively small areas in the mountain to the south. The piedmont and oasis plains are within the Cenozoic strata, including Tertiary and Quaternary deposits with a total depth of more than 5000 m in the piedmont area and 500–1000 m in the centre of the plain (Zhao, 2010). The vertical cross section (Fig. 1c) shows that the Quaternary deposits consist of pebbles, sandy gravel, and sand in the piedmont plain. The clay content in the Quaternary deposits increases from the overflow spring zone to the north oasis plain, which consists of silty loam and clay. The Huoerguosi–Manas–Tugulu thrust faults occurred in the early Pleistocene and cut the Tertiary strata with a total length of approximately 100 km in the piedmont alluvial fan (Fig. 1); these are water block features. These faults were intermittently active in the middle to late Pleistocene and then were more active from the late Holocene (Cui et al., 2007).
In the mountain area, groundwater consists of metamorphic rock fissure
water, magmatic rock fissure water, clastic rock fissure water, and Tertiary
clastic rock fissure water (Cui et al., 2007; Zhou, 1992). In the piedmont
plain of the Shihezi (SHZ) zone, groundwater is from a single-layer
unconfined aquifer. From the overflow spring zone to the central oasis
plain, groundwater consists of shallow unconfined water and deep confined
water. The hydraulic gradient, hydraulic conductivity, and transmissivity
show a large range of variations due to changes in grain size and local
increases in clay content (Wu, 2007). The groundwater flow direction is
consistent with the Manas River flow direction. In the piedmont plain, the
unconfined aquifer with saturated thickness more than 650 m is recharged by
the Manas River water and is hydraulically connected to the hydrological
network in the piedmont plain and north oasis plain (Ma et al., 2018; Wu,
2007). The depth of the piedmont plain unconfined aquifer decreases
gradually from south to north and has relatively fresh groundwater with total dissolved solids (TDS)
of < 1 g L
Chemical–physical parameters, stable isotopes, CFC concentrations,
tritium (
Water sampling sites and unconfined groundwater head contours (in metres) in the headwater catchments of Manas River. UG: upstream groundwater, MG: midstream groundwater, DG: downstream groundwater.
A total of 29 groundwater samples (pumped from fully penetrating wells and three artesian wells) were collected along the Manas River between June and August 2015 (from G1 to G29 in Table 1 and Fig. 2). Locations were separated into three clusters based on the hydrochemistry and stable isotope data: the upstream groundwater (UG, south of the Wuyi Road), midstream groundwater (MG, area between the Wuyi Road and the West Main Canal–Yisiqi), and downstream groundwater (DG, north of the West Main Canal–Yisiqi). Groundwater was sampled from wells for irrigation and domestic supply, in which shallow wells were pumped for a minimum of 5 min before sampling and deep wells were active for irrigation for more than 10 days prior to sampling. Surface water sample data (river water, ditch and reservoir water) and groundwater sample data (sample ID are from G30 to G39) were reported by Ji (2016) and Ma et al. (2018).
Water temperature (
For CFC samples extreme precautions were taken to avoid contamination from
equipment such as pumps and tubing (Cook et al., 2017; Darling et al., 2012;
Han et al., 2012). After purging the wells the water samples were collected
directly from the borehole using a copper tube sampling pipe. One end of the
pipe was connected to the well casing, and the other end was placed in the
bottom of a 120 mL borosilicate glass bottle, inside a 2000 mL beaker. The
well water was allowed to flow through the tubing for 10 min, thoroughly
flushing the tubing. The bottle was submerged, and then filled and capped
underwater when no bubbles appeared in the bottle following the protocols
described by Han et al. (2007). In this study, five bottles were collected at
each well, three of which were analysed. A total of 10 wells were collected for
CFC analysis (CFC-11, CFC-12, and CFC-113). Unfiltered groundwater samples
for
The CFC concentrations were analysed within 1 month of sample collection at
the Groundwater Dating Laboratory of the Institute of Geology and
Geophysics, Chinese Academy of Sciences (IGG–CAS), using a purge-and-trap
gas chromatography procedure with an electron capture detector (ECD). The
procedure was reported by Han et al. (2012, 2015) and Qin et al. (2011), which is modified from Oster et al. (1996). The detection limit for
each CFC is about 0.01 pmol L
The
The cation, anion, and stable isotope measurements were performed at the
State Key Laboratory of Biogeology and Environmental Geology, China
University of Geosciences, in Wuhan. Cations were analysed using
inductively coupled plasma atomic emission spectrometry (ICP–AES) (IRIS
Intrepid II XSP, Thermo Elemental). Anions were analysed on filtered
unacidified samples using ion chromatography (IC) (Metrohm 761 Compact IC).
Analytical errors were inferred from the mass balance between cations and
anions (with
Knowledge of the history of the local atmospheric mixing ratios of CFCs in
precipitation is required for indicating modern water recharge. The
difference between the local and global background atmospheric mixing ratios
of CFCs in the Northern Hemisphere –
Concentrations of CFC-11, CFC-12 and CFC-113 (pptv) in
the groundwater of this study area sampled in 2015 compared with the time
series trend of Northern Hemisphere atmospheric mixing ratio at a recharge
temperature of 10
Measured CFC concentrations (in pmol L
Carbon-14 (
Depending on knowing the measured
Previous studies in the arid northwest of China (Edmunds et al., 2006; Huang et
al., 2017) have concluded that a volumetric value of 20 % “dead” carbon
derived from the aquifer matrix was recognised, which is consistent with the
value (10 %–25 %) obtained by Vogel (1970). Therefore, the initial
Groundwater mixing may occur both within the aquifer and in the long-screened
wells (Cook et al., 2017; Custodio et al., 2018; Visser et al., 2013). A wide
range of the groundwater residence times (ages) has been reported in an arid
unconfined aquifer because recharge occurs under various climate conditions
(Custodio et al., 2018). Furthermore, the groundwater residence time with
wide variabilities governed by the distribution of flow paths of varying
length cannot be measured directly (de Dreuz and Ginn, 2016; Suckow, 2014). A
lumped parameter model may be an alternative approach to describe the
distribution of residence times, which at the same time describes a mean
residence time for the mixtures of different residence times. With the aid of
gaseous tracers (e.g.
In this study, the CFC concentrations from the time series trend of the
Northern Hemisphere atmospheric mixing ratio (Fig. 3) and
Tritium concentration (TU) of the upstream groundwater (UG),
midstream groundwater (MG), and downstream groundwater (DG). Time series of
tritium concentration in precipitation at Ottawa, Urumqi, Hong Kong, and
Irkutsk were obtained by GNIP in IAEA (
Several residence time distributions have been described (Małoszewski and
Zuber, 1982; Jurgens et al., 2012) and have been widely used in studies of
variable timescales and catchment areas (Cartwright and Morgenstern, 2015,
2016; Cartwright et al., 2018; Hrachowitz et al., 2009; Morgenstern et al.,
2010, 2015; McGuire et al., 2005). The selection of each model depends on the
hydrogeological situations in the hydrologic system to which it is
applicable. The exponential piston flow model (EPM) describes an aquifer that
contains a segment of the exponential flow followed by a segment of piston
flow. The piston flow model assumes minimal water mixing from different flow
lines and little or no recharge in the confined aquifer; the exponential flow
model assumes that water mixing between flowlines in the unconfined aquifer
is minimal and that flowlines have an exponential distribution of transit
times
(Jurgens et al., 2012; Małoszewski and Zuber, 1982). The weighting function
of this model is given by
Each residence time distribution has one or two parameters. MRTs (
The
Groundwater deuterium excess values (
The hydrochemistry compositions of surface water and groundwater in the MRB
reflect the evolution from the fresh
Piper diagram highlights the
Stable isotopes (
Three groundwater clusters can be identified in the
The second group has the average
The third group, which is most depleted in heavy isotopes (
Table 1 shows that groundwater with well depths of 13–150 m contained
detectable CFC concentrations (0.17–3.77 pmol L
Calculated results for CFC atmospheric partial pressure (pptv), fraction of post-1940 water, modern precipitation recharge year, and mean residence times (EPM, DM, and EMM).
The groundwater aerobic environment (Table 1; DO values from 0.7 to 9.8 mg L
G5 and G7 are located in the east bank of the East Main Canal in the midstream area and are closer than G15 and G16 north of the reservoir, showing that the modern recharge is much earlier than that of G15 and G16 (Table 2). This can be explained by the lower groundwater velocities in the east bank of the East Main Canal, where the hydraulic gradient (Fig. 2) is much smaller than that in the west. Furthermore, groundwater recharge south of the reservoir (G25, G8, and G9) becomes much earlier with increasing well depth from 48 to 100 m, whereas that north of the reservoir becomes much later with increasing well depth from 23 to 56 m (G15 and G16; Table 2, Fig. 2). The different trends for the relationship between groundwater recharge year and well depth may be due to the different flow paths between the two sites.
Plots showing relationships of
Comparing CFC concentrations helps to identify samples containing young
(post-1940) and old (CFC-free) water (Han et al., 2007, 2012;
Koh et al., 2012) or exhibiting contamination or degradation (Plummer et
al., 2006b). The cross-plot of the concentrations for CFC-113 and CFC-12
(Fig. 7a) demonstrates that all of the groundwater can be characterised as
binary mixtures between young and old components, though there is still room
for some ambiguity around the crossover in the late 1980s (Darling et al.,
2012). As shown in Fig. 7a, all of the MG samples were located in the shaded
region, representing no post-1989 water recharge. The UG (G3) sample is
clearly relatively modern and seems to have been recharged in 1990 through
piston flow or mixed with old water and post-1995 water. Using the method
described by Plummer et al. (2006b) with the binary mixing model, the
fractions of young water are found to vary from 12 % to 91 % (Table 2) for
the MG samples with the relatively low young fractions of 12 % and 18 % in
the MG samples from the east bank of the East Main Canal (G5 and G7). These
two well water tables are deeper than 40 m, suggesting a relatively slow and
deep circulated groundwater flow. This hypothesis is also suggested by the
lower DO (3.7–4.6 mg L
CFC contamination and sorption in the unsaturated zone during recharge
considerably influence the interpretation of groundwater recharge. Points
off the curves in the cross-plot of CFC concentrations may indicate
contamination with CFCs from the urban air during sampling (Carlson et al.,
2011; Cook et al., 2006; Mahlknecht et al., 2017) or the degradation or
sorption of CFC-11 or CFC-113 (Plummer et al., 2006b). Figure 7
demonstrates that CFC contamination from the urban air, which generally
increases CFC concentrations above the global background atmospheric CFC
concentrations for the Northern Hemisphere, are unlikely. Elevated CFC
concentrations have been reported in the air of urban environments such as
Las Vegas, Tucson, Vienna, and Beijing (Barletta et al., 2006; Carlson et
al., 2011; Han et al., 2007; Qin et al., 2007), but not in the arid northwest of China (Barletta et al., 2006). Hence, the anomalous ratios of
CFC-11
The time lag for CFC transport through the thick unsaturated zone (Cook and Solomon, 1995) and degradation (especially as CFC-11 is common in anaerobic groundwater; Horneman et al., 2008; Plummer et al., 2006b) are both important considerations when interpreting groundwater recharge using CFC concentrations. The time lag for CFC diffusions through the deep unsaturated zone in simple porous aquifers, a function of the tracer solubility in water, tracer diffusion coefficients, and soil water content (Cook and Solomon, 1995), have been widely proved (Darling et al., 2012; Qin et al., 2011). The small differences in CFC-11 and CFC-12 recharge years (Table 2) demonstrate that the time lag should be short in the fault-influenced hydraulic drop alluvium aquifers with the deep unsaturated zone (Fig. 1c). Studies on the MRB (Ma et al., 2018; Zhou, 1992) have shown that groundwater is mainly recharged by fast river leakage in the upstream area and piedmont plain, where the soil texture consists of pebbles and sandy gravel (Fig. 1c). This suggests that the unsaturated zone air CFC closely follows that of the atmosphere, so the recharge time lag through the unsaturated zone is not considered.
Groundwater recharge was determined using
Both
Distributions of
Combined use of CFCs and
Because atmospheric
Tritium and CFCs (CFC-11, CFC-12, and CFC-113) output
vs. mean residence times for different lumped parameter models estimated
using Eqs. (2) to (5). The input
Residence time distribution functions (Eqs. 3 to 5) are suited to several
specific hydrogeological situations (Małoszewski and Zuber, 1982). EPM is
particularly useful for the interpretation of MRTs in aquifers that have
regions of both exponential and piston flow (Cartwright et al., 2017). The
unconfined aquifers adjacent to the rivers (Fig. 1c) are likely to exhibit
exponential flow, and the recharge through the unsaturated zone (Fig. 1c)
will most likely resemble piston flow (Cartwright and Morgenstern, 2015; Cook
and Böhlke, 2000). For the time series of
Figure 11 presents MRTs determined from the time series of
It is seen from Fig. 11 that MRTs determined from the time series of
Groundwater MRTs in the west bank of the East Main Canal show an overall increasing trend with the distance to the mountain in MG and DG (Fig. 11b, c). It has been proven that much longer as well as much deeper flow paths usually give rise to much longer MRTs (Cartwright and Morgenstern, 2015, 2016; McGuire et al., 2005). On the other hand, groundwater MRTs in the east bank of the East Main Canal are much longer than those in the west bank. As shown in Fig. 2, this phenomenon can partly be ascribed to the relatively short distance to the mountain and much smaller hydraulic gradient in the west bank. Previous studies pointed out that groundwater MRTs would vary on account of the interplay of factors, like the uncertainties of the input concentrations and different models (Cartwright and Morgenstern, 2015, 2016), and mixing and dispersion in the subsurface flow systems. Moreover, the assumption of the homogeneous aquifer with a simple geometry may result in significantly different MRTs that are calculated by lumped parameter models to the actual MRTs (Cartwright et al., 2017; Kirchner, 2016; Stewart et al., 2017). Nevertheless, the homogeneous aquifers, being at steady state, justify the use of lumped parameter models to calculate MRTs in this study.
Strong correlations between hydrochemical components and groundwater age
permit their use as proxies for, or complementary to, age via previously
established relationships in similar lithological conditions. For example, an
excellent correlation between silica (
The combination of hydrochemistry concentrations and groundwater age data is
also a powerful tool for investigating the groundwater flow processes and
flow-through conditions (McGuire and McDonnell, 2006; Morgenstern et al.,
2010, 2015), and for identifying the natural groundwater evolution and the
impact of anthropogenic contaminants (Morgenstern et al., 2015; Morgenstern
and Daughney, 2012). The pH of the groundwater decreases from 10.1 to 8.6
over the age range from 19 to 101 years, with a log law fit of pH
The soda waters with an overall pH higher than 8.1 (Table 1) are in
disequilibrium with primary rock-forming minerals of the host rocks. The
incongruent dissolutions of the albite and anorthite through hydrolysis
reaction are as follows:
In this study, we used environmental tracers and hydrochemistry to identify
the modern and paleo-meteoric recharge sources, to constrain the different
end-members mixing ratios, and to study the mixed groundwater MRTs in
fault-influenced hydraulic drop alluvium aquifer systems. The aquifer below
the Manas River downstream area is recharged by the paleo-meteoric
precipitation instead of the lateral flow from higher-elevation regions. The
relatively modern groundwater with young (post-1940) water fractions of
87 %–100 % is obtained on the south side of the fault, indicating only a
small mixing ratio between old and young water. The
The data are available upon request.
XL and JL were responsible for
the
The authors declare that they have no conflict of interest.
This research was financially supported by the National Natural Science Foundation of China (no. U1403282 and no. 41807204). The authors would like to thank Yunquan Wang for the valuable discussions and suggestions for this paper. We wish to thank Xumei Mao, Dajun Qin, and Yalei Liu for sampling and laboratory works. We also wish to thank the editor and anonymous referees for their valuable suggestions and insightful comments. Edited by: Christine Stumpp Reviewed by: Michael Stewart and one anonymous referee