Groundwater recharge sources using isotopes and tracers

Abstract

Groundwater recharge sources are local precipitation and surface-water leakage. Isotope exchange along with some evaporation takes place.

δ2H and δ18O show homogeneous results along with the groundwater in well depth, indicating inter-aquifer mixing processes. 87Sr/86Sr ratios and δ18O values were found to be useful in studying Cl, Na, Mg, Ca and Sr concentrations to distinguish groundwater (based on recharge sources)

Two main processes are identified:

1.     Leakage from the river and lateral mixing of groundwater 

2.     Vertical mixing 

The 87Sr/86Sr ratios and selected ion ratios showed that carbonate dissolution and groundwater mixing with silicate. 

Introduction

·        Stable isotopes of deuterium and oxygen-18 are natural tracers of water, which are supplied by precipitation.

·        Strontium isotopes (87Sr/86Sr) are used to verify the mineralization processes conditioned by water-rock interactions with carbonate  or with silicate-dominated aquifers, mixing between different groundwaters which flows through lithology with distinctive 87Sr/86Sr ratios and water-rock interactions

·        The reasons for using 87Sr/86Sr ratios in hydrogeology are, firstly, that dissolution and mineral precipitations have no effect on it.

·        Secondly, 87Sr is produced by the decay of 87Rb which has a half-life of 48.8 Ga (Giga-annum=10⁹ years) which means no change in 87Sr/86Sr ratios due to the very long half-life of 87Rb in groundwater compared to the groundwater residence time of <1 Ma (Mega-annum =10⁶ years)

·        Thirdly,  87Sr/86Srratios have a predictable range of surface and groundwaters minerals.

·       High 87Sr/86Sr ratios in groundwater are generally from silicates such as biotite, feldspar (Feldspars are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight) and plagioclase (a member of the feldspar group) that have interacted with these minerals.

·        While water interacting with Ca-rich minerals such as calcite or gypsum have low 87Sr/86Sr ratios.

·        Fourthly, the groundwater 87Sr/86Sr ratios also depend on time for which water is in contact of rock and type of rock in contact with. e.g. rocks involving granites, Low 87Sr/86Sr ratios have been reported with the hydrolysis of plagioclase.

·        Over 10,000 boreholes were drilled for abstracting groundwater with a groundwater-level decline of around 0.72–1.07 m/year since 1987.

·        The MRB (Manas River Basin) is recharged by five stream systems with a total mean surface runoff of 22.98 × 108 m³/year.

·        The MRB is recharged by five streams t

•     Taxi River, 
•     Manas River, 
•     Ningjia River, 
•     Jingou River 
•     Bayingou River.

Sampling and analytical methods

·        Wells for irrigation and domestic supply were pumped first for a minimum of 5 min before sampling.

Using calibrated Hach (HQ40d) conductivity and pH meters, Water temperature, pH values, electrical conductivity(EC) and dissolved oxygen (DO) were measured in the field.

    

·        Bicarbonate was determined by titration with 0.05 N HCl on site.

·   Samples to be measured for isotopic concentrations were filtered on-site through 0.45-μm milli-pore syringe filters and stored in pre-cleaned polypropylene bottles at 4 °C until analysis.

·      For cation and strontium isotope analysis, the samples were acidified to pH<2 with ultrapure HNO3.

Results

·        Groundwaters are vaiable chemically as compared to surace waters.

·        The percentages decrease from 55.0 to 1.8% for HCO3 and from 24.2 to 1.5% for Si, and increase from 23.8 to 96.7% for Cl + SO4 along the Manas River.

δ2H and δ18O values:

·        The hydrogen and oxygen isotopic compositions of the MRB vary from −75.9 to −70.2% and −11.6 to −9.8% for the surface water and vary from −114.9 to −68.5% and −12.7 to−9.9% for the groundwater.

·        The local meteoric water line (LMWL) (The line drawn with repect to average values of hydrogen and oxygen isotopes around the experimental site) was established by the monthly amount-weighted mean δ2H and δ18O values of precipitation.

·        Both the linear slope (7.3) and intercept (3.1) of the LMWL are lower than that of the Global Meteoric Water Line (The line drawn with repect to average values of hydrogen and oxygen isotopes around the globe) (GMWL, 8 and 10, respectively; Craig 1961).

·        The monthly amount-weighted mean δ2H and δ18O values for local rainfall vary from −34.7 to −147.1% and from −5.7 to−21.0%, respectively, showing much greater variations than the surface water and groundwater samples collected in the MRB.

·        In addition, the rainfall samples display more enriched δ2H and δ18O values in the summer than those in the winter, with amount-weighted mean values of −52.6 % and −7.7% in summer (May to September) and −103.2 and−15.1‰ in winter (October to April).

Strontium isotope (87Sr/86Sr):

·        Surface-water samples contain the lowest strontium (Sr) concentration, varying from 0.20 to 0.49 mg/L (mean 0.34 mg/L).

·        Sr concentrations in ground-water samples increase along the Manas River (0.62 mg/L to 0.99 mg/L to 1.99 mg/L).

·        The strontium isotope (87Sr/86Sr) in groundwater samples show a range in the MRB, varying from 0.70837 mg/L to 0.70937 mg/L with a total mean of 0.70892 mg/L.

·        The lowest value was observed in Manas River water 0.70191 mg/L.

Conclusions

·        The study of hydrogeochemistry and isotope (2H, 18O, 87Sr/86Sr) changes of surface water and groundwater along the groundwater flow paths in the Manas River Basin showed a detailed and comprehensive picture of water re-charge sources, mixing characteristics and mineralization processes at a drainage basin scale.

·        The spatial investigation of the isotopes  in water, as well as the GMWL and the local rain-water characteristics, indicate different recharge sources for the groundwater among Manas River Basin.

·        Groundwater recharge's one source is local precipitation and surface-water leakage.

·        Another source is Lateral flow recharged by the local precipitation and snowmelt with effect of little evaporation.

·        Both paleo-water (recharged during a period of colder and drier climate) and lateral flow (recharged by the adjacent mountains affected by strong evaporation) are the possible recharge sources for the groundwater.

·        The Isotope compositions selected ion concentrations, and 87Sr/86Sr ratios show two groundwater mixing characteristics along the groundwater flow direction.

·        Vertical recharge and lateral mixing in the piedmont plain and lateral flow recharge and vertical mixing in the north oasis plain and north desert are evidenced (Piedmont plain, north oasis plain and north desert are parts of Manas River Basin).

·        Both major ions and 87Sr/86Sr ratios indicate that water-rock interaction processes (like mineral dissolution and precipitation, and cation exchange) control the groundwater hydrogeochemistry along the flow direction.

·        The selected ion concentrations and 87Sr/86Sr ratios suggest that carbonate dissolution in the sedimentary layer of the upper MRB in the mountain area is one of the major hydrogeochemical processes controlling the MRB groundwater evolution.

·        This validates the concept of lateral-flow recharge from the southern mountain area, which is the main recharge source of the groundwater.

Reference:

·        Groundwater mixing and mineralization processes in a mountain–oasis–desert basin, northwest China: hydrogeochemistry and environmental tracer indicators Bin Ma, Menggui Jin, Xing Liang & Jing Li  Hydrogeology Journal volume 26, pages 233–250 (2018).