Using Tritium Isotope in Groundwater to Estimate the Reservoir Capacity at Erbil, Northern Iraq

Abstract


Introduction
The tritium is heavy water moderated reactor and radioactive was discovered in 1950 in natural water.It is a hydrogen isotope that has a half-life of 12.43 years (Clark & Fritz, 2013) which is only radioisotope that genuinely dates groundwater because it is incorporated directly into the water molecule.Tritium concentration naturally created by cosmic radiation, although substantially more was produced during atmospheric nuclear weapons testing starting in 1953.Atmospheric nuclear device testing produced a massive amount of atmospheric tritium.This large amount of tritium was deposited in the stratosphere, contaminating rainfall patterns systems for almost four decades.Tritium concentrations in precipitation increased which was noticed their peak point in the spring of 1963 (Clark & Fritz, 2013).The presence of thermonuclear tritium in groundwater has been touted as clear evidence for active recharging for the past three decades.While thermonuclear tritium can still be found in some slowly moving groundwaters, current groundwaters are now dated using the primarily natural Tritium ( 3 H) signal.During the late 70s the isotope hydrology became a scientific and applicable discipline.Many hydrological studies used this maximum concentration from 1963 as a reference point.

Theory of Radioactive Decay Equation
The spontaneous radioactive hydrogen decay prosses is when a tritium sample (T) undergoes to change their number of particles with time.The following is the equation of tritium decay rate (White, 2015). Where: dT/dt is the rate of decay, N is the particle number, λ is the decay constant (its unit is time -1 ), and the negative sign before lambda means that the number of particles decreasing with time.Separating the variables and solving, it yields: (2) Equation 2 represents the radioactive decay equation, which the number of particles exponentially decreases with time.T o is the initial tritium concentration expressed in tritium units (TU) and T t is the residual activity (measured in a sample) remaining after decay over time t.Solving for (t) yields The half-life (t ½) of any given nuclear spices defines as the time period in which the sample decreases half to its original number (Mook, 2001).
Groundwater dating based on tritium decay implies that the tritium input (from precipitation) is known and that the "residual" tritium measured in a groundwater is the result of decay alone.Using tritium's half-life which is t1/2 = 12.43 years (Gat et al., 2000), the decay constant  = 0.0557.
The global distribution of tritium in precipitation was found by measuring the monthly precipitation composites and 3 H data from the GNIP (global network of isotopes in precipitation ) by IAEA (international atomic energy agency) database from 233 stations used for the purpose of high spatial resolution forecast map.The value of input tritium concentration in precipitation over the researched area is evaluated to be approximately 4-6 TU based on the global distribution map of tritium in precipitation (Terzer-Wassmuth et al., 2022).The feasible value to be used as an initial tritium is (T o = 4.20 TU) for groundwater dating, this gives the decay series of 70 years (1950s-2020s) in Table 1 which is the groundwater age estimation with tritium by using the tritium decay equation.(Telloli et al., 2022) indicated that source of natural tritium in the hydrosphere is meteoric supply, which can vary worldwide from 2 to 25 TU and, in the Central Mediterranean area, is around average annual value 5-8 TU.

Study Site
As shown in the Fig. 1-a, the Kurdistan region located at the northern part of Iraq country consists of Duhok , Erbil and Sulaymaniyah province.The study area map inside the Erbil city is zoomed at Fig. 1-b.The study area is also named as Dashty Hawler which is groundwater basin of Erbil city.This basin covers an area of 2660 km 2 which consists of three sub-basins named as Kapran at the north (720 km 2 ), Central sub-basin at center of Erbil city (1315 km 2 ) and Bashtapa sub-basin at the southern part (625 km 2 ).The maximum elevations ranged from 800 to 1087 m.asl for some catchments at the east of Kapran and Central sub-basins, while the minimum elevation ranged from 213 to 345 m.asl covers some location near the west.The aquifer conditions are confined, semi-confined, or unconfined as they relate directly to the recharge patterns.The groundwater of the Erbil Basin contains small amounts of soluble salts and harmful ions that could have negative impacts on human health (Dizayee, 2018).The water table has been found to decrease around 1.24 meters annually.For the purpose of tritium investigation at this catchment area, the total of 27 wells were selected that covered over three sub-basins.The groundwater samples drawn and collected in well-sealed plastic bottles.

Field Work and Sampling
In the present investigation, the total of 27 samples of groundwater are collected.Firstly, it is important to accurately record the coordinates of a sampling location on-site.Second, a well's static water level should be measured.Then well cap is removed carefully as shown in Fig. 2-a  Before collecting samples, three well volumes should typically be pumped out and the bottle washed with it (Sampling & Hydrology, 2000).This is due to the sample represents the groundwater inside the aquifer.Then two liters of water is drawn (1 liter was sufficient for isotope analysis as previously contacted with the laboratory).The sampling time is starting from 17 th July to 2 nd August of 2022.The field measurements of temperature were recorded using temperature measurement device (type: CheckTemp.) it is a digital thermometer with a 0.2°C accuracy.The device was turned on, then its rod was penetrated to water sample as shown in the Fig. 2-b.The water samples used in the current study's water isotope analysis range in temperature from 26.2 to 32.2 o C.

Laboratory Measurements
After sample collections, the bottles were transferred to the water supply laboratory of civil engineering/Salahaddin university for the purpose of measuring the pH value and electrical conductivity (EC).pH is the negative logarithm of hydrogen ions.Distilled water has pH equals to 7 at (25 °C).In this study, pH meter device type (Eutech™ pH 700) was used to measure pH value as shown in Fig. 3a of all groundwater samples.Is has digital display that record both temperature and pH value at the same time.Conductivity is measured in micro-siemens per centimeter (μS/cm) which describe the relative quantity of minerals dissolved in water.Distilled water has a lower conductivity because through the process of distillation the impurities were removed such as calcium, sodium, and magnesium.For the current study, EC was measured using a conductivity meter for all samples (CON-510) shown in Fig. 3-b.Stainless steel electrode washed with distilled water then penetrates to the water sample, after 3 to 5 minutes the readings of temperature and EC will be ready to record.

Oxygen-18 and Tritium Analysis
This step involved analyzing Oxygen-18 and Radioactive hydrogen (tritium) in water samples that transferred to the DSI laboratory at Ankara/ Turkey.The oxygen-18 in water samples will found out with cavity ringdown spectrometer CRDS (type: L2130-i isotopic H2O-Picarro) machine that runs groundwater samples as shown in Fig. 4-a.
While for determining the radioactivity of tritium in groundwaters, the liquid scintillation spectrometer (type: 1220 ultra-low level-Quantulus) were used as depicted in Fig. 4-b (Davies, 2006) explained that the activity of low energy radioisotopes, mostly beta and alpha emitting isotopes, is called liquid scintillation counting (LSC).The instrument consists of the sets of vails that install on the microplate inside the liquid scintillation spectrometer.The photomultiplier inside the photomultiplier tube transformed this light into an electric signal, which the pulse height analyzer analyzed and the data transferred to the desktop.

Measured Data
In this section, the overall field data including the well coordinates, elevations in (m) above mean sea level (AMSL) and groundwater sample temperature as well as the water qualities like pH and electrical conductivity of all groundwater samples were written in Table 2.

Results and Discussions
The results of isotope analysis of 27 samples of groundwater at different location over the study area were summarized in Table 3.The only radioactive element used in this study is tritium, their results over the Dashty Hawler basin were analyzed for groundwater dating.A map of the research region was contoured with these data.Following sections are descriptions of the key tritium results.

Tritium Distribution Over the Study Area
As was previously mentioned, the radioactive hydrogen isotope is tritium (T or 3 H), has a half-life of 12.43 years.Tritium concentrations are expressed in tritium units (TU), where 1 TU equals 1 tritium in 10 18 hydrogen atoms (IAEA, 2001).Small amounts of naturally occurring tritium are created in the earth via the alpha decay of lithium-7.The tritium atoms interact with oxygen to create water, which eventually turns into precipitation.The spatial distribution of tritium concentrations in groundwater over the Dashty Hawler basin is shown in Fig. 5.

Fig. 5. Distribution of tritium over Erbil groundwater basin
The average tritium concentration of groundwater samples for Kapran, Central and Bashtapa subbasins were found to be 1.30, 1.16 and 0.76 TU respectively, which were ranged from the tritium free up to maximum 6.1 TU.The lowest concentration which is tritium free were found for the groundwater at central towards the bashtapa sub-basins at wells No. 15, 20 and 25 located at Grda-rasha, Qushtapa and Shekhanan respectively.While its higher value was analyzed at the groundwater at well No. 10.The higher tritium indicated the young groundwater that recharged from recent precipitation while, lowest tritium, the older groundwater noticed (explained in next section).

Age Dating and Basin Capacity
The wide range of the tritium in groundwater refers to different residence times (groundwater ages), where the groundwater of high tritium value refers to low residence lime.The decay equation may be used to young groundwaters more easily since tritium values in modern precipitation are getting closer to natural values.Relationship between tritium and δ 18 O for groundwater in the study area were plot in Fig. 6, three age dating groups can be differentiated based on the determined tritium concentrations.This can find the dwell time of water in the reservoir.The following describes results of each group: Group (1): Two wells (No. 7 and 9) located at Kapran and Central sub-basins have tritium values (0.07 ± 0.62 TU) and (0.02 ± 0.68 TU), means that their age dating are about 73.5 and 96 years respectively (see Table 3).In this study, when tritium analysis is performed, the value less than 0.085 TU (limit of detection is 0.085 TU), which means a pre-bomb groundwater is estimated to be older than 70 years (as the tritium found in 1950s till now 2020s).On the other hand, three other wells (No. 15, 20 and 25) located at central and Bashtapa sub-basin, have no tritium values in their groundwater samples.The groundwater at these wells were older than 100 years, this finding indicated that the water are recharged from precipitation before the time of the nuclear test weapons.In case of this proportionality, long retention of groundwater reservoir of the aforementioned five wells can suggest inside their basins.therefore the groundwater at these basin should be remain as a groundwater reserve for Erbil city.
Group (2): Groundwater at well no. 10 with tritium value of 6.1 ± 0.82, TU cannot find their residence time with tritium isotope.Because this value is greater than the tritium of atmospheric precipitation ( 3 H = 4.2 TU) over the study area.This indicating that a well is contaminated with a source therefore the uncertainty of experimental works of this well's groundwater (± 0.82) is higher than other groundwater from wells.This type of tritium is geogenic for the geological formation of the well section.The geogenic tritium appears mainly due to fission of lithium in rocks that was absorbed directly into the groundwater with support of porosity of soil layers.Another reason is that due to water-rock interaction at high temperature, the exchange of an isotope with rock was forms minerals that affects the water isotope contents.
Group (3): Other 21 wells were estimated to be a mixture of groundwater at different recharge years.The minimum and maximum tritium value between these wells is 0.35 ± 0.66 TU at wells No. 17,18 and 22and (2.15 ± 0.74 TU at well No. 26).This concluded the mixture of water aging roughly between 44.6 to 12 years.This result indicated different types of basins, including long retentions of old groundwater and lower retention of younger groundwater at basins.The youngest groundwater with dwell time of 12 years is noticed in well No. 26 located at Rulka village within Bashtapa sub-basin.It indicates that the aquifer at the well position has small retention reservoir.Motzer (1982) indicated that the modern water is recharged from 5 to 15 years.Furthermore, if two wells at the Kapan sub-basin taken for comparison, like: well No. 1 at Kalak region has tritium value equals to 1.7 ± 0.72 TU that has groundwater ages roughly 16.2 years.While well No. 6 at Grd-Jutyar has lower tritium value equals to 0.72 ± 0.68 TU means greater age dating about 31.6 years.The size of reservoir of these two wells are different, retention of basin that save the groundwater of well No. 6 is longer than that of well No. 1 .
One of the reason for this high difference in residential time that both well that lie within the same groundwater aquifer is that the Kapran Sub-basin aquifer at position of well No. 1 with elevation of 266 m.AMSL is narrower that well No. 6 with an elevation of 440 m.AMSL.Second reason is that, there is an interfere between groundwater at these two wells layer because the δ 18 O values are close to each other which are for No. 1 and 6 is -5.95 and -5.69 respectively.But the well No. 1 with smallest δ 18 O is from deeper depth than well No. 6.

Water Quality and Salinity Source of Groundwater
The range of pH in 27 groundwater samples was 7.3 -8.31 and their mean value is 7.89.Higher values recorded nearly over bashtapa sub-basin at wells No. 22, 24, 25 and 27.As well as wells over central 12,15,16,17 and 18).The water is salinized from a variety of sources, some of which are natural and others are industrial.The spatial distribution of electrical conductivity (EC) value over the Dashty Hawler basin was shown in Fig. 7. It's clear that the maximum electrical conductivity recorded for well (No.18) at Aliyawa village which is 3670 μS/cm.The population do not use the groundwaters as their water supply.Because according to the Iraqi standards for showed by (Alanbari, 2015), the electrical conductivity for drinking water is 2000 μS/cm.therefore well No. 18 has a contaminated groundwater.The green shaded areas having the EC values less than 756 μS/cm which are very safe for drinking.
Stable isotopes can determine the primary mechanism of groundwater salinization for the source origin of salt.This will do by analyzing the relationship between δ 18 O and EC.On a semi-logarithmic graph, the results for tritium samples with an EC measurement were displayed in Fig. 8. Notable changes between δ 18 O and EC were detected, where the salinity origin resulted from groundwater at wells (No. 9, 18 and 21) that the electrical conductivity was (1022, 3670 and 1350) μS/cm and groundwater residence times are (95.9a, 44.6 a and 24 a) respectively.The higher the concentration of dissolved salts in the water, the greater electrical current can be conducted.As groundwater moves through rock in the aquifer it dissolves some of the minerals and salts along the way.The longer the groundwater stays in contact with the rock the more ions it dissolves and the higher conductivity achieved.According to Hassan (2022) the groundwater flow direction is from east to west of the study area.The groundwater at highest altitude of well No. 12 which is 910 m.asl flows towards the south-west of the groundwater basin.This illustrates that the EC collected at well No. 18 as a higher value.Groundwater mixing, evaporation processes and dissolution mechanism of Erbil landfill near well No. 9 at Kaniqrzhala are the source of the salinity in the research area.
The above result indicates that the EC is a measurement of amount the dissolved salts in water.Precipitation for tritium is probably more variable than for EC.EC is unlikely to be as high in precipitation as it is in surface water or groundwater since precipitation is not subject to weathering or water-rock reactions (Harms et al., 2016).

Conclusions
The main conclusions of the above findings were summarized in the following points: • The age dating of groundwater directly encourages regarding basin capacity.In the Dashty Hawler basin, it is reasonable to assume that there were several sizable subsurface retention reservoirs.• In case of old groundwater at Erbil basin, it is required for attention in controlling groundwater supplies so that the amount of water extracted does not exceed the amount of water recharged.This finding is important for underground water balance.• The dating with atomic weapon tritium ranges is used for dwell times of groundwater less than 100 years.Therefore, for accuracy of dating at wells (No. 7,9,15,20 and 25), carbon-14 measurements are recommended as its lower dating limit lies around 1000 years.

Fig. 6 .
Fig. 6.Relationship between Tritium and δ 18 O for groundwater in the study area.

Table 1 .
Tritium values at different times

Table 2 .
Field data of Selected wells and measurement of PH and EC of groundwaters.

Table 3 .
Results of isotope analysis of groundwater samples.