The radioactive isotope Tritium (δ3H) is a short-lived isotope of hydrogen with a half-life of 12.32 years. Tritium concentrations are represented in tritium units (TU). One tritium unit is equal to one molecule of δ3H per 1018 molecules of 1H and has an activity of 0.118 Bq/kg (3.19pCi/kg). The large pulse of tritium, that entered the hydrologic cycle from the middle of 1950’s to early 1960’s during atomic bomb test in the atmosphere, could be used to establish the age of groundwater recharge. Tritium concentration in ground water provides an indication for the mean residence time of water. Accurate dating of water based on tritium is not possible, because of the spatial and temporal variation in the tritium composition of the precipitation and the complicated mixing-processes that take place in each aquifer.
High levels of tritium (≥30 TU) indicate water that was recharged during the late 1950’s or early 1960’s; moderate concentrations indicate modern recharge; levels close to (~1 TU) are likely sub-modern or paleo-groundwaters that have mixed with shallow modern groundwaters.
Clark and Fritz (1997) estimated the following rough general guidelines for tritium:
<0.5 TU, sub-modern age (pre-1952),
0.5 - 5 TU, sub-modern and modern age (pre-1952 and post-1952), 5 - 15 TU modern age (<5 to 10 years age),
15 - 30 TU some bomb tritium,
30 - 50 TU recharge in the 1960‘s to 1970‘s,
>50 TU recharge in the 1960‘s (Bomb Tritium).
According to Carmi and Gat (1973), the tritium concentration in the precipitation is similar for the whole West Bank and Israel. During the 1950’s the tritium concentration in the precipitation of these areas was about 5 TU (Kaufman and Libby 1954). After 1953 the testing of nuclear weapons had increased the tritium level in the rainfall and consequently, ground water. This δ3H content of the atmosphere were peaking in 1963, after that the surface nuclear tests stopped and the tritium concentrations in precipitation decreased in general.
Carmi and Gat (1973 and 1978) stated that the tritium concentration in Israel was 30 TU in 1960, 522 TU in 1962, and 26, 24, 30 and 27 TU from 1975 to 1978. Tritium reaches the Dead Sea from the atmosphere by direct and delayed surface runoff, by molecular exchange through the surface of the lake, and from springs. The surface layers of the Dead Sea were first analyzed for tritium in 1960, when they contained 8.5 TU. A maximal value of 200 TU was
measured in 1964, and not more than 50 TU were measured in 1975; this further decreased to 30 TU in 1977 and 12 in 1979 (figure 6.5). During the course of this study (2007) one sample from the surface layer of the Dead Sea was analyzed for tritium, the tritium value was 3.6 TU.
The deeper waters contained 1.8 TU in 1960 and up to 7 in 1975-1977, but these measurements are less reliable than later ones (Ilana Steinhorn 1985). Ein Feshcha springs near the Dead Sea shore contained up to 36 TU (Mazor and Molcho, 1972), while during this study the maximum value was 1.2 TU. The tritium content of the samples collected from the study area during the course of this study presented in table 6.2.
Based on table 6.2, water samples have tritium concentrations around 0.4 TU dates back to the early 1950’s. An example of this water in the study area is that of the Azarea wells, which discharges the lower confined aquifer. Water that has tritium concentrations 0.4 TU < δ3H <
3.5 TU dates back to the late 1950’s to 1960; an example of this group is the Herodion-1 with δ3H value of 1.5, which discharges the upper aquifer, Herodion-4 with δ3H content of 0.3 TU, tapping the Albian aquifer which discharges the lower confined aquifer, dates its water back to the early 1950’s, while 1.0 TU in the water of Beit Fajar-1a tapping the Turonian-Cenomanian aquifer dates it back to the late 1950’s. Tritium values greater than 4 is expected to have contact with the atmosphere after 1963, three springs in Jericho area (Wadi Qilt, Ein-Sultan and Ein-Dyouk) represents this group, the δ3H concentration of these springs indicate that their water discharge is out of the upper aquifer, suggests recent recharge and tritium of meteoric origin.
Depending on δ3H values for the groundwater in the Marsaba-Feshcha study area presented in table 6.2 and figure 6.6, it can be concluded that Ein Feshcha waters are originated from deferent aquifers in which some springs show water that has tritium concentrations < 0.4 TU, dates back to the early 1950’s, this water is considered sub-modern or paleo-groundwater, spring number 13 and 19 representing this group, and those with tritium values >0.4 TU indicating mixing of old seated waters with shallow modern groundwaters (figure 6.6). Kina spring and Ein Gadi spring shows tritium value of 0.7 TU, indicating possible mixing of sub-modern to sub-modern age waters (pre-1952 and post-1952), representing fresh water discharge from the upper and lower aquifer. These values could reflect a mixing process between two waters of different age, were the tritium units differentiate two different age end members of old to modern and sub-modern recharged water that represent this mixing.
The Ein Feshcha water samples has tritium values <0.5 TU dates back to the early 1950’s. The Ein Feshcha springs discharge from the mixture of the fresh water coming from upper and lower aquifer and mixes with saline water of the regional aquifer.
The low tritium concentration from the Ein Feshcha wells and springs emphasizes the presence of such mixing process. The lower tritium units ranging between 0 - 1.2 TU might reflect a mixing of recent age water infiltrated from wadis runoff through the alluvium fane with deep old water characterized by high salinity. The low δ3H values indicate that transport time from the recharge zone in the mountain areas is more than 40 years.
According to the figures 6.3, 6.4and 6.6 a mixing process between freshwaters of meteoric origin and significantly evaporated and isotopically modified brines (δ18O – δ2H) because of salts dissolution, seems to better explain the observed δ3H, δ2H vs. δ18O trend for the water samples.
Figure 6.5: Results of the tritium measurements at the Dead Sea (after Ilana Steinhorn 1985).
Figure 6.6: Scattered plot of 18O versus 3H for the samples of the study area.
Table 6.2: The tritium (δ3H) concentration in the groundwater and springs of the samples collected during the courses of this study.
Borehole,
Spring Sample# Description Coordination δ3H
(TU)