Periodic Table--Hydrogen

Hydrogen has two stable isotopes: protium (¹H) and deuterium (²H, D). Deuterium comprises 0.0184-0.0082% of all hydrogen (IUPAC); ratios of deuterium to protium are reported relative to the VSMOW standard reference water. A radioactive isotope, tritium (T or ³H) is discussed at the end of the section.

Craig (1961a) observed that the d¹⁸O and d²H (dD) values of precipitation that has not been evaporated are linearly related by:

dD = 8 d¹⁸O + 10

Natural processes can cause waters to plot off the GMWL. Water that has evaporated or has mixed with evaporated water typically plots below the meteoric water line along lines that intersect the MWL at the location of the original un-evaporated composition of the water; evaporation lines with slopes in the range of 2 to 5 are common. Low temperature diagenetic reactions involving silicate hydrolysis can sometimes cause increases in the d¹⁸O and dD values of waters.

The two main factors that control the isotopic signature of precipitation at a given location are 1) the temperature of condensation of the precipitation and 2) the degree of rainout of the air mass (the ratio of water vapor that has already condensed into precipitation to the initial amount of water vapor in the air mass). Most water vapor in the atmosphere is derived from evaporation of low-latitude oceans. Precipitation derived from this vapor is always enriched in D and ¹⁸O relative to the vapor, with the fractionation between the rain and vapor a function of condensation temperature. Therefore, progressive rain-out as clouds move across the continent causes successive rain storms to become increasingly isotopically lighter (more negative d values).

Precipitation is the ultimate source of ground water in virtually all systems. Hence, knowledge of the factors that control the isotopic compositions of precipitation before and after recharge allows the use of oxygen and hydrogen isotopes as tracers of water sources and processes. On a regional scale, the distribution of isotopic compositions are controlled by several factors:

• Altitude effect: On the windward side of a mountain, the d¹⁸O and dD values of precipitation decrease with increasing altitude. Typical gradients are -0.15 to -0.5 ‰ per 100m for ¹⁸O, and -1.5 to -4 ‰ per 100m for D. This pattern is often not observed in interior mountains, for snow, or on the leeward side of mountains.
• Latitude effect: The d¹⁸O and dD values decrease with increasing latitude because of the increasing degree of "rain-out".
• Continental effect: Isotopic ratios decrease inland from the coast.
• Amount effect: The greater the amount of rainfall, the lower the d¹⁸O and dD values of the rainfall; this effect is not seen in snow.

Once the rain or snowmelt passes into the saturated zone, the d values of the subsurface water change only by mixing with waters that have different isotopic contents. The homogenizing effects of recharge and dispersive processes produce ground water with d values that approach uniformity in time and space, and approximate a damped reflection of the precipitation over a period of years. It is important to remember that although an individual storm may be large and isotopically very different from the "old" water in the catchment, the amount of precipitation that infiltrates will likely be small compared to the amount of "old" water in storage. Although there may be significant storm-to-storm and seasonal isotopic variations in precipitation, baseflow d- values remain relatively uniform in most streams in humid, temperate areas.

In small catchments, temporal and spatial variability in ground-water and baseflow d- values may reflect seasonal variability in precipitation. However, these variations are less extreme and usually delayed relative to the temporal variations that occur in precipitation (Turner et al., 1987). These variations can be utilized for water residence time calculations. Superimposed on the seasonal cycles in precipitation are storm-to-storm and intra-storm variations in the d ¹⁸O and d D values of precipitation. These variations may be as large as the seasonal variations. It is this potential difference in d values between the relatively uniform "old" water and variable "new" water that permits isotope hydrologists to determine the contributions of old and new water to a stream during periods of high runoff (Sklash et al., 1976).

Hydrogen isotopic ratios can be determined for many H-bearing compounds, including organics, silicates, and gases. The d D values of all these compounds are reported relative to VSMOW. For a brief review of the compositions of these materials and the processes that affect their compositions, see Hoefs (1987). A "dual" or "multi-isotope" approach to determining sources or geochemical processes takes advantage of the fact that compounds consist of several elements, many of which can be easily analyzed for their isotopic ratios. For example, organic matter can be analyzed for the major elements C, H, N, O, S -- and for trace elements.

Organic-H:

Silicate-H:

Tritium (³H, T) is a radiogenic and radioactive isotope of hydrogen with a half-life of 12.43 years (IAEA, 1981) which decays to ³He. It is an excellent tracer for determining time scales for the mixing and flow of waters, and is ideally suited for studying processes that occur on a time-scale of less than 100 years. Tritium content is expressed in tritium units (TU) where I TU equals 1 ³H in 10¹⁸ atoms of hydrogen. Prior to the advent of atmospheric testing of thermonuclear devices in 1952, the tritium content of precipitation was probably in the range of 2-8 TU (Thatcher, 1962); this background concentration is produced by cosmic ray spallation. Since 1952, tritium produced by thermonuclear testing ("bomb tritium") has been the dominant source of tritium in precipitation. A peak concentration of several thousand TU was recorded in precipitation in the northern hemisphere in 1963, the year that the Atmospheric Test Ban Treaty was signed. After 1963, the tritium levels in precipitation began to decline gradually because of radioactive decay, mixing into the ocean, and the cessation of atmospheric testing.

Tritium concentrations in precipitation are affected by latitude, distance from the ocean and season. Water vapor over the ocean has a low tritium concentration due to exchange with oceanic surface water, which has a low tritium content. With progressive movement of air masses across the continent, tritium concentrations in precipitation increase due to additions of tritium from the stratosphere and evapotranspiration. The seasonal effect is primarily caused by the breakup of the tropopause between 30 and 60° N latitude each spring, causing a leakage of stratospheric water vapor with a higher tritium content into the troposphere. This leakage results in a north-south gradient in tritium concentrations in precipitation, with higher concentrations in the north and spring-summer high concentrating that are typically 2.5-6 times greater than the winter lows (Gat, 1980).

The simplest use of tritium is to check whether detectable concentrations are present in the water. For waters with higher tritium contents, some fraction of the water must have been derived after 1953; thus, the tritium concentration can be a useful marker for recharge since the advent of nuclear testing. There are a number of models in use for this application. The simplest is the piston-flow method, which assumes that ³H moves like a slug through the aquifer. To use the piston-flow approach, it is necessary to know the direction of the flow and to collect samples along the flow path; consequently, the main problem with this approach is the assumption that no dispersion or mixing occurs as the water moves through the aquifer. The opposite of this approach is the reservoir or box model, which assumes that the system is the same everywhere, and that the mixing of the reservoir occurs on a short time scale compared to the input (this rarely holds true for ground-water systems). Compartment models strike a balance between these two extremes. The aquifer is envisioned as a series of compartments, with water coming in from other compartments or the boundaries, becoming mixed internally, and flowing out to other compartments or discharge points (see Przewlocki and Yurtsever, 1974).

For storm and snowmelt runoff separations, distinct "old" and "new" water ³H values are required for storm and snowmelt runoff hydrograph separation. In some larger catchments with residence times (the average time it takes for precipitation to enter the ground and travel to the stream) longer than a few months, old and new waters may be distinctive as a result of the gradual decline in precipitation ³H values since 1963 and the even more gradual decline in ground-water ³H values.

Tritium measurements are frequently used to calculate recharge rates, rates or directions of subsurface flow and residence times. For these purposes, the seasonal, yearly and spatial variations in the tritium content of precipitation must be accurately assessed (see Michel, 1989) and Plummer et al. (1993).

Further information can be found in Clark and Fritz (1997), Environmental Isotopes in Hydrology (CRC Press):

• Tritium
• Tritium in Precipitation
• The Tritum-d¹⁸O Correlation in Precipitation