3.4  Identifying Recharge Processes

Peter Cook

3.4 Identifying Recharge Processes

Stable isotopes and noble gases can be used to infer recharge processes and/or locations. They are particularly useful for estimating the degree of evaporation prior to recharge, as well as recharge temperature, pressure and/or elevation. For example, in mountainous terrain, estimation of recharge temperature using either stable isotopes or noble gases can provide information on the elevation of recharge. In central Oregon, USA (Figure 27), measurement of ²H and ¹⁸O on melted snow samples collected from different elevations allowed a relationship between isotopic composition and elevation to be established for the region (Figure 28; James et al., 2000). The data indicated a depletion in δ ¹⁸O of approximately 0.18 ‰ per 100 m increase in elevation; values between approximately 0.15 and 0.50 ‰ per 100 m have been reported from other studies (Araguas-Araguas et al., 2000). Isotope ratios were then measured on nine springs in the same area of central Oregon and compared with the relationship determined on the melted snow samples. Several springs had ¹⁸O compositions indicating recharge at elevations only a few hundred meters higher than the spring location. In most cases, this corresponded with distances of approximately 10 km from the spring outlets, indicating relatively local flow systems. Springs at lower elevations had inferred recharge locations that suggested more remote recharge. Lower Opal Spring, for example, occurs at an elevation of approximately 600 m, but has an ¹⁸O composition indicative of recharge at an elevation of almost 2500 m, which would suggest that it is part of a large regional flow system. The authors also compared the calculated recharge elevation for each spring with the measured water temperature, to provide a qualitative indication of the depth of circulation of the groundwater along its flow path. The measured water temperature at Lower Opal Spring was 12 °C, whereas the mean annual surface temperature at the inferred recharge elevation of 2500 m, is less than 2 °C. Therefore, the authors concluded that this water must travel along a deep flow path, where it is heated geothermally. In contrast, many of the springs that drain local flow systems have water temperatures of between 3 and 4 °C, which is consistent with their inferred recharge elevation of 1500 – 1800 m.

Map showing location of spring sites in mountainous parts of central Oregon — **Figure** 27 – Location of spring sites in mountainous parts of central Oregon (white square) that were sampled by James and others (2000) in a study of recharge processes (Figure 28).

Figure showing determination of groundwater recharge elevations for groundwater discharging from springs. — **Figure** 28 – Determination of groundwater recharge elevations for groundwater discharging from springs. The solid line defines the relationship between δ ¹⁸O composition of precipitation and elevation and was determined experimentally from analysis of snow samples. The red circles show spring elevations and δ ¹⁸O compositions of spring discharge. Spring samples with δ ¹⁸O values below the precipitation trend line therefore indicate recharge from higher elevations. The recharge elevations are determined by drawing a horizontal line from the spring sample to the precipitation line, and then a vertical line to where it intersects the x-axis (After James et al., 2000).

The relationship between rainfall amount and stable isotopic composition can be used to determine the size of a rainfall event leading to recharge. In the arid Ti Tree Basin (Figure 29), central Australia, stable isotope samples of groundwater plot below the local meteoric water line, suggesting evaporation prior to recharge (Figure 30). Fitting a regression line to the data and extrapolating to its intersection with the local meteoric water line, gives a mean composition of ²H = -62 ‰ and ¹⁸O = -9.3 ‰ for the precipitation water that recharges the groundwater system. This corresponds to the isotopic composition of rainfall events collected during months that receive more than about 90 mm of precipitation. On average, such periods occur about once every 1 – 2 years, indicating that groundwater recharge occurs regularly in this arid environment. However, the analysis is limited by the lack of isotope data on rainfall during such wet periods.

Map showing location of the Ti Tree Basin in central Australia — **Figure 29** – Location of the Ti Tree Basin in central Australia, where the isotopic composition of groundwater has been used to infer the episodic nature of groundwater recharge (Figure 30).

**Figure** 30 – Stable isotopic composition of groundwater in the Ti Tree Basin, central Australia, compared with the local meteoric water line (LMWL) for Alice Springs rainfall. Rainfall data are only available as mean monthly values, and black circles denote the mean, amount-weighted isotopic composition of rainfall for months with differing magnitudes of large precipitation totals. Groundwater data appear to fall on an evaporation line (with a slope of approximately 3) that intersects the local meteoric water line at an ¹⁸O value of approximately -9.3 ‰. This composition reflects the isotopic composition of rainfall events for months that receive more than 90 mm of rain. On average, such events occur about once every 1 – 2 years, indicating that groundwater recharge occurs regularly in this arid environment (After Calf et al., 1991).

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