3.5 Estimating the Recharge Rate of River Infiltration
Where groundwater recharge is dominated by infiltration from a river, groundwater beneath the river will be young, and groundwater further from the river will be older. The rate of river infiltration can be estimated by calculating the velocity of water flow away from the river, based on this increase in age.
The Danube River in Hungary (Figure 31) is a major recharge source for adjacent coarse-grained alluvial aquifers, and groundwater dating of these aquifers permits estimation of the rate of river infiltration. In this study, groundwater age was estimated using 3H/3He dating. 4He and Ne data were used to estimate the recharge temperature and excess air, which was required to distinguish tritiogenic 3He (3He derived from 3H decay) from 3He originating from other sources (i.e., atmospheric solubility and excess air components). Since the river dominates recharge to these aquifers, groundwater flow away from the river is essentially horizontal, and plots of groundwater age versus distance appear as straight lines (Figure 32), even though samples are collected from wells with long screens. Based on the rate of increase in age with distance, the groundwater velocity in shallow wells (screened 5 to 15 m below surface) is estimated to be approximately 800 m/y, and in deeper wells (screened 50 to 100 m below surface) it is estimated to be approximately 530 m/y.
One of the best examples of the application of this technique to ephemeral rivers (those that flow for only a short period during the year) is the study of the Finke River, central Australia (Fulton, 2012). Carbon-14 activities in groundwater show a general decrease with distance from the river, reflecting an increase in groundwater age (Figure 33). At distances beyond 40 km, 14C activities are less than 22 pmC, equivalent to groundwater ages of more than 12,000 years. At 20 km from the river, the mean 14C activity is approximately 60 pmC, equivalent to a groundwater age of approximately 4000 years. Assuming an age of zero at the river (distance of zero), this gives a mean velocity of 20 km in 4000 years, or 5 m/y. Assuming an aquifer thickness of 200 m and porosity of 0.22, the author thus calculated mean recharge rates of 5×106 to 12×106 m3/y for a 35 km reach of the river. This is equivalent to a recharge rate 0.71 m/y, assuming a nominal river width of 200 m.
