Experiences based on some existing plants, where Pleistocene gravels are artificially replenished by polluted river water and which have recharge capacities of 1-25 mgd, demonstrate the importance of retention time and of oxygen balance in the highly permeable aquifers.

The retention time of the single water particles has considerable variation and depends on hydraulic dispersion which may be proved by tracer investigations. In one case, for a distance of 1,350 ft, the minimal flow time is 3.4 days, the average 22 days. The flow times of the different water particles prove a logarithmic normal distribution function.

The oxygen content of the artificially recharged groundwaters depends on the oxygen concentration and on the biochemical oxygen demand (BOD) of the raw water. From the data the oxygen balance of a groundwater can be evaluated. For example, on an average, a groundwater with 3.4 ppm O₂ and only 0.5 ppm full BOD was obtained from a river water with 12.6 ppm O₂ and 9.7 ppm full BOD (equal to a 5-day BOD of 4.5 ppm) after a retention time of 7 days. As

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to the oxygen balance, there are no significant differences between a percolative and a direct infiltration, probably because decisive oxygen gas quantities are no longer present after the first percolative seepage of a polluted raw water in an unsaturated zone above the water table. Absence of oxygen in a groundwater will involve the solution of iron and possibly also of manganese. Low oxygen concentrations also retard the elimination of bacteria and of tastes and odors. This elimination results preferably from aerobic biologic filtration. Furthermore, if the oxygen content does not exceed about 5 ppm no sufficient protective scales on the interior of the ferrous water supply pipes are formed. Therefore, poor oxygen balance systems must be improved by different processes of raw water pretreatment.

After the infiltration, the aerobic decomposition of organic matter and the elimination of bacteria mainly take place in short flow times. A case in point is a reduction of an average coliform content from 10⁶ per 100 ml to 10 per 100 ml after about 1 day minimal and 7 days average retention time. Other empirical results confirm the excellent biologic filtration of the water during its lateral flow through the gravels. After the beginning of an artificial recharge and the following elevation of the water table, the biologic activity in the inundated natural filter rises only step by step in function of operating time. In the mentioned case, optimal coliform results were obtained at first after an operation time of about 70 days.

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