MAR with Untreated River Water: Clogging of Basins and Coliform Removal Rates

The Dresden-Hosterwitz water utility works operates five open managed aquifer recharge (MAR) basins supplied with pretreated Elbe River water and is located on the floodplain of the Elbe River. The quaternary aquifer consists of sand and gravel, and has a thickness of 9–14 m. The infiltration of untreated river floodwater and its effects on basin clogging and pathogen removal were investigated within a risk-based framework. Column experiments were performed to simulate infiltration in the basin during a flood event. Three vertical profiles were evaluated, which ranged from coarse filter sand to sandy gravel, and installed in sequence in an experimental flow-through column setup. Turbidity, total coliform, and E. coli were measured in the inflow and outflow at infiltration rates from $0.8–3.6\mathrm{m}/\mathrm{day}$. Experimental data was supplemented with clogging and pathogen measurements at the basin during a flood event, and infiltration of untreated river floodwater. The hydraulic conductivity of the filter sand decreased exponentially due to external clogging by two orders of magnitude from 126 to $4\mathrm{m}/\mathrm{day}$. It was $0.4\mathrm{m}/\mathrm{day}$ in field measurements during the flood event due to a higher particle load in the flood water. Total coliforms and E. coli numbers were reduced by $>4$ and $>3{\log }_{10}$ units after 2.5–4.4 m total travel distance. E. coli removal was greater than total coliform removal but E. coli counts in the river water were one order of magnitude lower than total coliform count. Mean turbidity was reduced from 14–0.9 nephelometric turbidity units after passing only 0.6–0.9 m filter sand. In field measurements, no breakthrough of turbidity and E.coli was observed during flooding and infiltration of untreated river water into the aquifer. In conclusion, a flood event of the infiltration basin and the subsequent infiltration of untreated river water into the aquifer poses no serious threat to water quality in terms of E. coli, total coliform, and turbidity at the abstraction wells at 70-m flow distance (4–5 days travel time), provided that the well heads are made flood-proof.