Characterization of the Ezousas aquifer in south-west Cyprus for the storage and recovery of treated sewage effluent

This thesis reports on research from a full-scale demonstration project to recharge a depleted aquifer with treated sewage effluent from the Paphos (Cyprus) wastewater treatment plant. The project artificially recharged the Ezousas river basin, located in the south-western coastal plain of Cyprus, with tertiary treated, disinfected effluent through a network of artificial infiltration ponds. The aims of the research were to determine the capacity of the aquifer to provide a suitable buffer for the flow from the wastewater without flooding and to measure the changes in reclaimed water quality. The aquifer hydraulics and treatment capacity, including recharge basins, were analysed using field and laboratory measurements. A geological field survey and modelling was used to assess, both by practice and theory the effectiveness of the Ezousas river aquifer for storage and recovery of the reclaimed water. The aquifer was found to be mainly composed of alluvium with typical hydraulic characteristics. The average porosity was 20% and hydraulic conductivity around 90 m/day, it was concluded the aquifer would be able to accept all the annual output of the treatment plant which was 5 Mm3 /a. The recharge network consisted of five groups of infiltration basins arranged on both banks of the River Ezousas about 2km upstream of the wastewater treatment plant. Each infiltration basin contained two, four or six recharge ponds, each basin was 2,000m2 in area with a depth of 1.5 m. A recharge pattern consisting of alternating weeks of wet-fill and drying cycles was found necessary to maintain the unsaturated zone below the ponds in order to maximise the amount of water that could be recharged whilst optimising water quality. The hydraulic impact of the artificial recharge and extraction from the field measurements of borehole water levels indicated recharged water down to 15m below the surface. Tracer tests on the groundwater flow, capture zone, residence times and mass balances of recharged and native waters gave widely varying residence times between 30 days and 5 years, these were attributed to the complex flow patterns found. Recharged water was sampled using a series of extraction wells located along the downstream river basin, starting at the infiltration ponds and then at stages downstream. Eight production and monitoring wells were tested including control samples up-gradient (upstream) from the ponds, to about 7km down-gradient (downstream). Water quality was analyzed for the standard wastewater constituents including indicator organisms, organic matter, nutrients N and P and the metals. It was one of the recommendations of the thesis however that attention also be paid to the persistent organics, including the pesticides, biocides, plasticizers and pharma residues. The chemical data was used to build and validate a solute transport model of the ponds and surrounding area to predict the transport and fate of priority contaminants. In this way, the geo-chemical potential for the retardation, attenuation and chemical or biochemical degradation processes taking place in the unsaturated and saturated zone were assessed. From the results it was concluded that for most analytes, which included metals, nitrate and common salts, the main processes were mixing and dilution by the native ground water. The extracted water was then a mix of waters according to the different residence times and flow of natural groundwater, giving a stable water quality for irrigation. A third reaction involving cation exchange with the local geology was however identified which reduced the concentrations of copper and phosphate beyond what was expected from just mixing. It was also concluded that denitrification did not occur because of a combination of the high quality of the effluent, the operational cycling of the ponds and the high porosity of the vadose zone. Previous work has found denitrification if the recycled water still contains organic matter, further work was recommended to determine the critical organic concentrations. The renovated water from the Ezousas wastewater reuse Project was able to meet the health and agronomic requirements for unrestricted irrigation. The risk of flooding with sewage effluent resulting from hydraulic mounding was also investigated to define the growth and decay of the mound. It was possible to report that after more than fifteen years of operation and a total infiltration of 40Mm3, there have been no signs of reduced hydraulic capacity or water quality.