The problem of salt recycling and seawater intrusion in coastal irrigated plains: an example from the Kiti aquifer (Southern Cyprus)

In coastal aquifers which are exploited for agricultural purposes, salinisation by salt recycling from irrigation is superimposed on the effects of seawater intrusion. Water quality degradation of irrigation pumping wells caused by seawater intrusion further enhances salinisation by irrigation, as the extracted solute mass is recycled and is not withdrawn from the system. The main objective of this study is the investigation and quantification of the impact of solute recycling from irrigation relative to seawater intrusion. A solute mass budget was established by expressing the solute mass return flow as fraction of the extracted solute mass from wells by means of a solute mass return flow ratio (rr). The obtained expression for the relative contribution of solute recycling from irrigation is an exponential function of the return flow ratio rr and normalised time t̄ only (time versus system turnover time). This expression was applied to an example, the Kiti aquifer (Southern Cyprus), where field observations suggest that solute return flow is a super-imposed salinisation mechanism. The contribution from solute recycling normalised with the solute mass flux entering from the sea after 20 years was found to be 1.5–8.5% in the extracted solute mass flux, depending on the estimation of the system turnover time. Subsequently, a coupled finite element model, reflecting the main features of the Kiti aquifer was used as a possible ‘synthetic reality’, to test the relative impact of solute recycling on the spatial salinity distribution in a complex hydrogeological and geometrical setting. This was done by running two simulation scenarios: (1) recycling all the extracted solute back into the system and (2) leaving solute recycling aside and comparing the results of these two scenarios relative to each other and to patterns observed in the field. The results showed, that by introducing solute recycling into the numerical model as coupled boundary condition does not only respect the overall solute mass balance but can have an important impact on the salinity distribution, leading to a significant spreading of the mixing zone, similar to what was observed in the field.