Coastal fresh water aquifers are an increasingly desirable resource. In a karstic aquifer, sea water intrusion occurs as a salt water wedge, like in porous media. However, preferential flow conduits may alter the spatial and temporal distribution of the salt water. This is typically the case when the outlet of the aquifer is a brackish spring. This paper shows that salinity and flow rate variations at a spring, where salinity is inversely proportional to discharge, can help to understand the hydrodynamic functioning of the aquifer and to locate the fresh water-sea water mixing zone deep inside the aquifer. The volume of water-filled conduit between the sea water intrusion zone and the spring outlet is calculated by the integral over time of the flow rate during the time lag between the flow rate increase and the salinity decrease as measured at the spring. In the example of the spring at Almyros of Heraklio (Crete, Greece), this time lag is variable, depending on the discharge, but the volume of water-filled conduit appears to be constant, which shows that the processes of salt water intrusion and mixing in the conduit are constant throughout the year. The distance between the spring and the zone where sea water enters the conduit is estimated and provides an indication of the position where only fresh water is present in the conduit.
Two of the main concerns for wastewater reuse in agricultural irrigation are environmental and human security. Different research studies and practices have been developed recently in order to quantify the risk of possible environmental contamination of surface and groundwater resources as well as the risk to public health from enteric viruses. Other, far from negligible, related issues are largely of a socio-economic and technical character. For example, the social acceptance, especially by farmers, of wastewater reclamation and reuse in agriculture is influenced by specific local cultural, religious and socio-economic conditions. Economic and technical factors should also be taken into consideration, such as the water and wastewater treatment costs, the cost of maintenance, the employment of rural labour, the structure of irrigation networks and crop patterns. In this paper, the concept of sustainability in wastewater reclamation and reuse is formulated by using a general risk analysis framework and by taking into account all the above factors. The paper suggests a methodology for sustainable wastewater reuse in agriculture by considering not only technical and economic factors, but also environmental and social risks. Alternative strategies based on different treatment and irrigation technologies are evaluated using the multicriteria decision analysis technique. The methodology is illustrated in a case study of wastewater reclamation and agricultural reuse in the city of Thessaloniki, Macedonia, Greece.
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