Seawater intrusion in coastal aquifers: Effects of seasonal variations in extraction and recharge rates

Over-exploitation of coastal aquifers causes seawater intrusion and eventually contamination of groundwater wells in many parts of the world. Artificial recharge of treated wastewater can be used to avoid or control seawater intrusion. A common characteristic of many coastal areas in Southern Europe is that most of the annual water demand, and hence the amount of water that can be used for artificial recharge, is concentrated to a few summer months, whereas natural groundwater recharge takes place during the winter months. This paper addresses the effects of seasonal variations in pumping, as well as in artificial and natural recharge rates on the dynamics of saltwater intrusion and especially on the evolution of salinity in extracted groundwater. Specifically, numerical simulations are performed to examine the seasonal variability effects for a given set of extraction and recharge locations, but for scenarios with different artificial recharge rates. Parameter values for describing the aquifer and its hydraulic-hydrological properties are taken from a coastal aquifer in Israel. We show that predicted salinity in extracted groundwater under assumed temporally constant pumping and recharge conditions differs from the mean value of salinity under seasonally variable conditions. For long-term predictions, the assumption of temporally constant conditions may yield relatively small (less than 10%) overor underestimation of average salinity during the periodic steady-state stage that results from seasonally variable conditions. For short-term predictions, up to the common planning period of 25 years after starting a new groundwater management practice, the same assumption may lead to considerable (50-55%) underestimation of maximum salinity values under seasonally variable conditions. INTRODUCTION Saltwater intrusion jeopardizes the freshwater supply from many coastal aquifers in the world. Seawater may intrude into freshwater aquifers whenever groundwater levels are low due to the occurrence of dry periods and/or overexploitation of the aquifer. These conditions are normally observed in semi-arid regions like those located around the Mediterranean Sea at least during the summer period. The climatic conditions typical of these areas and the intensive summer tourism cause a pronounced decrease in groundwater recharge and increase in the water demand during the summer season. However, during winter, groundwater levels may be restored due to higher frequency of recharge incidences and lower water demand. These seasonal variations in pumping and recharge rates may have significant implications on the dynamics of seawater intrusion and, as a consequence, on salinity of pumped groundwater.