Abstract: The inverse problem of seawater intrusion(SWI) is reviewed. It represents a challenge because of both conceptual and computational difficulties and because coastal aquifer models display many singularities:(1) head measurements need to be complemented with density information; (2) salinity concentration data are
very sensitive to flow within the borehole. Data problems can be reduced by incorporating the measurement process within model calibration; (3) SWI models are extremely sensitive to aquifer bottom topography; (4) the initial conditions may be far from steady state and depend on the location and type of sea-aquifer connection. Problems with aquifer geometry and initial conditions can be addressed by parameterization, which allows for modification during inversion. The four sets of difficulties can be partly overcome by using tidal response and electrical conductivity data, which are highly informative and
provide extensive coverage. Still, SWI inversion is extremely demanding from a computation point of view. Computational improvements are discussed.

Abstract: A Coordinated Research Project (CRP) on “Origin of salinity and impacts on fresh groundwater resources: Optimisation of isotopic techniques” was initiated in 2000 within the framework of the IAEA Water Programme. Research groups from Australia, China, France, Israel, Italy, Jordan, Korea, Morocco, Pakistan, Sweden, Tunisia and United Kingdom of Great Britain participated in and contributed to the project. Two Research Co-ordination meetings were held in Vienna respectively in December 2000 and June 2003. The current publication is a compilation of final reports of six individual studies carried out under the CRP. The IAEA officer in charge of designing and coordinating all related work in this CRP and responsible for this publication was Cheikh B. Gaye of the Division of Physical and Chemical Sciences. Salinization is a global environmental problem that affects various aspects of our life such as changing the chemical composition of natural water resources (lakes, rivers, and groundwater), degrading the quality of agricultural and domestic water supplies, contributing to loss of biodiversity, loss of fertile soil, collapse of agricultural and fishery industries, and creating severe health problems (e.g., the Aral Basin). In Australia, for example, continuous soil salinization has become a massive environmental and economic disaster requiring drastic resource management changes. High levels of total or specific dissolved constituents associated with saline water other than sodium and chloride, may limit the use of the water for domestic, agriculture, and industrial applications. For instance, in some parts of Africa, China, and India, high fluoride content is often associated with saline groundwater and causes severe dental and skeletal fluorosis. Consequently, the “salinity” problem is only the “tip of the iceberg”. High levels of salinity often associated with high concentrations of sodium, sulphate, boron, fluoride, and bioaccumulated elements such as selenium, and arsenic. High salinity groundwater may also be associated with high radioactivity. Water salinization is a global problem but it is more severe in water-scarce areas, such as arid and semi-arid zones, where groundwater is the primary source of water. The increasing demand of groundwater has created tremendous pressure on the use of the resources resulting in lowering of water levels and an increase in salinization. In the Middle East for example, salinity is the main factor limiting the continued use of groundwater, and future reliance on groundwater in the region is further diminished as groundwater levels decline, creating increases in salinity and in exploitation costs. The CRP participants have addressed the following categories of salinity problems: River salinization (River Murray, Australia, and River Souss, Morocco); Salinization due to damming and base flow in the arid zone (River Souss, Morocco); Time of recharge/replenishment (Murray Basin, Australia, Disi aquifer, Jordan and Nubian sandstone aquifer, Israel); Time frames of salinization: past flushing versus modern mixing (Murray Basin, Australia, Disi aquifer, Jordan and Nubian sandstone aquifer, Israel); Times scale of salt accumulation (Murray Basin, Australia); Identifying the extent of seawater intrusion (Karachi, Pakistan, Souss coastal plain, Morocco, and Cheju Island, South Korea); Distinction between present and past seawater intrusion and evolution of salinity (Karachi, Pakistan, Souss coastal plain, Morocco, and Cheju Island, South Korea); Leaching of evaporites (Souss coastal plain, Morocco, Guanzhong Basin, China, Nubian sandstone aquifer, Israel, and Disi aquifer, Jordan); Mixing with formation water and/or brines (Nubian sandstone aquifer, Israel and Guanzhong Basin, China); Modification and salinity build-up by water-rock interactions (Souss coastal plain, Morocco, Guanzhong Basin, China, Nubian sandstone aquifer, Israel, Disi aquifer, Jordan, Murray Bain, Australia, Cheju Island, South Korea, and Karachi, Pakistan); Geothermal influence (demonstration study at Abano thermal basin, Italy and Cheju Island, South Korea); Urban environment – sewage contamination (Karachi, Pakistan); Agricultural environment – seepage of agricultural return flows (Souss coastal plain, Morocco, and Cheju Island, South Korea); Dry land salinization (Murray Basin, Australia, Nubian sandstone aquifer, Israel, Disi aquifer, Jordan, Souss coastal plain, Morocco, and Guanzhong Basin, China). The major objective of the CRP was to explore and develop isotopic tools that can be used to determine salinity sources and processes in aquifer systems. It was based on the implementation of several coordinated regional studies and a central “flagship” study in the Souss coastal aquifer of western Morocco. The research sites represent a large variety of examples of the salinization problem. These include salt-water intrusion into coastal aquifers (Morocco, Pakistan, Cheju Island in South Korea), dry land and inland salinization (Australia, Jordan, Israel, China); salinization of fossil groundwater (Australia, Israel, Jordan), and anthropogenic salinization (Pakistan, Morocco). In addition to individual efforts of the different member countries to investigate the origin of the salinization phenomena in their own country, special efforts were given to the integration of the isotopic techniques and crosslaboratories measurements. The integration approach enabled measurements of a large suite of isotopic tools in the selected research site in Morocco that include major and minor dissolved constituents, and the isotopic compositions of oxygen (18O/16O), hydrogen (2H/1 H), 3tritium (3H), sulphur (34S/32S), oxygen in the sulphate molecule (18O/16O), boron (11B/10B), strontium (87Sr/86Sr), carbon (14C and 13C/12C), chlorine (36Cl) and iodine (129I). The different case studies have indicated that aquifers can be impacted by both geogenic (natural) and anthropogenic salinity sources and often many basins are salinized by multiple sources of salinity. The CRP demonstrated that using the different isotopes and close integration with geochemical tools can provide key information on the origin and mechanisms of the multiple salinity sources. Isotope results from the pilot site in Morocco, confirm the existence of at least 3 salinity sources in the Souss plain: marine intrusion (present day and/or Pliocene sea water); Jurassic and Cretaceous evaporites; local contribution from the unsaturated zone; anthropogenic pollution. The high SO4/Cl ratio combined with low δ11 B, and very low 87Sr/86 Sr ratios (~ 0.7076), indicate dissolution of evaporites. The water composition at Bou lbaz;(TDS=8300, mg/l) characterized by Na/Cl ratio of 0.9, a low δ11B (24‰), and very high radiogenic 87Sr/86Sr ~ 0.711, suggests interaction of seawater/brine with silicate rocks for obtaining a non-marine signature. The δ13C TDIC values ranging from – 6 ‰ –13 ‰ could be attributed to contribution of pollution to groundwater through seepage from polluted rivers and local septic tank systems/ minor sewage drains. Agriculture return flows are characterized by high nitrate contents, high δ11 B (45‰), and high 87Sr/86Sr ratios (~ 0.711). Investigations carried out in Australia show that in addition to the groundwater salinization processes observed, the process of enhanced recharge following land clearing is resulting in water table rises close to the River Murray. In this area, groundwater is saline and water table rise is likely to increase the flow of the saline groundwater into the River Murray. Isotope data from the saline groundwater lens occurring in the northeast Guanzhong basin, China, is consistent with evaporation and mixing processes. The data from Israel shows that multiple sources of salinity affect the solute composition in the Nubian sandstone of the Negev. Based on integration of hydrochemical and isotopic data it was possible to distinguish between different water groups, to distinguish between “pristine” and “secondary” salinity sources, and identify modern versus paleo-recharge components. In the coastal aquifer of Karachi (Pakistan), anthropogenic sources are found responsible to affect the quality of local groundwater. The shallow / phreatic aquifers are recharged by a mixture of fresh waters from the Indus and Hub rivers as well as polluted waters from Layari and Malir rivers and their feeding drains both under natural infiltration conditions and artificially induced infiltration conditions, and to a much smaller extent, from direct recharge of local precipitation. Investigations carried out in Korea indicate clearly that seawater intrusion is the main source of groundwater salinity in Cheju Island.