Salt Water Intrusion

Groundwater from the La Paz arid coastal aquifer in Baja California, Mexico, is essentially the only source of drinking water for the local population and tourists, as well as irrigation water for agricultural needs. The intensive exploitation of the aquifer and water cycling has resulted in groundwater abatement (up to 10 m) and high salinity (up to *5800 mg l-1 ). A study using hydrochemistry, isotopic (deuterium, oxygen-18 and car- bon-14) and gaseous tracers (chlorofluorocarbons CFC-11, CFC-12, CFC-113), as well as multivariate statistics, was developed to elucidate groundwater composition, flow and occurrence. Groundwater is of meteoric origin, and a large proportion is subject to evaporation. The primary natural recharge is generated in the El Novillo and Las Cruces ranges, and groundwater subsequently flows in a SE–NW direction toward the coast. The initial water type is the result of discordant dissolution of silicate minerals and ion exchange on soils. In the lower plain portion, the aquifer system is recharged from irrigation return flow and sea- water intrusion, which significantly affects groundwater chemistry. Nitrate and chloride concentrations indicate that groundwater is highly affected by an overuse of fertilizers in agricultural activities, but there is little effect from urban activities. Seawater intrusion has progressed rapidly during the past decade, and the impact currently extends 13 km inland. Radiocarbon residence time calculations suggest that groundwater is modern, with the exception of Cha- metla and El Centerario sites in the central and western lowlands with ages of up to 5000 years. These waters indicate an additional recharge source for the upconing of fossil groundwater or regional flow.

Bangladesh is densely populated and it is expected that the population increases significantly in the coming decade, up to 60% more by 2050 according to IIASA (2013). Demand for drinking water will increase accordingly. These developments may cause significant changes in the hydrological system, e.g. leading to a drop of groundwater pressures. Moreover, climate change and a sea level rise are predicted by the scenarios drawn by the International Panel of Climate Change (IPCC, 2013). This leads to, among others, an increase of salt water intrusion, in surface water as well as in groundwater.
The goal of the BRAC WASH II programme is to contribute to the attainment of the Millennium Development Goals by providing integrated water services, sanitation and hygiene promotion expanding to hard-to-reach areas and to under-served populations, in collaboration with government and other stakeholders. As part of the programme, innovative research is addressed in six relevant themes, one of which is the theme salt water intrusion. This project, SWIBANGLA – Managing salt water intrusion impacts in Bangladesh, contributes to the theme salt water intrusion.

In this project, the focus is on salt water intrusion in coastal groundwater systems, as groundwater is the main resource of drinking water. So, intrusion of sea water into the coastal surface water courses that are under the influence of tidal effect and storm surges are not studied here.

When the salt water intrusion processes and dynamics in groundwater systems are understood, adequate water management measures are feasible. Otherwise, a shortage of sufficient, clean fresh water at the right moment for domestic water supply as well as for agricultural use will occur. Currently, a blue print for accurate policy measures on the issue of salt water intrusion does not exist. In current water safety planning practices in Bangladesh, the issue of salt water intrusion in groundwater systems is underexposed.
The Department of Public Health and Engineering (DPHE) is the main authority in developing and improving Water Safety Plans for water supply technologies in Bangladesh. The outputs of SWIBANGLA are mainly meant for DPHE, which has been the main stakeholder during the project, but they are also meant for all governmental agencies, NGOs, universities and others that study, manage or have to deal with salt water intrusion impacts in Bangladesh. SWIBANLGA has two main general objectives which contribute to the achievement of the Millennium Development Goals:

increasing WASH sector stakeholders’ awareness of the salinization of drinking water resources and the threats posed by this process; increasing WASH sector stakeholders’ knowledge and skills necessary to anticipate on the salinization of drinking water resources.

The specific additional objectives leading to the main ones are:
-Create a better understanding of the process of salinization of drinking water resources in Bangladesh
-Provide recommendations for monitoring
-Provide recommendations for adaptation and mitigation
-Achieve an effective, tailored knowledge transfer between the Netherlands and Bangladesh
-Advise on the integration of the salinization issue in Water Safety Planning

Holocene marine transgressions are often put forward to explain observed groundwater salinities that extend far inland in deltas. This hypothesis was also proposed in the literature to explain the large land-inward extent of saline groundwater in the Nile Delta. The groundwater models previously built for the area used very large dispersivities to reconstruct this saline and brackish groundwater zone. However , this approach cannot explain the observed freshening of this zone. Here, we investigated the physical plausibility of the Holocene-transgression hypothesis to explain observed salinities by conducting a palaeohydrogeological reconstruction of groundwater salinity for the last 32 ka with a complex 3-D variable-density groundwater flow model, using a state-of-the-art version of the SEAWAT computer code that allows for parallel computation. Several scenarios with different lithologies and hypersaline groundwater provenances were simulated, of which five were selected that showed the best match with the observations. Amongst these selections, total freshwater volumes varied strongly, ranging from 1526 to 2659 km 3 , mainly due to uncertainties in the lithology offshore and at larger depths. This range is smaller (1511-1989 km 3) when we only consider the volumes of onshore fresh groundwater within 300 m depth. In all five selected scenarios the total volume of hypersaline groundwater exceeded that of seawater. We also show that during the last 32 ka, total freshwater volumes significantly declined, with a factor ranging from 2 to 5, due to the rising sea level. Furthermore , the time period required to reach a steady state under current boundary conditions exceeded 5.5 ka for all scenarios. Finally, under highly permeable conditions the marine transgression simulated with the palaeohydrogeological reconstruction led to a steeper fresh-salt interface compared to its steady-state equivalent, while low-permeable clay layers allowed for the preservation of fresh groundwater volumes. This shows that long-term transient simulations are needed when estimating present-day fresh-salt groundwater distributions in large deltas. The insights of this study are also applicable to other major deltaic areas, since many also experienced a Holocene marine transgression.