Hydrology Of Bangladesh

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Tom Donahou

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Aug 3, 2024, 5:10:39 PM8/3/24

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Salinity is a widespread problem along the Asian coast, mainly in reclaimed lands where most people live. These low-lying areas are vulnerable to impacts from tropical cyclone induced storm surges. The role of such surges on the long-term salinity of water resources, particularly the salinisation of drinking water ponds, a key water resource, requires further investigation. Here we show, using high-resolution measurements of pond hydrology and numerical modelling, that episodic inundation events cause the widespread salinisation of surface water and groundwater bodies in coastal areas. Sudden salt fluxes in ponds cause salinity build-up in the underlying sediments and become a source of salinity. Rapid clean-up of drinking ponds immediately after a surge event can significantly minimize these salinity impacts, which are likely to increase under climate change. Our study has implications for coastal land use and water resources management in tropical deltas.

Of particular concern, however, are episodic influxes of saline water due to storm surges and their long-term effect on pond salinities. Such occurrences have major impacts on drinking water quality and on the health and well-being of those affected27. Whilst no such event occurred during the period the pond was monitored, there was a sudden and sustained increase in salinity that occurred between 14 March 2014 and 24 May 2014 (blue line (BS01) in Fig. 2b). This was due to a failure in a pond levee that separated the drinking water pond from an adjacent shrimp farm pond35 with salinities around 15,000 mg/L. A major inflow of saline water occurred between 16 March and 24 May 2014 shown by a 0.3 m rise in pond water level during the dry season. This was accompanied by a sharp rise in pond salinity (BS01) from 2000 mg/L to a peak of over 8000 mg/L in June 2014. At this point the monsoon season started, causing dilutions during rainfall events, which were followed by rises in salinity during intervening dry periods. This sequence of falls and rises continued until September 2014, when steps were taken to remediate the pond. This was achieved, during the following two months, by flushing the pond with river water during high tide and the draining it during low tide. This process enabled pond salinities to be reduced to around 800 mg/L, so that it could be used for drinking water once again. Figure 2c shows groundwater salinities distributions from the pond base down to a depth of 1.5 m within the underlying sediment which implies the build-up of salinity in shallow groundwater due to the influx of saline water.

The steady state model (Supplementary Fig. S.6b) provided the initial conditions for the dynamic simulation model. Rainfall, abstraction, and evaporation data provided the main hydrological drivers. As discussed above, there are a range of controls on pond salinity. In addition to a reduction by rainfall dilution and an increase through evaporation, there are more complex exchanges, via diffusion and advection, with the groundwater36 directly beneath the pond. The interaction between pond water and groundwater is largely controlled by the hydraulic head gradient, which was generally vertically upward, resulting in a downward flow from the pond to the underlying groundwater. Under these conditions there was minimal influence of groundwater on pond salinity. This, however, could be reversed during periods of pond remediation. The locations of the pond stilling well (BS01) and the underlying groundwater piezometers (BP02 and BP03) are shown in Supplementary Figs. S.5c and 1d. The simulated pond water level data (BS01) confirm that a rise in water level is largely due to rainfall, whereas decreases are caused by evaporation (especially during the dry season), abstraction or seepage into the subsurface. We reproduce the hydraulic heads of BS01, BP02 and BP03 and salt concentrations of BS01, BP02 and BP03 (Fig. 2a,b) in a 2D model. Modelling of the pond water and salt balance provide a reasonable fit to the field data. Discrepancies between the model and the observed data are thought to be due to uncertainties in field measurements, e.g., abstractions by local users. The model parameters (Supplementary Table S.1) were calibrated manually by comparing model outputs to measured hydraulic heads and salinity measurements of BS01, BP02 and BP03 over the simulation period. The hydraulic heads in the underlying groundwater (BP02 and BP03) had a similar trend to the pond water level (BS01) but with fluctuations due to tidal variations in river level (which were not included in the model due to the absence of measured river levels at the site). Sensitivity analysis of the model was carried out for hydraulic conductivity, effective porosity, specific storage, longitudinal dispersity, transverse dispersity, evaporation, river level and pond abstraction by local users. This showed that, whilst changes in hydraulic conductivity and abstraction have a rapid and marked response on pond water level and groundwater hydraulic heads, they have little effect on pond and groundwater salinity. The groundwater hydraulic heads are the most sensitive to changes in the river level and evaporation. Groundwater salinity is sensitive to longitudinal dispersivity, whilst changes in effective porosity and specific storage have minimal influence on pond water level or salinity.

Conceptual mechanism: The salinisation of drinking water ponds in coastal deltas is linked to episodic inundation: (a) Normal conditions, water levels in the drinking water ponds are higher than the underlying groundwater, causing a downward flow, leading to freshening of the immediate underlying groundwater. (b) Following a storm surge, these ponds are inundated with saline water, which leads to a buildup of salinity in the groundwater immediately below the pond base due to downward infiltration caused by the downward head gradient. (c) When saline water is pumped out to remediate the pond, the natural downward gradient changes to an upward gradient for a brief period, bringing back saline water from the underlying groundwater, which mixes with accumulating rainwater in the pond, resulting in significant salinity. (d) If, however, the pond is remediated immediately after a storm surge, salinity buildup at the pond base cannot take hold and hydraulic reversal does not become a reason for salinity buildup.

In the long-term (2-year) remediation scenario (Fig. 4c), we simulate a surge inundation with saline river water (salinity 12,000 mg/L) in May 2014. The salinized pond is remediated by emptying after 2 years (May 2016) and allowed to fill with rainwater (salinity 30 mg/L). During the years of high pond salinities, the results show a progressive build up in salinity (due to downward flow from the pond) in the underlying sediments below the pond-base (red and yellow lines in Fig. 4c). Following its remediation through emptying the pond there is a reverse hydraulic gradient which allows the underlying saline groundwater to re-enter the pond (blue line in Fig. 4c and yellow line in Fig. 4d). Although the pond is progressively filled with fresh rainwater during the rainy season (May-Oct), the additional salt from the underlying sediments (over the duration of the reversed hydraulic head gradient, as explained in Supplementary Fig. S.8) maintains a raised level of salinity in the pond (a peak located at May 2016 in Fig. 4c). In contrast, if remediation is undertaken immediately following inundation (in this case, after 7 days in Fig. 4b) salinization is significantly reduced (red line in Fig. 4d) ensuring long-term reductions in pond salinity over many years. This example demonstrates the need for rapid response to pond salinization (pond salinities with no remediation and remediations were compared in Fig. 4d) in order to ensure low salinity pond drinking water in southern Bangladesh, and probably in other deltaic coasts in Asia22,37 as well. The need for rapid responses to polder and pond management is likely to become even more critical in future years as climate change results in projected increases in tropical cyclone intensity and frequency24,25,38, as explored below.

To explore the impacts at the polder scale of increased cyclone frequency due to climate change on groundwater salinity a further set of simulations are run for a period of 80 years with storm frequencies of every 3 years, and every year41 (Supplementary Fig. S.9). The results show (Fig. 5b,c) that groundwater salinities progressively increase and develop from the surface and infiltrated into the near-surface area over time. In the extreme case (every year), the surge-induced vertical salt infiltration penetrated into lateral saltwater intrusion and pushed the saltwater toe toward inland further (Fig. 5c). These models indicate the effect of climate change on both vertical saline inundation and lateral saltwater intrusion42. The results show that episodic storm surges are causing build-up of salinity in shallow groundwater and affecting surface water salinity and drinking water ponds. This is likely to worsen due to projected increased frequency of storm surges25 affecting coastal deltas. These cases are all based on one assumption: surge water overtops the embankments. It means that embankments play a crucial role in groundwater and surface water salinities. Therefore, if embankments work adequately to protect polders from tidal and surge water inundations, this could decrease groundwater salinisation considerably.

Our results indicate that historic tropical cyclone induced storm surges have contributed to the high groundwater salinities observed in coastal regions of Bangladesh. These saline groundwaters are also enhanced by the presence of saline ponds (e.g., due to shrimp farming). Highly salinity groundwater means that communal ponds are the main alternative low-cost source of drinking water for many living in these coastal regions. These ponds, however, are vulnerable to the impacts from saline water inundation due storm surges. This can result in either the loss of these important local water sources or to prolonged exposure to high salinity drinking water and the consequent health impacts10,14,15,17. Simulations show that if these ponds are left to freshen naturally by rainwater dilution it can take around 5 years or more (Fig. 4a). Conversations with pond owners in the three polders in the Dacope Upazila support this result. Residents reported that it typically took between 7 and 10 years for pond salinities to return to levels similar to those prior to a surge event. Rapid clean-up immediately after an inundation event, however, can reduce the salinity problem significantly. As climate change is likely to exacerbate these problems, polder management strategies are therefore vital both by protecting from saline inundation through effective embankment maintenance and repair and by discouraging activities that promote ponding of saline water and irrigation with salty water. Furthermore, when such inundations have occurred, rapid clean-up of drinking water ponds (i.e. within weeks) can greatly reduce impacts from exposure to high salinity drinking water. Whilst this work has focussed on Bangladesh, the results are applicable to other Asian deltas, such as the Mekong and Irrawaddy22, where similar drinking water sources are vulnerable to impacts from tropical cyclone storm surges.