Conjunctive use of saline and non-saline coastal aquifers for agriculture

Abstract

In a coastal region and the regions where the fresh water aquifer domain is overlain or underlain by saline water domain, the availability of usable water is limited due to the presence of salinity and unplanned use of the aquifers. In the current study, concepts and technologies have been developed for augmentation of usable water by combined use of saline and non-saline aquifers where the non-saline aquifer domain is underlain or overlain by saline aquifer domain. The developed technologies are the multi-well and the multi-strainer models that evaluate the design criteria for formulation of multi-well and multi-strainer well field development schemes. The models require salinity concentration data from non-saline and saline aquifer domains, salinity concentration of mixed discharges from saline and the non-saline aquifer domains, and the pre-determined acceptable level of concentration based on the purpose of well field development schemes. These data are generated by installation of monitoring wells in saline and non-saline aquifer domains and the monitoring well screening both the domains at the area where the well development schemes are required. The multi-well model generates the design criteria to formulate a multi-well field development scheme and the multi-strainer model generates the design criteria to formulate a multi-strainer well field development scheme. Both the models have been applied at two locations in the southwest coastal region of Bangladesh to formulate the schemes for irrigation water supply. The saline and the non-saline geologic formations are identified in the two selected locations of the region at a depth 10 m to 110 m at one location and 140 m to 300 m at another location. In the first location, the non-saline aquifer domain is overlain by saline aquifer domain and the salinity varies from 0.56 dS/m to 1.18 dS/m in the non-saline aquifer domain and from 1.8 dS/m to 5.01 dS/m in the saline aquifer domain. In the second location, the non-saline aquifer domain is underlain by saline aquifer domain and the salinity varies from 0.75 dS/m to 1.18 dS/m in the non-saline aquifer domain and from 1.6 dS/m to 5.38 dS/m in the saline aquifer. In both locations, the salinity in the saline aquifer domain is significantly higher than the salinity limits recommended for agriculture in the region. At both locations, the pH and the SAR are found to be good to excellent for crop production. The recommended design ratio of discharges between saline and non-saline aquifer domains for the multi-well scheme is 1:4 at the first location and 2:5 at the second location. The design ratio of discharges can be maintained either by adjusting the well numbers of equal capacity or different capacity or combination of both in saline and non-saline aquifer domains. The recommended design ratio of strainer lengths between saline and non-saline aquifer domains for the multi-strainer scheme is about 1:2 at the first location and about 2:1 at the second location. At the first location, only about 37 % of the traditionally designed length of the saline aquifer domain can be screened using the traditionally designed full length of strainer in non-saline aquifer domain to maintain the discharge of the multi-strainer scheme with salinity at acceptable level. At the second location, the scenario is reverse where full length of the traditionally design well can be screened with screening less than 50 % of the traditionally designed screen length of the non-saline aquifer domain to maintain the discharge of the multi-strainer scheme with salinity at acceptable level. Evaluation of an existing water resources management project in south west region shows that introduction of multi-well and the multi-strainer well schemes can augment availability of usable water supply to help eliminate the conflict among the farmers due to limited usable water. The current relevant policies enforced in Bangladesh should be modified to introduce the concepts and technologies for conjunctive use of saline and non-saline aquifers to achieve the full benefit of the developed schemes.