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

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 unpl anned us e o f t he a quifers. In t he current s tudy, concepts and t echnologies have be en developed for augmentation of usable water by combined use of saline and non-saline aquifers where t he non -saline a quifer dom ain i s unde rlain or ove rlain b y s aline aquifer dom ain. T he developed technologies are the multi-well and the multi-strainer models that evaluate the design criteria f or f ormulation of mul ti-well a nd multi-strainer w ell f ield development s chemes. The models require salinity concentration data f rom non-saline and saline aquifer domains, salinity concentration of mixed discharges from saline and the non-saline aquifer domains, and the predetermined acceptable l evel of c oncentration b ased on t he pur pose of well f ield d evelopment schemes. These data ar e generated by ins tallation of moni toring wells in saline and non-saline aquifer domains and the monitoring well screening both the domains at the area where the well development schemes a re required. T he mul ti-well m odel generates the design criteria t o formulate a mul ti-well f ield development scheme and t he m ulti-strainer m odel g enerates the design criteria to formulate a mul ti-strainer w ell f ield development s cheme. B oth t he m odels have been applied at two locations in the southwest coastal region of Bangladesh to formulate the schemes for i rrigation 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 t o 300 m a t a nother l ocation. In the f irst loc ation, the non -saline a quifer dom ain i s overlain by saline aquifer domain and the salinity varies from 0.56 dS/m to 1.18 dS/m in the nonsaline a quifer dom ain a nd f rom 1.8 dS /m t o 5. 01 dS /m i n t he s aline a quifer dom ain. In t he second l ocation, t he non -saline aqu ifer dom ain i s unde rlain b y s aline a quifer dom ain a nd t he 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, t he pH a nd t he S AR a re f ound t o b e g ood t o e xcellent f or c rop pr oduction. T he 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 o r c ombination of bot h i n s aline a nd n on-saline a quifer dom ains. T he r ecommended design ratio of s trainer lengths b etween s aline and non -saline a quifer d omains f or the mul tistrainer 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 di scharge of the mul ti-strainer s cheme w ith salinity at a cceptable l evel. 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 a quifer dom ain to ma intain the discharge of t he m ulti-strainer s cheme w ith salinity at acceptable l evel. Evaluation of a n e xisting water r esources management p roject in s outh w est region s hows t hat introduction of m ulti-well and the mul ti-strainer w ell s chemes can augment availability of usable w ater s upply t o he lp e liminate t he c onflict a mong t he f armers due t o 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.