A new modified chloride mass balance approach based on aquifer hydraulic properties and other sources of chloride to assess rainfall recharge in brackish aquifers

A reasonable estimation of aquifer recharge can assist in the sustainable management of groundwater resources. The methodology to calculate aquifer recharge presents many challenges due to geological complexity and the existence of different water qualities in the aquifer. The classic chloride mass balance (CMB) method has been used widely for aquifer recharge estimation but the assumptions in the equations neglect other sources of chloride than rainfall; chloride accumulation due to evaporation and the hydraulic properties of the recharged aquifer. This research aims to introduce modifications and derive a new CMB equation that covers the shortcomings in the earlier classic CMB approach. The newly developed approach was verified mathematically to identify the sensitive parameters that affect the results of aquifer recharge. Also, it was verified by independent data and method (groundwater modelling) and showed that it is approaching reality about recharge estimation than the earlier classic method which underestimates aquifer recharge especially for brackish aquifers. This article applies the new modified method to sedimentary aquifers of brackish nature and carbonate aquifers predominantly with fresh water and brackish water. In particular, numerical modelling showed that the optimum percentage of aquifer recharge from rainfall for brackish aquifers is between 10% and 20%. Application of the classic method in brackish aquifers in Kuwait estimates the aquifer recharge to be 0.5%–3% due to rainfall, while the modified method yields aquifer recharge of 9.3%–20% from total rainfall which agrees with the results of numerical modelling approach. The application of the classic CMB method for the carbonate aquifers containing fresh to brackish groundwater in the West Bank, gave recharge percentage of 25% from rainfall while the modified method indicated the recharge percentage is 32% from rainfall. The results of the study obtained by the modified CMB technique are proved promising. A new chloride mass balance (CMB) approach for aquifer recharge is developed including chloride concentration in rainfall and accumulated in aquifer storage. The new CMB approach was validated mathematically and numerically and proved to be promising. New CMB estimated rainfall recharge in sedimentary aquifer to be as 9.3%–20% while classic CMB reflected 0.5%–3%. New CMB also determined that carbonate aquifers had a 32% rainfall recharge and 25% from classic CMB.