Estimating groundwater recharge based on mass balance evaluation of unsaturated zone in a coastal catchment characterized by tropical rainforest weather conditions

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Estimation of recharge is necessary for evaluating water resources in the north Kelantan catchment of Malaysia due to rapid development and consequently over-drafting of available groundwater. In order to determine the amount of recharge, a study of natural salts (i.e., chloride and sulfate) was conducted to estimate groundwater recharge rates. Studies of the chloride mass balance (CMB) and sulfate mass balance (SMB) were carried out in the unsaturated zone under rainforest weather conditions with a relatively shallow groundwater table. Ten soil profiles in different parts of the catchment representing a range of soil textures were selected for the study. The CMB results showed that the mean recharge rate is 527.87 mm/year. The highest and lowest calculated recharge rates in the study area were 8 and 31 % of local rainfall. Times required to accumulate chloride/sulfate beneath the natural ecosystem highly depend on profile texture and the CMB age is maximized at KP94 and KT121 sites to be 15.01 and 14.15 years, respectively. The SMB method produced lower recharge rates than those based on chloride, yielding an average of 364.60 mm/year, with values ranging between 7 and 26 % of local rainfall and the SMB age is 21.01 years at KP94 site. Mobilization decreases with fine texture and low permeability and increases with coarse texture and high permeability. In this way, low permeability of surface geology not only controls the recharge rate but also causes high buffering in holding salts. Nevertheless, higher sulfate concentration in profile may be due to different dissolution or sorption processes which are required to be studied thoroughly. Based on two types of scenarios representing natural water movement in unsaturated zone using HYDRUS-1D, recharge rates were simulated to be 419 mm in profile with overlain fine texture soil and 636 mm in profile characterizing with single coarse texture for a total time of 213 days. Our study suggests the feasibility of using the mass balance methods in tropical rainforest weather conditions. The information may improve the accuracy of prediction of possible groundwater pollution and groundwater management in the future.