Unraveling controlling factors of concentration discharge relationships in a fractured aquifer dominant spring-shed: Evidence from mean transit time and radium reactive transport model

Abstract

This study unravels the controlling factors of concentration-discharge (C-Q) relations and solute export regimes of spring water based on long term monitoring of physicochemical parameters, radium and 222 Rn. A simplified analytical solution is derived to quantify radium based mean transient time (MTT) based on radium disequilibrium. Key hydrogeological and lithological parameters such as retardation factor (R f,Ra ) and MTT of radium nuclides, and effective surface areas (S e ) of subsurface are therefore quantified based on the long term monitoring records and discussed with the C-Q relation and solute export regimes. The study leads to the findings: 1) The spring water is characterized by very short MTT (days to weeks). Lower MTT, higher effective surface areas and R f,Ra are observed during the wet seasons than the dry seasons. 2) The slow mass transfer between immobile and mobile phases controls the solute exports of Cl − , Na + , K + , Ca 2+ , and Mg 2+ . Export regimes of theses solutes are negligibly influenced by dynamics of S e and retardation processes. 3) Lower MTT, higher S e and activation of NO 3 − and SO 4 2− lead to higher concentrations of NO 3 − and SO 4 2− in wet seasons, and vice versa in dry seasons. 4) SO 4 2− export is additionally influenced by co-precipitation of sulphate precipitates. HCO 3 − is controlled by dynamics of CO 2 dissolution caused by fluctuations of water temperature and water table. This study demonstrates that the disequilibrium of radium transport in the fractured aquifer can be deployed to investigate the metrics of the critical zone and the solute export regimes of a springshed.