Estimation of Optimal Groundwater Substitution Volumes Using a Distributed Parameter Groundwater Model and Prediction Uncertainty Analysis

Abstract

This paper describes the development of a methodology that can be used for determining the spatial and temporal distribution of additional water volumes required to meet a defined groundwater target, such as an historical peizometric surface. The methodology is demonstrated on a case study concerned with mitigating declining groundwater levels in an alluvial aquifer intensively used for agriculture in the Lockyer Valley, Queensland, Australia. The proposed mitigation measure is the importing of large volumes of purified recycled water (PRW) from a large scale indirect potable reuse scheme into the aquifer system. The developed methodology employs both a groundwater flow model together with linear uncertainty analysis. Therefore for the case study, a distributed parameter numerical groundwater flow model was developed for the Lockyer valley alluvial aquifer system using MODFLOW, calibrated to observed groundwater levels, and further constrained by estimates of diffuse and river recharge from water balance studies. The model was used to simulate groundwater levels in the aquifer over a 20-year period. Optimal spatial and temporal distribution of volumes of imported water required to mitigate declining groundwater levels over that period were then estimated, using a modified version of the MODFLOW General Head Boundary (GHB) package. Uncertainty in the predicted import volumes was estimated using linear bayesian analysis principles. The relative worth of data from each observation bore within the groundwater monitoring network was also assessed in terms of the extent to which predictions of import water volumes were made more reliable when furnished with that data. Application of the methodology to the Lockyer alluvial aquifer system illustrated the suitability of the developed methodology for estimating the additional water volumes required for managed aquifer recharge or groundwater substitution schemes in similarly over-exploited aquifers.