Open Cut » Environment
The purpose of this project was to estimate the rate of diffuse recharge via rainfall to groundwater systems. The rate of recharge (usually measured in mm/year) is a key ingredient to mines' environmental impact assessments.
Water is added to groundwater systems by processes that are collectively called “recharge”. Water that is recharging the groundwater system may come from various sources such as streams that lose water to groundwater, irrigation, or overbank flooding of streams. In contrast to these local sources of recharge, rainfall potentially recharges groundwater systems over the entire continent of Australia, and the focus of this project is to quantify the diffuse recharge via rainfall.
The groundwater recharge rate critically influences the predictions of numerical groundwater models used in mines' environmental impact assessments, and to a lesser extent, the requirements of mine de-watering operations. It is also critically influential when devising water sharing plans. However, the numerical value of recharge is often poorly constrained in Australia, so estimates such as “between 2% and 5% or rainfall” may be used. As a result, recharge remains a major source of uncertainty in groundwater impact assessments, which sometimes leads to debates during and after the mine approval process.
- The rate of groundwater recharge may be estimated using a variety of techniques. Some examples are: by employing tracers;
- by careful measurement of hydraulic conductivity (“Darcy method”);
- by measuring fluctuations in the water table;
- by measuring the recession-curve displacements for streams;
- or by creating a water-balance groundwater model.
This project relied on the Chloride Mass Balance (CMB) method to estimate recharge in the vicinity of boreholes where chloride concentration has been measured. It then upscaled these point-scale results to the entire study area by using regression equations and kriging, so that the predicted recharge at any point is dependent on local observations, rainfall, soil type, vegetation and surface geology. This process naturally allows uncertainty in the results to be quantified.
The results are presented on a 0.005° (approximately 500m) grid and summarised into recharge to the regions in the NSW water sharing plans and to the predominant geology types present in each coal field and basin.