Coupled Baseline Estimation and Trend Analysis Approach to Differentiate Natural and Mine-Related Stresses on Groundwater Levels

Abstract

Permitting and regulatory obligations require mine operators to periodically report the spatial extent of mine-induced water-level changes, and to provide water-level monitoring data for review, along with explanations of all hydrologic stresses causing water-level trends. This study couples a baseline-estimation method with a curve-fitting trend-analysis approach to determine all natural, mining, and non-mining aquifer stresses affecting water levels in wells. The approach was applied to the Twin Creeks Mine monitoring network in north-central Nevada, USA. Stresses identified in wells were used to delineate the approximate extent of pumping effects and indicated that drawdowns have not coalesced between mining and non-mining pumping areas. Trend-analysis results indicated that ten study-area well hydrographs have natural trends, and statistical methods identified three of these wells as having statistically significant downward trends. Thus, caution should be used when interpreting the meaning of statistically significant downward trends, because the downward trend may be climate driven rather than pumping related. The curve-matching approach requires the development of a baseline water-level trend to understand expected natural fluctuations. The baseline trend assumes a dynamic-equilibrium natural condition, where long-term net changes in groundwater levels are zero. The baseline trend and trend analysis require a recharge proxy for the study area. This study developed recharge proxies using winter precipitation data, peak reservoir-storage volumes, and metered mine-water discharge to a surface-water channel. The curve-matching approach also can be used to identify and remove erroneous data, reconcile water-level and pumping datasets, or build hydrologic conceptualizations.