Quimico

GEOTEST: development of field geochemical cells for on-site acid mine drainage monitoring

M. Carolina Soto1, Natalia Farfán1, Patricio Galaz1 and Lissette Flores2

1Water & Environment Area, Fundación Chile, Chile, msoto@fundacionchile.cl
2Environment Area, Compañía Minera Doña Inés de Collahuasi, Chile, lgflores@collahuasi.cl


Abstract
CURRENTLY, ACID MINE DRAINAGE IS OFTEN PREDICTEDBY A SERIES OF LABORATORY STATIC AND KINETIC TESTS WHICH ARE DEVELOPED ON SMALL-SCALE SAMPLES, DURING WEEKS OR MONTHS ON CONTROLLED LABORATORY CONDITIONS. AS WATER DRAINS FROM FULL-SCALE MASSIVE MINE WASTES, UNDER NATURAL AND VARIABLE CONDITIONS, PREDICTION OF DRAINAGE CHEMISTRY AND CONTAMINANT MOBILITY IS A MAJOR CHALLENGE OF SCALE AND TIME.

GEOTESTs are geochemical on-site medium-scale cellsdesigned for determining the capacity of acid/neutral drainage generation from waste rock dumps and for monitoring the geochemical dynamic evolution of the system through time under real minesite conditions. Additionally, they can be used to evaluate the behaviour of massive mine waste; to validate laboratory results; to quantify acid or neutral drainage (chemical and water balance); and, tovalidate prevention and control methods proposed initially.

A pilot GEOTEST is operating in a minesite at the north of Chile. After every rain or storm event, leachate is collected and analyzed in situ for critical physical-chemical parameters. Samples are collected for further laboratory geochemical analysis. Results of these tests will give important insights on drainage chemistry and will help tobetter understand and predict its changes through time.

Key Words: massive mine waste, prediction, waste rock dump, medium-scale cells, minesite, on-site kinetic test

INTRODUCTION
NOWADAYS, THE GEOCHEMICAL CHARACTERIZATION OF MASSIVE MINE WASTE ACID MINE DRAINAGE (AMD) PREDICTION IS OFTEN DEVELOPED BY A SERIES OF PRELIMINARY LABORATORY STATIC AND KINETIC TESTS THAT ALLOW FOR DETERMINATIONSIN THE FIRST INSTANCE OF ACID DRAINAGE POTENTIAL RISKS AND MOBILITY OF CONTAMINANTS. THESE TESTS CORRESPOND TO ACID BASE ACCOUNTING (ABA), NET ACID GENERATION (NAG) AND SYNTHETIC PRECIPITATION LEACHING PROCEDURE (SPLP) IN THE CASE OF STATIC TESTS, AND HUMIDITY CELLS AND LEACH COLUMNS IN THE CASE OF KINETIC TESTS, WHICH IN TOTAL ARE DEVELOPED ON SMALL-SCALE SAMPLES, WITH FINE MATERIAL, OVER APERIOD OF WEEKS OR MONTHS, AND CONDUCTED UNDER CONTROLLED LABORATORY CONDITIONS.

One of the greatest challenges in minesite drainage chemistry is adjusting predictions from small-scale samples to full size mine components, such as mined-rock piles, tailing impoundments, and mine walls (Morin and Hutt, 2007), where water drains from components that are millions to billions of tonnes in size,reaching up to 100 meters high and developed under natural and variable conditions.

Field on site tests are carried out to evaluate the behavior of sulfidic wastes under field conditions and can allow for calibration of the result of kinetic testing completed in the laboratory. They usually involve field measurements and observations of an application of interest with in situ monitoring includinginternal physical-chemical parameters measurements, runoff and groundwater monitoring, waste characterization, and other sampling and analysis (MEND Manual, 2009).

There are different scales in which prediction of AMD can be studied, and observations can be made at micro, meso and macro-scale. A micro-scale approach can be as complex as considering physical aspects like water flow paths passingover the surface of every particle of material or sequences of minerals. In meso-scale observations (hand-size or intermediate size), predictions based on test like ABA are typically checked against results of similarly scaled kinetic tests like humidity cells or leach columns (Morin and Hutt, 2007). In a full-scale approach (minesite components) several studies have found that there are many...