Effect of Intermittent Flow on the Mobility of Metals from Abandoned Uranium Mine Waste Sites on Native American Land

Abstract:

Column experiments were conducted to study the effect of intermittent flow on the mobility of metals from abandoned uranium mine waste sites in Blue Gap Tachee (BGT), AZ and Laguna, NM. Intermittent flow represent the rainfall patterns in the southwestern United States, involving alternate wet and dry cycles. In order to simulate these rainfall patterns shorter wet periods of 15, 30, 60, 120 and 360 minutes, followed by longer dry periods of 24 hours, were adopted for the column experiments. The experiment involved sequential leaching of sediments from Laguna and BGT with 18MὨ water (pH 5.4), Synthetic Rain Water (SRW, pH 5.6), 10mM bicarbonate solution (pH 7.9) and 10mM acetic acid (pH 3.4) solution that represent the environmentally relevant conditions as witnessed in BGT water samples (pH 3.8 and 7.4). These reagents were specifically chosen to target most metal species through various transport mechanisms which include advective-dispersive forces, ion-exchange, desorption and dissolution. With just 18MὨ water and SRW almost 90 µg/L of U, 4500 µg/L of V and 20 µg/L of As were released from BGT mine waste while the Laguna sample showed the release of 380 µg/L of U, 2 µg/L of V and 40 µg/L of As. The released U concentrations were 3-13 times its EPA MCL for U which under natural circumstances could threaten the proximate communities. Bicarbonate and acetic acid extractions on the other hand released 3500-6000 µg/L of U, 50–3000 µg/L of V and 14-35 µg/L of As from both Laguna and BGT mine waste respectively. Based on our previously published results, U and V from the uranyl-vanadate (U-V) species within BGT mine waste samples were only partially released with bicarbonate unlike the column experiments where almost all of the U and V from the U-V species were dissolved and released using 10mM bicarbonate solution. For reference, the columns were also leached continuously with bicarbonate and acetic acid for a week (each), to identify if the phases were kinetically or thermodynamically controlled. A 1D reactive transport model is also being applied to better interpret the role of these interplaying mechanisms.Column experiments were conducted to study the effect of intermittent flow on the mobility of metals from abandoned uranium mine waste sites in Blue Gap Tachee (BGT), AZ and Laguna, NM. Intermittent flow represent the rainfall patterns in the southwestern United States, involving alternate wet and dry cycles. In order to simulate these rainfall patterns shorter wet periods of 15, 30, 60, 120 and 360 minutes, followed by longer dry periods of 24 hours, were adopted for the column experiments. The experiment involved sequential leaching of sediments from Laguna and BGT with 18MὨ water (pH 5.4), Synthetic Rain Water (SRW, pH 5.6), 10mM bicarbonate solution (pH 7.9) and 10mM acetic acid (pH 3.4) solution that represent the environmentally relevant conditions as witnessed in BGT water samples (pH 3.8 and 7.4). These reagents were specifically chosen to target most metal species through various transport mechanisms which include advective-dispersive forces, ion-exchange, desorption and dissolution. With just 18MὨ water and SRW almost 90 µg/L of U, 4500 µg/L of V and 20 µg/L of As were released from BGT mine waste while the Laguna sample showed the release of 380 µg/L of U, 2 µg/L of V and 40 µg/L of As. The released U concentrations were 3-13 times its EPA MCL for U which under natural circumstances could threaten the proximate communities. Bicarbonate and acetic acid extractions on the other hand released 3500-6000 µg/L of U, 50–3000 µg/L of V and 14-35 µg/L of As from both Laguna and BGT mine waste respectively. Based on our previously published results, U and V from the uranyl-vanadate (U-V) species within BGT mine waste samples were only partially released with bicarbonate unlike the column experiments where almost all of the U and V from the U-V species were dissolved and released using 10mM bicarbonate solution. For reference, the columns were also leached continuously with bicarbonate and acetic acid for a week (each), to identify if the phases were kinetically or thermodynamically controlled. A 1D reactive transport model is also being applied to better interpret the role of these interplaying mechanisms.