STORM AND DIEL SURFACE-WATER CHEMISTRY VARIATIONS IN THE RED RIVER NEAR QUESTA, NEW MEXICO

Mining operations and runoff from hydrothermal scars can affect the Red River water quality. Three water samples were collected at the USGS Questa gage, the first was collected 6 hours before a rainstorm that occurred on 9/17-18/02, the second close to the peak discharge about 14 hours after the rainstorm began (9/18/02), and the third during the falling limb of the hydrograph the following day (9/19/02). The discharge at the USGS Questa gaging station increased from 7 to a maximum of 90 cfs. Runoff from hydrothermally altered scars reached the Red River causing it to become acidic and highly turbid. From the first to the second sample, pH decreased from 7.8 to 4.8, alkalinity decreased from 50 to <1 mg/L, SO₄ increased from 160 to 320 mg/L, dissolved Fe increased from to 0.01 to 0.60 mg/L, dissolved Al increased from 0.19 to 2.9 mg/L, and dissolved Zn increased from 0.056 to 0.61 mg/L. The total recoverable concentrations (unfiltered and acidified with HNO₃) of Fe and Al increased from the first to the second sample by nearly 2 and 1 orders of magnitude, respectively; and the second sample contained mostly particulate Fe (99%) and Al (85%). When the third sample was collected, the total recoverable concentrations of Fe and Al had decreased by 7- and 4- fold from the second sample, but a much higher proportion of the Fe (44%) and Al (77%) were dissolved. The proportion of total dissolved Fe that was Fe(III) decreased from 67% to 29% from the first to the second sample and increased to 90% when the third sample was collected. The next month’s sample demonstrated that a longer time was required for total recoverable concentrations, proportions of dissolved to particulate, and proportions of Fe(III) to total Fe to decrease to pre-storm levels.

Diel samples were collected during high-flow (5/03) and low-flow (10/03) conditions. Diel sampling was conducted at 2 sites: the USGS gage and just upstream of the mill above the influence of any mining activities. At both sites during high-flow and at the gage during low-flow, concentrations of chemical constituents changed minimally. Above the Questa mine mill during low-flow, the pH increased at night from 8.15 to 8.34 due to CO₂ degassing. Only dissolved Zn concentrations had a diel pattern (and only for low-flow) decreasing to a minimum of 16 µg/L before sunrise and increasing to a maximum of 40 µg/L by about noon.